What Darwin Didn't Know


What Darwin Didn't Know

Evolutionary biologist Armand Marie Leroi tells the story of evolution theory since Darwin postulated it in 1859 and how the new science of 'evo devo' might predict its future.


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We live in a world of exquisite diversity,

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with more species than we can possibly count.

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Here in Lake Malawi, for instance,

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there are hundreds of different fish that are found nowhere else.

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Why so many?

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Why so different?

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150 years ago Charles Darwin published On The Origin Of Species.

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And in that one great book he asked the right question...

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..and gave the right answer...

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Where, asked Darwin, does all this diversity come from?

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And answered that it must be the product of evolution.

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Species, he argued, give rise to other species

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and as they do so, they change.

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The changes are minute and subtle,

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but given enough time the results could be spectacular.

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And so they are.

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Darwin's explanation for life on earth

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was so seductive and so simple that it seems obvious today.

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And yet, Darwin's explanation of how evolution works

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was riddled with holes.

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Its logical foundations were shaky.

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His evidence was weak.

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There was so much he did not,

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could not, know.

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Darwin trusted that future generations of scientists

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would complete his work and prove the essential truth of his vision.

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And for 150 years that is what we have been doing.

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In this film, I'll chart the decline, fall

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and ultimate triumph of Darwin's ideas.

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And I'll show how evolutionary theory has itself evolved,

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so that it is now far more vast and subtle than ever he imagined.

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In September 1835

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Charles Darwin arrived in the Galapagos Archipelago...

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..and did what he always did when arriving in a new place -

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he got out his gun and began to collect.

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Among the many inhabitants of the Galapagos that Darwin

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pinned, pickled, shot, or stuffed are these four birds.

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They don't look like much, but look at them closely.

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Look at them as Darwin looked at them,

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and you can see the beginnings of evolution.

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They are mocking birds.

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Each comes from a different island,

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and each is subtly different from the others.

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They differ in the shape of their bills and the size of their bodies,

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and the colour of their plumage.

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It was these differences that first caused Darwin to wonder

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whether species might transform over time.

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Darwin surmised that the birds were variants of the same species

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and must therefore descend from a common ancestor -

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a mocking bird which had somehow

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found its way to the Galapagos many years earlier.

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That was Darwin's hunch,

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but how to prove it?

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He certainly couldn't produce the hypothetical ancestor -

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it was lost in time.

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So he did what scientists do when they don't have the data -

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he appealed to an analogy.

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Darwin bred pigeons.

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They were, for him, a microcosm of evolution.

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They showed how any creature could, given enough time,

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be transformed into something very different from its ancestor.

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For, implausible though it may seem,

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these gorgeous, monstrous, inbred aristocrats of the avian world -

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the Scandaroon, the Frillback, the Jacobin, not to forget the Mookee -

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are all descended from this - the plebeian rock pigeon.

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All pigeons are, at birth, subtly different from each other.

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Breeders select those with desirable features to survive and reproduce...

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..and they cull the rest.

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The desirable features accumulate from generation to generation

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and become exaggerated.

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And so, remarkably quickly, the birds evolve.

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Nature, Darwin said, works like that.

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It favours some features and permits others to whither away.

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He called this process natural selection.

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All this explains why the first chapter of The Origin Of Species is

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not about the wonders of the natural world, but rather about pigeons.

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Understand the pigeon, he is saying, believe the pigeon,

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and all the rest follows.

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Or does it?

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For Darwin had a problem.

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Natural selection was the cornerstone of his theory.

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It was, for him, the engine of evolution.

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And yet it was by no means clear

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that natural selection really worked.

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There is, he said, a war of nature.

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Famine, violence and death are everywhere.

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Species and individuals are locked in a struggle for existence.

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The strong survive and reproduce...

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..while the weak go to the wall.

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Given enough variation, this selective pressure

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is enough to bring about slow, incremental change.

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This was the theory of evolution by natural selection

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that Darwin unveiled in The Origin Of Species.

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While the idea of evolution was not in itself new,

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no-one had argued it more forcefully,

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or documented the evidence for it, with greater rigour.

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But was it right?

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Had Darwin really made his case?

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Of course, many religious types hated the very idea of evolution.

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But some of Darwin's fellow scientists weren't too keen either.

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Notably Richard Owen, who wrote one of the first reviews of The Origin.

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Richard Owen, premier palaeontologist,

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coined the term 'dinosaur', helped design these things.

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Rampaging through a South London park,

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these marvellous reconstructions were built in the 1850s.

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They are a tableau of dinosaur life based on Owen's research.

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Owen had vague evolutionary leanings.

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He thought that species change intermittently,

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under the influence of some divine law

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and that periodically,

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they are swept away in some great catastrophe.

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He loathed Darwin's godless evolutionism.

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Owen was a thoroughly nasty piece of work.

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His review of The Origin, rich in malice, dripping with sarcasm,

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damns Darwin even as he praises himself.

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All anonymously of course.

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"Mr Darwin's rash speculations degrade science.

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"He's as bad as the French.

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"And", continues Owen, "he doesn't know anything about fossils.

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"If he did, he would know that

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"ichthyosaurs appear in the lower Jurassic,

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"stay there pretty much unchanged, and then just disappear -

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"no sign of evolution there."

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Owen's venom was probably born from mere spite.

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Still, he did seem to have the fossil record on his side.

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According to Darwin's theory,

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gradual change should be visible in the rocks.

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But it wasn't.

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Instead, species seemed to arrive and depart,

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leaving little in between.

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Such gaps in the fossil record would haunt Darwin's theory.

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The evidence for natural selection simply wasn't there.

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Even his friends had their doubts.

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Thomas Henry Huxley worked here at Imperial College London,

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where I now work.

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A firebrand and a populist, they called him Darwin's bulldog.

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Huxley also reviewed The Origin.

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"It's a magnificent work.

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"It makes the case for evolution."

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But then he turns to natural selection.

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"Yes, it's logical, yes, it's simple, but has Mr Darwin

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"actually seen a species originate by natural selection?

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"Can he even prove that it really exists?"

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" Well, no. It's a hypothesis,

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"perhaps even the best one going, but, and I say this as a friend,

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"you understand, Mr Darwin really hasn't proved his point."

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Huxley said natural selection can't be seen.

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Others said it doesn't work.

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They claimed that it was logically flawed.

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That it was inconsistent with Darwin's account of inheritance,

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of how species transmit their features

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from one generation to the next.

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This is how Darwin thought inheritance works.

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Suppose one parent has dark feathers, fur or skin,

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the colour of black coffee, while the other is milky white,

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their progeny would be a mix of the two.

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They would be a blend.

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It seems like an innocuous idea.

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Quite a reasonable one, too.

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After all, isn't this how human skin colour is inherited?

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But Darwin had walked into a theoretical trap,

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and a Scottish engineer called Fleeming Jenkin sprang it.

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This is how Jenkin phrased the argument.

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"Imagine that a white man arrives on an island of negroes.

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"He would, no doubt make himself king.

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"He would take many negro wives and father many mulatto children.

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"Yet no matter how successful our hero is, no matter how superior,

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"his coffee-coloured descendants would become progressively darker.

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"Within a few generations, all trace of his presence would disappear."

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Let's ignore, if we can, the casual racism.

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This is Scotland in the 1860s, and Jenkin had a point.

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Variation is the stuff of evolution,

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and if variation blends then it disappears.

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And as it disappears, so the power of natural selection ebbs away.

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Jenkin's challenge was serious, and Darwin had no response.

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How could he? His theory required some system, some law of inheritance

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in which variation did not blend,

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but remained stable over the generations.

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And yet no-one really knew how such a system could work.

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The thing is, Darwin knew all this.

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And we know that he knew because he told us so.

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Perhaps the most wonderful chapter

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of The Origin Of Species is Chapter VI.

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It's called Difficulties Of The Theory.

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Charles Darwin exposes, with unbearable candour,

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devastating honesty, all the weaknesses of his theory.

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He tells us all the reasons he may be wrong,

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the reasons that his critics pointed out, and more,

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but then, appeals to future generations of scientists

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to draw inspiration from his book,

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solve the difficulties with which his theory is riddled.

