0:00:08 > 0:00:13We live in a world of exquisite diversity,
0:00:13 > 0:00:17with more species than we can possibly count.
0:00:18 > 0:00:21Here in Lake Malawi, for instance,
0:00:21 > 0:00:26there are hundreds of different fish that are found nowhere else.
0:00:26 > 0:00:29Why so many?
0:00:29 > 0:00:30Why so different?
0:00:37 > 0:00:43150 years ago Charles Darwin published On The Origin Of Species.
0:00:43 > 0:00:48And in that one great book he asked the right question...
0:00:48 > 0:00:50..and gave the right answer...
0:00:56 > 0:00:59Where, asked Darwin, does all this diversity come from?
0:01:00 > 0:01:06And answered that it must be the product of evolution.
0:01:06 > 0:01:10Species, he argued, give rise to other species
0:01:10 > 0:01:13and as they do so, they change.
0:01:13 > 0:01:16The changes are minute and subtle,
0:01:16 > 0:01:19but given enough time the results could be spectacular.
0:01:19 > 0:01:22And so they are.
0:01:24 > 0:01:26Darwin's explanation for life on earth
0:01:26 > 0:01:32was so seductive and so simple that it seems obvious today.
0:01:32 > 0:01:37And yet, Darwin's explanation of how evolution works
0:01:37 > 0:01:40was riddled with holes.
0:01:40 > 0:01:43Its logical foundations were shaky.
0:01:43 > 0:01:45His evidence was weak.
0:01:45 > 0:01:48There was so much he did not,
0:01:48 > 0:01:50could not, know.
0:01:52 > 0:01:56Darwin trusted that future generations of scientists
0:01:56 > 0:02:00would complete his work and prove the essential truth of his vision.
0:02:00 > 0:02:06And for 150 years that is what we have been doing.
0:02:07 > 0:02:10In this film, I'll chart the decline, fall
0:02:10 > 0:02:13and ultimate triumph of Darwin's ideas.
0:02:15 > 0:02:18And I'll show how evolutionary theory has itself evolved,
0:02:18 > 0:02:24so that it is now far more vast and subtle than ever he imagined.
0:02:38 > 0:02:41In September 1835
0:02:41 > 0:02:45Charles Darwin arrived in the Galapagos Archipelago...
0:02:47 > 0:02:52..and did what he always did when arriving in a new place -
0:02:52 > 0:02:56he got out his gun and began to collect.
0:02:59 > 0:03:03Among the many inhabitants of the Galapagos that Darwin
0:03:03 > 0:03:08pinned, pickled, shot, or stuffed are these four birds.
0:03:10 > 0:03:15They don't look like much, but look at them closely.
0:03:15 > 0:03:19Look at them as Darwin looked at them,
0:03:19 > 0:03:23and you can see the beginnings of evolution.
0:03:25 > 0:03:28They are mocking birds.
0:03:28 > 0:03:30Each comes from a different island,
0:03:30 > 0:03:33and each is subtly different from the others.
0:03:33 > 0:03:38They differ in the shape of their bills and the size of their bodies,
0:03:38 > 0:03:41and the colour of their plumage.
0:03:41 > 0:03:46It was these differences that first caused Darwin to wonder
0:03:46 > 0:03:50whether species might transform over time.
0:03:52 > 0:03:57Darwin surmised that the birds were variants of the same species
0:03:57 > 0:04:01and must therefore descend from a common ancestor -
0:04:01 > 0:04:03a mocking bird which had somehow
0:04:03 > 0:04:07found its way to the Galapagos many years earlier.
0:04:09 > 0:04:13That was Darwin's hunch,
0:04:13 > 0:04:15but how to prove it?
0:04:15 > 0:04:19He certainly couldn't produce the hypothetical ancestor -
0:04:19 > 0:04:22it was lost in time.
0:04:22 > 0:04:27So he did what scientists do when they don't have the data -
0:04:27 > 0:04:30he appealed to an analogy.
0:04:39 > 0:04:42Darwin bred pigeons.
0:04:42 > 0:04:45They were, for him, a microcosm of evolution.
0:04:52 > 0:04:56They showed how any creature could, given enough time,
0:04:56 > 0:05:01be transformed into something very different from its ancestor.
0:05:06 > 0:05:08For, implausible though it may seem,
0:05:08 > 0:05:13these gorgeous, monstrous, inbred aristocrats of the avian world -
0:05:13 > 0:05:19the Scandaroon, the Frillback, the Jacobin, not to forget the Mookee -
0:05:19 > 0:05:24are all descended from this - the plebeian rock pigeon.