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It's easy to forget that Darwin was not the first evolutionist.

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50 years before The Origin Of Species,

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a Frenchman had proposed a theory of evolution,

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albeit less coherent and comprehensive than Darwin's.

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His name - Jean-Baptiste Lamarck.

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Lamarck was Professor of Zoology at the Museum d'Histoire Naturelle.

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He published his ideas on evolution in the year Darwin was born.

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They were, however, very different.

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When an animal, any animal, uses an organ, Lamarck argued,

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it becomes strengthened and enlarged.

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That's fairly obvious.

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We all know that exercise modifies the shape of our body.

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It's the same for other creatures as well.

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But Lamarck went further.

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He argued that these changes,

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acquired in one's lifetime, were passed on.

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And it is this, the inheritance of acquired characteristics,

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as it came to be known, that's the engine of evolution.

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The icon of Lamarckism is the giraffe.

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According to Lamarck, some ancestral giraffe had stretched its neck

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reaching for leaves on the highest branches.

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That stretched neck had been passed on to its offspring, who, in turn,

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had stretched their necks even further,

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so that now all giraffes have long necks.

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There is a seductive, intuitive, quality to Lamarck's logic,

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one that Darwin, confronting the inadequacies of his own theory,

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found increasingly hard to resist.

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The giraffe's neck is a cliche.

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It's in every textbook that explains the difference

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between Darwinian and Lamarckian evolution.

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But if you actually read what Darwin says about the giraffe

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in the sixth and final edition of The Origin,

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what you find is something rather different.

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First he talks about natural selection.

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That's what gives you the giraffe's neck.

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But then he adds another line

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about the inherited effects of the increased use of parts.

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Together, they give you the giraffe's neck.

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"Increased use of parts"?

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What's going on here?

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That's pure Lamarck.

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Can it be that Darwin, in his dotage, is becoming less Darwinian?

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Well, yes.

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Perhaps natural selection is not as powerful as once he had thought.

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A recantation? No.

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Just the candour of an old man

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who had spent his life trying to understand the world.

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Darwin died in April 1882.

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He had wanted to be buried quietly near his house in Kent,

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but his supporters arranged a funeral here at Westminster Abbey.

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They turned the agnostic into a saint

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of the new secular materialist age.

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It was the apotheosis of Charles Robert Darwin.

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He had become a great Briton.

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But the eulogies rang hollow.

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Darwin had shown that life on earth was the result of natural laws.

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But what were those laws?

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Everyone - everyone, that is, who mattered -

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agreed that evolution was a fact.

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But natural selection?

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No thanks.

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Even as Darwin lay in state,

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some Darwinians were breaking ranks.

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Chief among them, Dutch botanist, Hugo De Vries.

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Inspired by Darwin, he was searching for a suitable organism

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with which to investigate the workings of inheritance...

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and found one.

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He chose a plant called Oenothera lamarckiana.

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Gardeners will know it as the evening primrose

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for it blooms at dusk.

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It is found throughout the dunes that protect Holland from the sea.

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And it's really just a weed.

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Though lovely for all that.

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De Vries discovered that Oenothera lamarckiana

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occasionally produces progeny that looked very different from itself,

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that have different stems, leaves, flowers.

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These new variants he found did not blend

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but were stable, as stable as new species.

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Coining a term, De Vries called these dramatic variations mutations.

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Following his discovery,

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De Vries was made director of the botanical gardens in Amsterdam.

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He bred and crossbred more Oenotheras, 53,000 of them.

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It seems like a lot, but then again, he was Dutch.

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This is the palm house that Amsterdam built for De Vries.

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And these are some of his flowers pressed for posterity.

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They are ancient and desiccated

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but you can still see the differences in growth and form.

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Mutation, it seemed, could produce radically new plants.

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It could even, said De Vries, produce new species.

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This was all very unDarwinian.

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What of the vertiginous time scales, the infinity of incremental steps,

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the grandeur of Darwin's view of life?

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Irrelevant, said De Vries.

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The origin of species requires only one thing: mutation.

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He called it his Mutation Theory.

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It made him famous.

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For Lamarck, he said, the origin of species was a natural phenomenon.

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For Darwin, the object of scientific investigation,

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for De Vries, he liked to talk of himself in the third person,

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it was the object of experimental enquiry.

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Lamarck, Darwin, De Vries.

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No doubts about HIS place in the pantheon, then.

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What De Vries or no-one else realised at the time

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was that Oenothera lamarckiana was a genetic freak.

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Few other organisms mutate so spectacularly.

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He had based his entire theory on one, very peculiar, species.

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That, however, didn't stop the rise of mutationism.

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Others began to investigate the oddities of nature.

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In Britain, a Cambridge biologist, William Bateson,

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published materials for the study of variation.

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A collection of two-headed turtles,

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girls with four ears,

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and eight-fingered hands.

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It was a medieval monsters and marvels book,

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reworked for the evolutionary age.

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Mutation was the real creative force behind evolution

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and natural selection, said the mutationists, just wasn't needed.

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London 1909.

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Darwin is long dead, and his theory is 50 years old.

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The British Museum of Natural History celebrates

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with an exhibition of Darwiniana:

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specimens, letters, manuscripts.

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It's a magnificent celebration,

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a worthy commemoration of the man who gave us evolution.

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But something is missing,

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something upon which the organisers refuse to be drawn.

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Natural selection.

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You might have expected that the South Kensington museum,

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now a temple of evolutionism,

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would have wanted to tell the public about Darwin's theory.

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But no, that's all too controversial for the keepers and curators who'd

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much rather not commit themselves in the great evolution debate.

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Hardly courageous, but understandable.

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By 1909 scientific consensus had shifted against Darwin's theory.

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Evolution by natural selection was almost extinct.

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Just as Darwinism was at its nadir, a revival was underway.

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For Darwin's critics were, themselves, coming under attack.

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And leading the vanguard was a German scientist, August Weismann.

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Weismann was a doctor, a biologist and above all a great Darwinian.

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He would revive the case for natural selection.

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His key exhibit, an insect called Papilio dardanus.

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Papilo dardanus is a butterfly that lives throughout Africa,

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and the females of the species are mimics.

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This female Papilo dardanus here

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mimics this altogether unrelated species.

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And this one over here mimics something completely different.

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And it does so in every detail.

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Over many generations, Papilio dardanus females have evolved.

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The shapes and colours of their wings have transformed...

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..and the reason why is very obvious.

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Birds eat butterflies,

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so many butterflies have evolved offensive chemicals

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that make them taste repugnant.

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Papilio dardanus doesn't,

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but by mimicking those that do, they can fool the birds.

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They are the cardboard tanks in the battle of nature.

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Here, said Weismann, is evidence that Darwin was right.

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Only natural selection, that slow and subtle craftsman,

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working in infinitesimally small steps,

0:27:180:27:22

could make two unrelated butterfly species so very much alike.

0:27:220:27:27

Hugo De Vries and his fellow mutationists had argued that species

0:27:320:27:37

originate instantaneously by single dramatic mutations.

0:27:370:27:42

No, said Weismann, they evolve gradually

0:27:470:27:51

by the accumulation of a great many tiny mutations.

0:27:510:27:55

Natural selection is a subtle force -

0:27:550:27:58

as subtle as the markings on the wings of a butterfly.

0:27:580:28:03

But Weissman's real ire was reserved

0:28:060:28:09

for that other great anti-Darwinian theory, Lamarckism.

0:28:090:28:14

He knew that sperm and eggs carry the material of inheritance.

0:28:160:28:22

But where, he asked, do they come from?

0:28:220:28:25

By tracing the origin and fate of the cells in the embryo,

0:28:270:28:30

Weismann realised that the cells that give rise to sperm and eggs

0:28:300:28:35

were quickly isolated from the rest of the body's cells.

0:28:350:28:38

That they formed a separate lineage.

0:28:380:28:42

That, said Weissman, is why acquired characteristics

0:28:450:28:48

could not be passed on to future generations,

0:28:480:28:52

why Lamarck was wrong, and why the giraffe stretches its neck in vain.

0:28:520:28:59

A body could strive, suffer, stretch and sacrifice

0:29:010:29:06

and none of it would matter.

0:29:060:29:08

All bodies must die was Weismann's message,

0:29:080:29:13

only your eggs and sperm have even a shot at immortality.

0:29:130:29:17

But Weissman did more.

0:29:190:29:22

He reasoned that the material of inheritance

0:29:220:29:25

was something physical in the nucleus of each sperm and egg cell.

0:29:250:29:29

He called this material germ plasm.

0:29:290:29:32

Looking closer, his contemporaries saw distinct particles

0:29:340:29:38

within the germ plasm - chromosomes.

0:29:380:29:41

With reproduction, chromosomes mix, mingle, and recombine,

0:29:440:29:49

but they never blend.

0:29:490:29:51

They are always passed on intact.

0:29:530:29:57

This dance of the chromosomes

0:29:580:30:00

confirmed what an Augustinian monk has supposed three decades earlier.

0:30:000:30:07

Gregor Mendel, he's the archetypal scientific hero:

0:30:090:30:14

works away breeding peas in a Moravian monastery,

0:30:140:30:17

publishes two luminous papers in an obscure journal that no-one reads,

0:30:170:30:24

give up science and becomes,

0:30:240:30:27

like so many great scientists, an administrator.

0:30:270:30:30

Mendel was appointed abbot of Brno monastery.

0:30:340:30:38

He abandoned his experiments; his publications were forgotten

0:30:380:30:43

for 34 years, and when he died his papers were burned.

0:30:430:30:48

When, in 1900, his experiments were rediscovered and republished,

0:30:480:30:53

they became, however, the stuff of scientific legend.

0:30:530:30:57

Think about it, you're breeding peas,

0:31:030:31:06

green peas, yellow peas, wrinkly peas, smooth peas.

0:31:060:31:11

You count the numbers of peas in each generation,

0:31:110:31:17

calculate a few ratios, and you discover what everyone else,

0:31:170:31:22

what Darwin himself had missed.

0:31:220:31:25

The laws that rule the inheritance of nearly every living thing.

0:31:250:31:31

Here, among Mendel's peas, were mathematical laws

0:31:320:31:36

that explained how traits are passed down the generations.

0:31:360:31:41

And, rather wonderfully, these laws, mere statistical abstractions,

0:31:410:31:48

were the very system of inheritance that natural selection needed.

0:31:480:31:53

They gave natural selection

0:31:530:31:56

the supply of heritable variation that it needed to work.

0:31:560:32:00

Rothamsted Agricultural Station in Hertfordshire

0:32:100:32:13

is an unlikely landmark in the history of evolutionary biology.

0:32:130:32:19

What it's really famous for is an experiment aimed at estimating

0:32:190:32:23

the effects of fertilizers on crop yields.

0:32:230:32:26

The experiment had begun in the 1840s.

0:32:280:32:31

Every year, samples had been collected and stored

0:32:310:32:34

and there they lay.

0:32:340:32:35

Until 1919 when they hired a young Cambridge mathematician

0:32:350:32:41

to analyse them.

0:32:410:32:42

His name was Ronald Aylmer Fisher.

0:32:420:32:47

Fisher was a prodigy.

0:32:490:32:51

Profoundly myopic,

0:32:510:32:53

he had learnt to visualize mathematical problems in his head.

0:32:530:32:58

You wouldn't think that such a clever man

0:33:020:33:05

would be happy calculating agricultural yields.

0:33:050:33:09

But he liked it.

0:33:100:33:12

There were lots of numbers to crunch,

0:33:140:33:17

and no-one knew how to do it.

0:33:170:33:19

No problem, said Fisher,

0:33:190:33:22

and invented some new statistics.

0:33:220:33:24

Some new statistics?

0:33:290:33:30

Fisher invented just about every statistical test I've ever used.

0:33:300:33:35

And not just the tests. When Fisher wanted to solve a problem,

0:33:350:33:39

he would invent a whole new branch of mathematics.

0:33:390:33:43

But Fisher was interested in more than crop yields.

0:33:450:33:50

He was also rather keen on eugenics.

0:33:500:33:54

NARRATOR: In institutions such as this all over the country,

0:33:540:33:57

mental defectives are cared for.

0:33:570:33:59

But it would have been better by far,

0:33:590:34:02

for them and for the rest of the community,

0:34:020:34:04

if they had never been born.

0:34:040:34:06

Fisher was worried that the British were becoming thick.

0:34:060:34:11

That the poor, feckless and stupid

0:34:110:34:14

were outbreeding the rich, thrifty and smart.

0:34:140:34:18

So you see, that in a mere matter of four generations

0:34:180:34:21

individuals below the average

0:34:210:34:23

have become more than five times as abundant as those above it.

0:34:230:34:26

And so if we want to maintain the race at a high level,

0:34:270:34:31

everybody sound in body and mind should marry

0:34:310:34:34

and have enough children to perpetuate their stock

0:34:340:34:37

and carry on the race.

0:34:370:34:39

Fisher fathered eight children himself -

0:34:390:34:42

his own personal eugenics programme.

0:34:420:34:46

It was, of course, absurd.

0:34:480:34:50

There's no evidence that the nation's collective IQ

0:34:500:34:54

was in terminal decline.

0:34:540:34:56

Yet out of Fisher's eugenical obsessions came something wonderful.

0:34:560:35:00

For in the evenings between calculating correlation coefficients

0:35:000:35:04

and fathering children, he thought about natural selection.

0:35:040:35:09

For Fisher, natural selection was a force

0:35:140:35:17

rather like the waves that beat against a beach.

0:35:170:35:20

Just as they may at times pound the shore without relent

0:35:220:35:27

and at other times lap gently,

0:35:270:35:30

so too natural selection may gust or whisper, but it never disappears.

0:35:300:35:35

And just as the waves winnow, sift and sort the pebbles on this beach,

0:35:390:35:44

so natural selection winnows, sifts and sorts

0:35:440:35:47

the variation within species.

0:35:470:35:49

And it is this sorting that is evolution itself.

0:35:510:35:55

All this Fisher described with a single equation.

0:35:590:36:04

He called it his Fundamental Theorem of Natural Selection,

0:36:040:36:10

and said that it was supreme among the laws of biological science.

0:36:100:36:16

He even compared it in its scope and power and generality

0:36:160:36:21

to the Second Law of Thermodynamics.

0:36:210:36:24

Fisher turned natural selection into a formula.

0:36:280:36:32

But a formula without data isn't much good.

0:36:320:36:37

You have to show that it actually works -

0:36:370:36:39

that it says something useful

0:36:390:36:41

about living, breathing, copulating creatures.

0:36:410:36:45

Which brings us to a rather dull-looking moth.

0:36:470:36:51

Here is a story of natural selection in action.

0:36:540:36:58

The story of Biston bestularia - the peppered moth.

0:36:580:37:03

Once upon a time, it was the colour of speckled ivory.

0:37:050:37:09

This colour was an adaptation,

0:37:090:37:12

camouflaging the moth as it rested on woodland lichens,

0:37:120:37:16

protecting it from the birds that would prey on it.

0:37:160:37:19

But then came the Industrial Revolution.

0:37:190:37:23

Soot killed the lichens and turned the trees black.

0:37:230:37:27

The moths were no longer camouflaged,

0:37:290:37:32

they were exposed and vulnerable to attack.

0:37:320:37:35

So they began to evolve - fast.

0:37:370:37:41

A new, dark, form of the moth spread.

0:37:410:37:44

By the 1950s it was found across Britain.

0:37:440:37:48

In the woods, biologist Bernard Kettlewell conducted experiments,

0:37:510:37:56

calculated the rate of change and the strength of natural selection,

0:37:560:38:02

and he found that the equations worked.

0:38:020:38:07

Every character, in every species of insects, plant and man himself,

0:38:070:38:13

is constantly under collective pressure.

0:38:130:38:17

We have shown if the pressure is high enough within 50 generations

0:38:170:38:23

one character can nearly entirely substitute another.

0:38:230:38:28

It is due to such changes in many characters

0:38:280:38:32

that new species are gradually evolving.

0:38:320:38:36

And the moths have continued to evolve.

0:38:410:38:44

In 1956 the Clean Air Act scrubbed the soot from the nation's skies,

0:38:440:38:49

and from the bark of the nation's trees.

0:38:490:38:51

The dark moths began to disappear,

0:38:530:38:55

and the light moths returned.

0:38:550:38:57

Evolution went into reverse.

0:38:590:39:01

Natural selection not only existed, it was far more powerful,

0:39:040:39:09

and evolution far swifter, than Darwin had ever imagined.

0:39:090:39:14

But if there is a place of which Darwinians dream, it is Lake Malawi.

0:39:190:39:25

One of the three great lakes of Africa,

0:39:320:39:36

it was discovered by David Livingstone

0:39:360:39:38

just as The Origin of Species was going to press.

0:39:380:39:41

But it took the best part of a century before we realised

0:39:470:39:51

just how remarkable are the fish in these waters.

0:39:510:39:55

They are, quite simply,

0:39:570:39:59

the most beautiful vindication of Darwin's theory.

0:39:590:40:03

Dive among them, and the first thing you notice

0:40:040:40:07

is how astonishingly various are these fish.

0:40:070:40:12

They differ from each other

0:40:120:40:13

in the shapes of their bodies, their mouth and teeth -

0:40:130:40:17

in their colours and their breeding habits,

0:40:170:40:21

and yet they are all members of the same family.

0:40:220:40:27

They are all cichlids.

0:40:270:40:29

Two million years ago

0:40:330:40:35

a cichlid must have entered the lake and multiplied.

0:40:350:40:40

Over time it seems the lake levels rose and fell,

0:40:400:40:44

creating a universe of different habitats,

0:40:440:40:47

each with its own resources to exploit,

0:40:470:40:49

and each evolving its own set of cichlids.

0:40:490:40:53

And now there are 400, 500, maybe 600 species here -

0:41:010:41:07

more than all the species of fish

0:41:070:41:10

in all the lakes and rivers of Europe or America.

0:41:100:41:15

It's not just that this lake has so many species of cichlids,

0:41:270:41:31

it's how diverse they,

0:41:310:41:33

how many different ways in which they make a living.

0:41:330:41:36

This fish over here is Pseudotropheus elongatus and

0:41:360:41:40

it makes a living by scraping algae off rocks and combing through it.

0:41:400:41:45

This one does the same except it has an even more elaborate mouth.

0:41:450:41:50

It has its teeth arranged rather like a rasp

0:41:500:41:54

with lots of little teeth with which it files away

0:41:540:41:57

and scrapes the algae off the rocks.

0:41:570:41:59

This...this fish here buries it head in the sand, opens its mouth

0:41:590:42:05

and gobbles up little Chironomid midges that are living there.

0:42:050:42:10

And this thing is the most remarkable cichlid of all,

0:42:100:42:14

Rhamphochromis.

0:42:140:42:15

It is the uber-predator, the Tyrannosaurus of the lake,

0:42:150:42:20

gobbling anything that it can with those formidable sharp teeth.

0:42:200:42:25

What's so most amazing about these fish is that they're all descended

0:42:280:42:33

from one cichlid that entered this lake about two million years ago.

0:42:330:42:38

0 to 600 fish in about two million years.

0:42:380:42:43

It is one of the most astonishing evolutionary events

0:42:430:42:46

that has ever happened on this planet.

0:42:460:42:49

By the end of the 1950s evolution had a new formula.

0:42:530:42:58

A combination of natural selection,

0:42:580:43:01

isolation, Mendelian inheritance, and mathematical theory.

0:43:010:43:06

It was called the neo-Darwinian synthesis.

0:43:080:43:11

The formula wasn't entirely Darwinian, but that didn't matter.

0:43:140:43:19

On the centenary of The Origin of Species,

0:43:190:43:22

everyone agreed that Darwin's vision had triumphed.

0:43:220:43:25

Everyone was a Darwinian.

0:43:250:43:28

You could almost say that they were more Darwinian than Darwin himself.

0:43:280:43:32

And yet, as Darwinism entered its second century,

0:43:370:43:41

some anomalies remained.

0:43:410:43:43

Natural selection may have triumphed,

0:43:440:43:47

yet some animal behaviours were still hard to explain

0:43:470:43:51

in terms of natural selection.

0:43:510:43:53

Such as altruism.

0:43:580:44:01

In a world driven by competition,

0:44:050:44:08

why are some animals altruistic?

0:44:080:44:12

Termites, for instance,

0:44:130:44:15

cooperate relentlessly building vast mounds on the African plain.

0:44:150:44:20

A termite mound is filled with millions of altruists,

0:44:210:44:25

the soldiers and workers

0:44:250:44:33

whose lives

0:44:330:44:35

are devoted to defending and feeding the queen.

0:44:350:44:40

They are the eunuchs of the termite state

0:44:430:44:47

and their existence is surprisingly hard to explain.

0:44:470:44:52

According to Darwinian logic, creatures are engineered

0:44:550:44:59

by natural selection to increase their chances of reproduction.

0:44:590:45:03

Yet most termites are sterile, they don't reproduce at all.

0:45:090:45:13

They work for the colony, apparently without reward.

0:45:130:45:17

Why?

0:45:190:45:21

In 1964 zoologist, Bill Hamilton,

0:45:210:45:25

proposed a solution to the problem of altruism.

0:45:250:45:28

One that explained the existence of social insects.

0:45:280:45:33

This highly regimented move by hundreds of thousands of individuals

0:45:330:45:37

is a typically impressive achievement of the social insects.

0:45:370:45:40

Hamilton realised that the members of any colony

0:45:400:45:44

are very closely related.

0:45:440:45:45

And, as such, they share genes.

0:45:470:45:50

The queen alone is replicating the genes of the colony on its behalf.

0:45:500:45:55

That, said Hamilton, is the key.

0:45:560:45:58

If your altruistic act benefits a relative then you may pay a cost.

0:46:010:46:06

But at least some of your genes will reap the benefit.

0:46:060:46:10

Natural selection does not count the fates of individuals,

0:46:140:46:19

it counts the fates of genes.

0:46:190:46:21

Termite soldiers sacrifice themselves for their queen

0:46:270:46:31

because she shares many of their genes.

0:46:310:46:34

And by devoting their lives to her,

0:46:340:46:36

more of her genes, and hence their genes,

0:46:360:46:39

are passed on than they could possibly achieve by themselves.

0:46:390:46:43

This was a radical extension of Darwinism

0:46:470:46:50

and it spawned a new science.

0:46:500:46:52

At Harvard E O Wilson gave it a name - sociobiology.

0:46:520:46:56

At Oxford, Richard Dawkins gave it a slogan - The Selfish Gene.

0:46:560:47:02

Wherever sociobiologists looked, they explained

0:47:040:47:07

all the strange things animals do as the product of natural selection,

0:47:070:47:12

not on individuals, but on their genes.

0:47:120:47:16

Each individual is just obeying its own genes.

0:47:160:47:19

And humans, they said, are no different.

0:47:190:47:23

Our behaviours too, can be explained by genetic programmes

0:47:250:47:28

shaped by natural selection.

0:47:280:47:31

This is the fundamental principle of sociobiology.

0:47:310:47:34

The genes for particular social behaviour exist.

0:47:340:47:38

Applied to humans, sociobiology seems excessively reductive.

0:47:380:47:44

But there's no doubt that as an explanation of animal behaviour,

0:47:450:47:49

it triumphed.

0:47:490:47:52

And sociobiology's triumph was the triumph of natural selection.

0:47:520:47:57

A force, that in Darwin's time, seemed weak and ephemeral,

0:48:020:48:07

was now omnipotent and omnipresent.

0:48:070:48:10

Yet Darwin gave us more than natural selection,

0:48:150:48:18

more than a mechanism of evolution.

0:48:180:48:21

He also gave us a new narrative, or at least the promise of one.

0:48:230:48:28

He told us that the history of life was a tale of epic forces and scales

0:48:280:48:35

and that it was ours to discover.

0:48:350:48:39

If there is an icon of Darwin's theory, it is this.

0:48:480:48:52

A metaphor for all evolutionary history.

0:48:540:48:59

A tree.

0:48:590:49:00

The twigs, Darwin said, were species.

0:49:040:49:07

And they were connected to their ancestors by branches,

0:49:070:49:10

and those ancestors to theirs, reaching deep into the past.

0:49:100:49:15

So that the whole history of life

0:49:150:49:17

could be represented as a great tree.

0:49:170:49:21

Darwin first conceived this image in 1837.

0:49:240:49:29

He sketched a simple tree-like diagram to show how lineages

0:49:290:49:34

could originate from a single source and then diverge and proliferate.

0:49:340:49:39

Above it he scribbled the words, "I think".

0:49:390:49:43

22 years later, in The Origin,

0:49:460:49:49

he confidently asserts that just such a tree

0:49:490:49:53

could be constructed for any group of creatures.

0:49:530:49:57

Easy to say, hard to do, and Darwin didn't even try.

0:50:000:50:05

Why did Darwin, so bold and so visionary,

0:50:110:50:14

not give us the history of life that his theory implied?

0:50:140:50:18

Perhaps because he was so acutely aware

0:50:200:50:24

of the deficiencies of the fossil record.

0:50:240:50:28

The rocks ought to bear mute testimony to titanic conflicts

0:50:280:50:33

playing out over eons of time.

0:50:330:50:36

But, as Richard Owen had so cruelly exposed,

0:50:370:50:41

the reality was rather different.

0:50:410:50:44

Animal fossils were abundant, but there were also huge gaps.

0:50:460:50:50

And nowhere was the gap greater than at the base of the Cambrian.

0:50:500:50:55

That's when an explosion of animal life seems to have occurred.

0:50:570:51:02

New species, entire faunas emerged as if from nowhere,

0:51:020:51:07

their ancestors absent.

0:51:070:51:10

Those rocks over there are Cambrian.

0:51:140:51:17

That makes them around 525 million years old.

0:51:170:51:21

And they contain animal life, wonderful creatures such as

0:51:210:51:25

brachiopods, ostracods and trilobites.

0:51:250:51:29

These rocks are Precambrian, they're only about 30 million years older.

0:51:290:51:36

And yet they are empty.

0:51:360:51:39

There are no animal remains in them whatsoever.

0:51:390:51:42

But how could this be?

0:51:440:51:46

If these Precambrian rocks didn't have any fossils in them,

0:51:460:51:50

where did the animals come from?

0:51:500:51:52

Characteristically, Darwin did not shirk the problem.

0:51:520:51:55

"During these vast, and unknown periods of time", he wrote,

0:51:550:52:00

"the world must have swarmed with living creatures."

0:52:000:52:04

That he couldn't produce them was, he admitted, a grave difficulty.

0:52:040:52:10

Darwin despaired of being able to reconstruct the history of life.

0:52:110:52:15

Yet he did not doubt that his successors would do just that.

0:52:150:52:21

And so they have.

0:52:210:52:23

Enter one of Darwin's most ardent disciples -

0:52:290:52:32

a young German scientist, Ernst Haeckel.

0:52:340:52:37

Of all the scientists who followed Darwin,

0:52:410:52:44

Haeckel was the most protean.

0:52:440:52:47

A gifted artist who could reveal nature's exquisite geometries

0:52:490:52:52

with the stroke of a pen,

0:52:520:52:55

he was also a brilliant anatomist,

0:52:550:52:57

devoting months to the study of obscure sea creatures.

0:52:570:53:02

And he was a romantic,

0:53:050:53:06

of the sentimental, nature-loving Goethe-worshipping German kind.

0:53:060:53:11

For Haeckel loved his cousin, Anna.

0:53:140:53:17

She had golden hair and blue eyes.

0:53:170:53:20

He described her as "a true German child of the forest."

0:53:200:53:25

Haeckel was besotted with her and married her.

0:53:280:53:32

What bliss!

0:53:320:53:34

But for only a few months,

0:53:340:53:37

and then she died.

0:53:370:53:38

Anna's death left Haeckel unhinged.

0:53:440:53:46

He contemplated suicide.

0:53:460:53:49

But then he found religion.

0:53:490:53:51

Not the false consolations of Christianity,

0:53:510:53:54

but the harsh, godless clarity of Der Darwinismus.

0:53:540:53:58

He would become its greatest apostle.

0:53:580:54:01

He would take the good book to the German masses,

0:54:010:54:05

he would preach the truth and he would do what Darwin had

0:54:050:54:08

so conspicuously failed to do - he would re-write the history of life.

0:54:080:54:15

But how?

0:54:170:54:18

Haeckel needed a way of reconstructing the evolutionary past

0:54:180:54:23

that did not rely on fossils.

0:54:230:54:25

The answer, he said, was to look at the embryo.

0:54:250:54:29

The embryo of an animal contains, is,

0:54:310:54:34

a record of its evolutionary past.

0:54:340:54:37

The earlier in development you look,

0:54:370:54:40

the further back into their past you can see.

0:54:400:54:42

The embryo, Haeckel said,

0:54:420:54:46

is Ariadne's thread.

0:54:460:54:48

He began by comparing vertebrate embryos.

0:54:530:54:57

Just before birth they seem very different, as you'd expect.

0:54:580:55:04

But follow the embryos back in time, to when they are younger,

0:55:040:55:07

and less developed, they look remarkably alike.

0:55:070:55:11

They have the same dorsal nerve cords, the same pharyngeal slits.

0:55:110:55:18

But Haeckel looked further, deeper into the embryo,

0:55:220:55:26

earlier into its development.

0:55:260:55:28

Before the limbs appear,

0:55:280:55:31

before there's a head or a tail, and further yet,

0:55:310:55:35

to when it is but a ball of cells with the beginnings of a gut.

0:55:350:55:40

This is a stage of development called "gastrulation".

0:55:400:55:44

And here he thought he found something wonderful -

0:55:440:55:48

the ancestor of us all.

0:55:480:55:52

Here, said Haeckel, is a remembrance, a recollection,

0:55:550:55:59

a recapitulation of the very first animal.

0:55:590:56:03

A creature, no more than a ball of flagellated cells,

0:56:030:56:07

that had once whirled through the Precambrian seas.

0:56:070:56:11

He called it the gastrea

0:56:110:56:13

and said that it was his most important discovery.

0:56:130:56:17

Others said it was his most outrageous invention.

0:56:170:56:22

Haeckel used embryos to produce evolutionary trees.

0:56:240:56:28

Lots of them.

0:56:280:56:29

They look a bit like Darwin's tree, but they are not abstract metaphors,

0:56:310:56:36

they are the first attempt to put every living thing

0:56:360:56:40

in its evolutionary place.

0:56:400:56:42

All the animals are there, in more or less the right order,

0:56:440:56:48

and somewhere near the base of the trunk

0:56:480:56:51

leading to all the other animals is the gastrea -

0:56:510:56:55

Haeckel's hypothetical ancestral beast.

0:56:550:56:58

Haeckel's speculations were, no doubt, too bold.

0:57:010:57:05

The embryo does not contain a simple picture of the history of life.

0:57:050:57:09

And yet, there's no doubt that his guesses were,

0:57:090:57:13

more often than not, inspired.

0:57:130:57:16

Since the 1950s, a trickle of animal fossils

0:57:200:57:25

has been emerging from Precambrian rocks.

0:57:250:57:28

Some are little more than imprints, others are minute.

0:57:290:57:34

But by using Computed Tomography Imaging,

0:57:360:57:39

even individual cells can be seen.

0:57:390:57:41

And, what's more, some of these proto-creatures

0:57:420:57:46

are not so very different from Haeckel's gastrea.

0:57:460:57:50

But there's another reason to think that animals lived

0:57:540:57:57

long before the Cambrian and that's because DNA tells us so.

0:57:570:58:01

When Watson and Crick elucidated the structure of DNA they unified life.

0:58:100:58:16

The DNA story is without doubt one of the greatest success stories

0:58:160:58:20

in the history of science.

0:58:200:58:22

Because it can't be often that two newcomers to a field

0:58:220:58:25

make such a major discovery so quickly.

0:58:250:58:28

Nearly all living things use DNA as the stuff of inheritance.

0:58:300:58:35

So they must all be related,

0:58:350:58:37

and descend, much as Darwin had supposed, from a single ancestor.

0:58:370:58:44

But now we can go further.

0:58:470:58:49

We can sequence the genome of any living thing

0:58:490:58:54

and read it as if we were reading a book.

0:58:540:58:58

Genomes are documents written in billion of letters.

0:59:000:59:05

They are palimpsests, endlessly augmented, erased,

0:59:050:59:09

and rewritten by the hand of evolution.

0:59:090:59:13

And if you can read them

0:59:130:59:15

you can read the history of life.

0:59:150:59:19

By sequencing genomes we can now date

0:59:200:59:23

the origin of animals in the tree of life.

0:59:230:59:27

Some of them turn out to be astonishingly ancient.

0:59:270:59:31

Perhaps the simplest of all animals

0:59:340:59:37

is a microscopic creature called Trichoplax.

0:59:370:59:40

It doesn't have a gut, a mouth, a brain, or even sense organs.

0:59:400:59:46

Its genome suggests that its ancestors departed

0:59:460:59:49

from the main trunk of animal evolution,

0:59:490:59:52

perhaps a billion years old.

0:59:520:59:54

Just as Darwin had supposed,

0:59:560:59:58

there must have been animals in the Precambrian seas.

0:59:581:00:02

The sequencing machines are revealing new branches

1:00:041:00:07

on the tree of life.

1:00:071:00:08

They are giving us a new historical narrative.

1:00:081:00:13

But, we are also discovering new fossils.

1:00:141:00:17

And often, the story they tell is the same.

1:00:171:00:22

Consider the whale.

1:00:241:00:27

Whales obviously evolved from some land mammal but, if so,

1:00:271:00:31

where were the fossil half-whales?

1:00:311:00:34

Where were the whales with legs?

1:00:341:00:36

By Darwin's logic, they must have existed and they must have been big.

1:00:361:00:41

So where were they?

1:00:411:00:43

For years, the origin of whales was shrouded in obscurity.

1:00:461:00:51

Not any more.

1:00:511:00:53

In recent decades, the fossil record has become wonderfully complete.

1:00:541:00:59

Just as Darwin had predicted, just as Darwin had hoped,

1:01:021:01:06

we now have an astonishing array of fossils that show how a land mammal

1:01:061:01:10

makes a transition to one that lives in the sea.

1:01:101:01:13

They show how front limbs evolved into flippers,

1:01:131:01:18

and how hind limbs just wither away,

1:01:181:01:22

and how a whale comes to breathe

1:01:221:01:24

not through their nostrils in the tips of its snout

1:01:241:01:27

but rather a blowhole in the back of its head.

1:01:271:01:30

And they show us one more thing.

1:01:301:01:33

They tell us about the place of whales in the tree of life.

1:01:331:01:39

The evidence hinges, literally, on an ankle bone.

1:01:411:01:45

For primitive whales, it turns out, have ankle bones

1:01:451:01:49

that are remarkably similar to those of modern ungulates,

1:01:491:01:52

such as cows, sheep and pigs.

1:01:521:01:55

Yet we don't have to rely on fossilised bones

1:01:561:01:59

to tell us about the ancestry of whales.

1:01:591:02:01

We can use DNA too.

1:02:041:02:07

And to do that we have to go to Africa.

1:02:091:02:13

Compare the DNA of a cetacean to that of any other mammal

1:02:171:02:21

and something surprising emerges.

1:02:211:02:24

Their closest living relation is this, a hippo.

1:02:241:02:29

It's not that whales evolved from hippos,

1:02:381:02:41

or that hippos evolved from whales.

1:02:411:02:45

Rather, it is simply that hippos and cetaceans are, as it were, cousins.

1:02:451:02:51

They are descended from a common ancestor that lived, perhaps,

1:02:511:02:55

55 million years ago.

1:02:551:02:57

It is precisely this sort of agreement

1:03:051:03:08

between DNA and fossil evidence

1:03:081:03:11

that makes the case for evolution so utterly compelling.

1:03:111:03:15

And so, to the evolution of the one species

1:03:191:03:23

we care about more than any other.

1:03:231:03:26

One might have expected Darwin to say something about human evolution.

1:03:291:03:34

And, in The Origin Of Species, he does.

1:03:361:03:40

After 400 pages of ants and agoutis, bats and barnacles,

1:03:441:03:50

the whole bestiary, in fact, right through to zebras,

1:03:501:03:53

he settles down to consider humanity.

1:03:531:03:57

And what he says is this.

1:03:571:04:00

"Light will be thrown on the origin of Man and his history."

1:04:041:04:10

And that's it.

1:04:191:04:21

Well, thanks for that, Charles.

1:04:211:04:24

Darwin, of course, knew we were descended from apes,

1:04:271:04:30

but left others to spell it out.

1:04:301:04:33

Among them, Ernst Haeckel.

1:04:351:04:38

With a characteristic flourish,

1:04:421:04:44

he imagined humanity like Botticelli's Venus,

1:04:441:04:48

rising gloriously from the brutes that surround her.

1:04:481:04:52

At the time, there were no fossils linking man to apes.

1:04:551:04:59

And so he set to imagining what lay between.

1:05:011:05:05

No human fossils, no problem.

1:05:071:05:11

Let's just invent one.

1:05:111:05:13

Something between an ape and a man.

1:05:131:05:16

Let's give it a name. Ape man.

1:05:161:05:20

Let's give it a real, proper, Latin name, Pithecanthropus.

1:05:201:05:25

Like the gastrea, Pithecanthropus was an invention,

1:05:281:05:33

a hypothetical ancestor.

1:05:331:05:36

Yet Haeckel's reasoning was sound.

1:05:361:05:38

If we were descended from apes,

1:05:401:05:43

then sooner or later intermediates would be found.

1:05:431:05:47

In 1891, a Dutch physician, Eugene Dubois,

1:05:501:05:54

digging in the banks of the Solo River in Java

1:05:541:05:57

discovered this skullcap.

1:05:571:05:59

It wasn't human, it wasn't ape, it was an ape man.

1:05:591:06:04

In homage to Haeckel, Dubois called it Pithecanthropus.

1:06:041:06:10

In the centuries since,

1:06:161:06:18

Pithecanthropus has acquired a new name, Homo erectus,

1:06:181:06:23

and has been joined by a collection of other fossils,

1:06:231:06:26

some, apish humans,

1:06:261:06:30

others, human apes.

1:06:301:06:33

The family tree of humanity can now be richly filled with species

1:06:351:06:40

and there's a clear and unambiguous line

1:06:401:06:43

between the earliest apes and us, Homo sapiens.

1:06:431:06:49

But which of the great apes now alive is our closest relation?

1:06:541:06:58

That question, endlessly debated since Darwin's time,

1:07:001:07:04

hasn't been answered by fossils.

1:07:041:07:07

It required DNA.

1:07:071:07:09

By comparing DNA sequences from each of the great apes,

1:07:131:07:16

the order of evolutionary descent has become clear.

1:07:161:07:20

We're genetically closest to chimpanzees and bonobos.

1:07:231:07:27

Five to six million years ago our ancestor was theirs.

1:07:271:07:33

Seven million years ago, we shared an ancestor with the gorilla.

1:07:361:07:40

12 million years ago, with an orang-utan.

1:07:431:07:46

And so on back to the very beginning of life.

1:07:481:07:53

"Who do you think you are?" asked Haeckel.

1:07:581:08:01

And answered,

1:08:011:08:02

"You are an ape, a mammal, a reptile, a fish,

1:08:021:08:08

"a worm, a ball of cells and finally a single cell

1:08:081:08:13

"floating in the saline womb of the primordial seas."

1:08:131:08:18

150 years ago, Darwin spoke of a time

1:08:191:08:23

when the tree of life would be more than a metaphor,

1:08:231:08:26

when it would be an accurate historical record.

1:08:261:08:31

That time has come.

1:08:311:08:33

The tree of life stands before us,

1:08:331:08:36

its branches becoming clearer with every fossil and DNA sequence,

1:08:361:08:42

and our species is but a leaf on a twig,

1:08:421:08:46

buried within in its vast and ramifying canopy.

1:08:461:08:51

When we look at living things it is the differences that we first see,

1:09:041:09:10

the astonishing variety of form, colour and behaviour.

1:09:101:09:16

And yet, beneath this diversity runs a deep, unifying plan.

1:09:161:09:23

For most animals have much the same geometry,

1:09:231:09:27

the same basic body plan.

1:09:271:09:30

For Darwin, this paradox of unity within diversity

1:09:321:09:37

was the gift of the tree of life,

1:09:371:09:39

the consequence of species giving rise to species,

1:09:391:09:44

endlessly adapting over countless generations.

1:09:441:09:49

But what Darwin could never have imagined

1:09:561:09:58

is that such unity within diversity can be found in every cell,

1:09:581:10:04

every molecule and every gene of every living thing.

1:10:041:10:09

Well done, where's that?

1:10:091:10:11

Can you look at the light? Where's my light?

1:10:111:10:14

Where's that?

1:10:141:10:16

Ellie is a patient at Moorfields Hospital in London.

1:10:161:10:20

She was born without irises.

1:10:201:10:23

Her pupils are enormous for they cannot contract,

1:10:231:10:27

and she can barely see.

1:10:271:10:29

Where's it gone now? Where's it gone now?

1:10:291:10:32

The disorder is called Aniridia

1:10:321:10:35

and it's caused by a mutation that she inherited from her mother.

1:10:351:10:39

Well done, oh, you get a lovely view of your eye.

1:10:391:10:42

She's such a good girl.

1:10:421:10:43

Good, well done.

1:10:431:10:45

In 1992, geneticists identified the mutant gene.

1:10:451:10:49

Now that one... Good, good, good, good.

1:10:501:10:53

Located on chromosome 11, it's called PAX6.

1:10:531:10:57

Well done, well done. You're such a good girl.

1:10:571:11:00

PAX6 is a very special kind of gene.

1:11:001:11:04

It's a molecular switch, a gene that turns other genes on and off,

1:11:041:11:09

and this particular molecular switch

1:11:091:11:12

is needed in the construction of the human eye.

1:11:121:11:16

The really interesting bit, however, is what happens next.

1:11:161:11:21

In 1994, geneticists were studying fruit flies,

1:11:271:11:31

searching for the genes that make its different organs.

1:11:311:11:35

They screened thousands of flies for mutations that cause abnormality.

1:11:371:11:44

Some of the mutant flies were blind.

1:11:471:11:49

They had a mutation called eyeless.

1:11:511:11:54

Normal flies have large, red, compound eyes.

1:11:571:12:01

Eyeless flies have none.

1:12:011:12:05

Analysing the DNA of eyeless flies, they identified the mutant gene.

1:12:061:12:13

It was PAX6 -

1:12:131:12:15

the same gene, or at least its fly version,

1:12:151:12:19

that causes aniridia in humans.

1:12:191:12:22

I remember how amazed we all were when we heard about this result.

1:12:251:12:29

Everyone just knew that human eyes and fly eyes

1:12:291:12:33

had evolved independently.

1:12:331:12:35

How could it be otherwise - they just looked so very different?

1:12:351:12:40

But the discovery that they shared a gene

1:12:401:12:43

told us that they shared an evolutionary history.

1:12:431:12:47

Geneticists started looking into the eyes of other animals

1:12:541:12:57

and wherever they looked, they found the same thing.

1:12:581:13:04

The molecular circuit of which PAX6 is a part

1:13:041:13:06

is universal.

1:13:061:13:08

Eyes are so ubiquitous, so useful and so various in their design,

1:13:101:13:16

that for most of the 20th century,

1:13:161:13:17

zoologists had argued that they must have evolved many times.

1:13:171:13:21

This, however, must be wrong.

1:13:231:13:25

Eyes have evolved only once.

1:13:261:13:29

All the eyes, belonging to all the animals on earth,

1:13:311:13:36

can trace their origin to one very simple eye

1:13:361:13:40

that belonged to one, doubtless, very simple creature that lived,

1:13:401:13:44

perhaps, one billion years ago.

1:13:441:13:47

Here is another mutant fruit fly.

1:13:501:13:53

Its eyes are quite normal,

1:13:551:13:57

but it has an extra pair of legs where its antennae should be.

1:13:571:14:02

And here's a different mutant.

1:14:051:14:08

Instead of the usual two wings, it has four.

1:14:101:14:14

True, they're a little deformed,

1:14:141:14:16

but they're definitely an extra set of wings.

1:14:161:14:19

These flies have mutations in their Hox genes.

1:14:211:14:25

Like PAX6, Hox genes are molecular switches

1:14:291:14:32

that turn other genes on and off.

1:14:321:14:35

They determine a fly's basic geometry, the number and position

1:14:351:14:40

of its segments, limbs and wings.

1:14:401:14:43

And like PAX6, Hox genes are universal.

1:14:441:14:48

They can be found in all animals, including us.

1:14:481:14:52

Animal bodies seem so very different from each other.

1:14:551:14:59

And yet they are not.

1:14:591:15:02

A fly has wings and segments, we have arms and vertebrae.

1:15:021:15:09

A facile comparison, you may think, until you look at the embryo.

1:15:091:15:15

This is a fruit fly embryo.

1:15:181:15:21

And here, one by one, the Hox genes are being expressed.

1:15:211:15:26

They ensure that every segment, head to tail,

1:15:261:15:30

knows what structure to make.

1:15:301:15:32

They work with the exquisite Boolean logic of a computer programme,

1:15:321:15:36

directing cells to their fates.

1:15:361:15:39

And this is a human embryo.

1:15:421:15:45

Again the Hox genes are activated along the head-to-tail axis

1:15:451:15:49

ensuring that our parts are arranged in the right order

1:15:491:15:53

and end up in the right place.

1:15:531:15:55

The circuitry is more complex than a fruit fly's,

1:15:571:16:00

but the logic is the same.

1:16:001:16:02

The discovery that many important genes

1:16:071:16:10

are shared across the animal kingdom was thrilling.

1:16:101:16:14

A new science was born -

1:16:161:16:18

Evolutionary Developmental Biology,

1:16:181:16:21

Evo-Devo, for short.

1:16:211:16:24

Like Haeckel,

1:16:291:16:30

we would delve into the embryo to unravel our evolutionary past.

1:16:301:16:35

But now, instead of working out what our ancestors looked like,

1:16:361:16:41

we'd work out the genetic programmes that made them.

1:16:411:16:44

Evo-Devo would explain how organic structures can be, at once,

1:16:471:16:51

so similar and conservative and, yet, so promiscuously different.

1:16:511:16:58

There is a paradoxical quality to genes.

1:17:011:17:04

Look at any animal and you can see many of the same genes

1:17:041:17:07

doing much the same thing.

1:17:071:17:09

And yet, look at them closely, look at them working in the embryo,

1:17:091:17:15

and you can see the origins of all diversity.

1:17:151:17:19

This is the embryo of a tetrapod.

1:17:221:17:25

As it grows, Hox genes switch on and off in a kaleidoscopic pattern.

1:17:251:17:31

Molecular signals sweep across the limb buds, telling each cell

1:17:311:17:34

what it is and what it must become.

1:17:341:17:37

The cells of the limb condense into the outline of bones

1:17:381:17:42

and so a mole grows a paw.

1:17:421:17:46

Another embryo and the same thing happens...

1:17:481:17:51

..at least to begin with.

1:17:511:17:53

But there's far more bone growth signal in this embryo.

1:17:531:17:56

The bones of the limb grow longer and the cells between the fingers

1:17:561:18:00

do not die, they become webbing.

1:18:001:18:03

And so a bat grows a wing.

1:18:031:18:06

Two embryos that start their lives in much the way, but with just

1:18:081:18:12

a subtle shift in gene activity, become two very different creatures.

1:18:121:18:18

I could tell many such stories about the evolution of beaks and feathers,

1:18:211:18:28

scales and spots and snouts.

1:18:281:18:31

But is it enough?

1:18:311:18:33

Surely the point of science is not merely to tell stories,

1:18:331:18:37

but rather to reveal the laws of nature.

1:18:371:18:42

Haeckel believed that the symmetries shown by living things

1:18:441:18:48

were caused by simple laws,

1:18:481:18:50

not very different from those that explained

1:18:501:18:54

the symmetries of crystals.

1:18:541:18:56

Living things are, of course, much more complex than crystals,

1:18:581:19:02

yet they do have a geometry.

1:19:021:19:06

An internal one composed of vast and intricate networks

1:19:061:19:12

of genes, proteins and molecules, all working together to give life.

1:19:121:19:18

Perhaps then, the war of nature is not simply

1:19:201:19:24

a struggle among individuals, or even genes,

1:19:241:19:28

but a struggle among different ways of organising life.

1:19:281:19:33

A struggle among systems.

1:19:331:19:36

In the short run, success depends on being robust.

1:19:381:19:42

On being able to weather the vicissitudes of existence

1:19:421:19:46

and then reproduce.

1:19:461:19:47

In the long run, however, in the evolutionary long run,

1:19:481:19:52

success needs something else, it needs flexibility.

1:19:521:19:56

On being able to respond to a mutable and contingent world.

1:19:561:20:02

Let me put it another way -

1:20:021:20:04

in the short run, creatures evolve.

1:20:041:20:07

In the long run, they evolve evolvability.

1:20:071:20:12

About a billion years ago, a creature,

1:20:141:20:17

something like Trichoplax, crawled out of the pre-Cambrian ooze.

1:20:171:20:21

Superficially, it may have been unremarkable.

1:20:241:20:28

But it seems that there was something new and rather special

1:20:281:20:32

about its genetic network.

1:20:321:20:34

It had a structure that was robust enough to be strong,

1:20:351:20:39

yet flexible enough to change.

1:20:391:20:42

Over time, this way of organising life was copied

1:20:451:20:49

and modified countless times.

1:20:491:20:51

Parts could added, even rearranged,

1:20:561:20:58

and yet the system as a whole would continue to work.

1:20:581:21:03

It was an engine of innovation

1:21:031:21:08

and it went on to conquer the world.

1:21:081:21:10

The theory of evolution by natural selection

1:21:401:21:43

is one of the most beautiful products of science.

1:21:431:21:47

It provided an answer, the only rational one we have,

1:21:471:21:50

as to why living things bear the hallmarks of design.

1:21:501:21:55

It is so beguilingly simple, so powerful and so manifestly true,

1:21:581:22:05

that it is easy to forget just how inadequate it is.

1:22:051:22:10

There are so many questions that we can't answer...

1:22:101:22:14

Why are there 600 species of cichlid in this lake and not 60 or 6,000?

1:22:141:22:21

Why are there no anglerfish cichlids, or flying fish cichlids?

1:22:211:22:25

Or, cichlid sharks? What, in short, are the limits to evolution?

1:22:251:22:33

The theory is inadequate because it is not predictive.

1:22:351:22:39

It explains what has evolved, but not what will.

1:22:391:22:44

There are simply too many possible streams

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and there's no saying which one will be chosen.

1:22:501:22:53

We can only follow the journey and reconstruct the route

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once it is done. But must it always be so?

1:23:011:23:06

Compare Darwin's theory of evolution

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to Newton's account of the planetary motions.

1:23:121:23:15

Newton, after all, gave us celestial mechanics -

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a mathematical theory that enables us

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to make predictions far into the future.

1:23:271:23:30

Darwin did nothing of the kind,

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but can we, perhaps, make a predictive theory of evolution?

1:23:351:23:39

Can we compute

1:23:391:23:41

the future of life?

1:23:411:23:43

Many would say no.

1:23:451:23:48

The evolution of life, they say, is ruled by chaos and contingency.

1:23:481:23:53

Perhaps evolution is like the weather.

1:23:571:24:00

Given enough data and computing power,

1:24:031:24:06

forecasters can tell us whether or not it will rain at the weekend.

1:24:061:24:09

But ask them about a month of Sundays and their predictions

1:24:111:24:14

fall apart - defeated by imperfect data

1:24:141:24:20

and the chaos concealed in their own equations.

1:24:201:24:23

And yet, I still think

1:24:371:24:39

a predictive theory of evolution might be possible.

1:24:391:24:42

And the reason I do is because of the fish in Lake Malawi.

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Two million years of evolution have produced 600 species of cichlid here

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and an astonishing diversity of form.

1:24:541:24:57

But what's even more remarkable

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is that this great evolutionary experiment is not unique.

1:25:021:25:06

Follow the African rift north-west for just a few hundred miles

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and you come to Lake Tanganyika.

1:25:171:25:19

There, too, a cichlid arrived a few million years ago

1:25:221:25:26

and there, too, it multiplied and evolved

1:25:261:25:29

into hundreds of different species.

1:25:291:25:32

The fish in both lakes are only remotely related,

1:25:341:25:37

but they have similar colours, fins, teeth, diets, habits and habitats.

1:25:371:25:44

You can hardly tell some of them apart without DNA.

1:25:471:25:51

In other words, the tape of cichlid evolution has been run twice

1:25:551:26:00

and both times the outcome has been much the same.

1:26:001:26:05

It's this repeatability that makes me think

1:26:051:26:09

that much of evolution is indeed predictable.

1:26:091:26:12

Perhaps then evolution does not so much resemble the weather

1:26:161:26:19

as it does our climate.

1:26:191:26:21

At the grandest scales of space and time, the atmosphere is not chaotic.

1:26:231:26:28

The physics of our planet imposes order and, thus,

1:26:281:26:32

predictability upon it.

1:26:321:26:35

So, although we can scarcely tell what the weather will be

1:26:371:26:40

three weeks from now, we can predict, at least

1:26:401:26:43

probabilistically, what the climate will be three centuries hence.

1:26:431:26:48

I think evolution is like that.

1:26:521:26:56

As we explore the internal geometry of living things,

1:26:561:26:59

I think we'll discover that there are deterministic laws

1:26:591:27:03

that impose order upon the capricious designs

1:27:031:27:07

of natural selection and that limit the paths of evolution.

1:27:071:27:12

And when we discover those laws, we'll be able to construct

1:27:141:27:18

a predictive theory of evolution.

1:27:181:27:21

We'll be able to complete Darwin's great project.

1:27:221:27:26

We'll explain life itself.

1:27:271:27:31

"There is grandeur in this view of life, with its several powers

1:27:361:27:41

"having been originally breathed into a few forms, or into one.

1:27:411:27:46

"From so simple a beginning, endless forms most beautiful

1:27:461:27:51

"and most wonderful have and are being evolved."

1:27:511:27:57

That is how The Origin ends.

1:27:571:28:02

And it is how our science begins.

1:28:021:28:05

Subtitles by Red Bee Media Ltd

1:28:131:28:16

E-mail [email protected]

1:28:161:28:19

Documentary which tells the story of evolution theory since Darwin postulated it in 1859 in 'On the Origin of Species'.

The theory of evolution by natural selection is now scientific orthodoxy, but when it was unveiled it caused a storm of controversy, from fellow scientists as well as religious people. They criticised it for being short on evidence and long on assertion and Darwin, being the honest scientist that he was, agreed with them. He knew that his theory was riddled with 'difficulties', but he entrusted future generations to complete his work and prove the essential truth of his vision, which is what scientists have been doing for the past 150 years.

Evolutionary biologist Professor Armand Marie Leroi charts the scientific endeavour that brought about the triumphant renaissance of Darwin's theory. He argues that, with the new science of evolutionary developmental biology (evo devo), it may be possible to take that theory to a new level - to do more than explain what has evolved in the past, and start to predict what might evolve in the future.


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