0:05:27 > 0:05:32All pigeons are, at birth, subtly different from each other.
0:05:32 > 0:05:38Breeders select those with desirable features to survive and reproduce...
0:05:38 > 0:05:41..and they cull the rest.
0:05:43 > 0:05:48The desirable features accumulate from generation to generation
0:05:48 > 0:05:50and become exaggerated.
0:05:50 > 0:05:56And so, remarkably quickly, the birds evolve.
0:05:56 > 0:06:00Nature, Darwin said, works like that.
0:06:00 > 0:06:06It favours some features and permits others to whither away.
0:06:06 > 0:06:10He called this process natural selection.
0:06:12 > 0:06:16All this explains why the first chapter of The Origin Of Species is
0:06:16 > 0:06:22not about the wonders of the natural world, but rather about pigeons.
0:06:22 > 0:06:26Understand the pigeon, he is saying, believe the pigeon,
0:06:26 > 0:06:28and all the rest follows.
0:06:31 > 0:06:33Or does it?
0:06:34 > 0:06:38For Darwin had a problem.
0:06:38 > 0:06:42Natural selection was the cornerstone of his theory.
0:06:42 > 0:06:46It was, for him, the engine of evolution.
0:06:46 > 0:06:50And yet it was by no means clear
0:06:50 > 0:06:54that natural selection really worked.
0:06:57 > 0:07:01There is, he said, a war of nature.
0:07:05 > 0:07:09Famine, violence and death are everywhere.
0:07:10 > 0:07:15Species and individuals are locked in a struggle for existence.
0:07:15 > 0:07:18The strong survive and reproduce...
0:07:20 > 0:07:23..while the weak go to the wall.
0:07:25 > 0:07:28Given enough variation, this selective pressure
0:07:28 > 0:07:32is enough to bring about slow, incremental change.
0:07:32 > 0:07:37This was the theory of evolution by natural selection
0:07:37 > 0:07:40that Darwin unveiled in The Origin Of Species.
0:07:42 > 0:07:46While the idea of evolution was not in itself new,
0:07:46 > 0:07:49no-one had argued it more forcefully,
0:07:49 > 0:07:53or documented the evidence for it, with greater rigour.
0:07:53 > 0:07:55But was it right?
0:07:55 > 0:07:59Had Darwin really made his case?
0:08:04 > 0:08:11Of course, many religious types hated the very idea of evolution.
0:08:11 > 0:08:16But some of Darwin's fellow scientists weren't too keen either.
0:08:16 > 0:08:21Notably Richard Owen, who wrote one of the first reviews of The Origin.
0:08:23 > 0:08:26Richard Owen, premier palaeontologist,
0:08:26 > 0:08:31coined the term 'dinosaur', helped design these things.
0:08:33 > 0:08:36Rampaging through a South London park,
0:08:36 > 0:08:41these marvellous reconstructions were built in the 1850s.
0:08:42 > 0:08:48They are a tableau of dinosaur life based on Owen's research.
0:08:49 > 0:08:52Owen had vague evolutionary leanings.
0:08:52 > 0:08:55He thought that species change intermittently,
0:08:55 > 0:08:59under the influence of some divine law
0:08:59 > 0:09:00and that periodically,
0:09:00 > 0:09:05they are swept away in some great catastrophe.
0:09:05 > 0:09:08He loathed Darwin's godless evolutionism.
0:09:10 > 0:09:14Owen was a thoroughly nasty piece of work.
0:09:14 > 0:09:18His review of The Origin, rich in malice, dripping with sarcasm,
0:09:18 > 0:09:22damns Darwin even as he praises himself.
0:09:22 > 0:09:24All anonymously of course.
0:09:24 > 0:09:29"Mr Darwin's rash speculations degrade science.
0:09:29 > 0:09:31"He's as bad as the French.
0:09:31 > 0:09:35"And", continues Owen, "he doesn't know anything about fossils.
0:09:35 > 0:09:37"If he did, he would know that
0:09:37 > 0:09:40"ichthyosaurs appear in the lower Jurassic,
0:09:40 > 0:09:44"stay there pretty much unchanged, and then just disappear -
0:09:44 > 0:09:46"no sign of evolution there."
0:09:49 > 0:09:53Owen's venom was probably born from mere spite.
0:09:53 > 0:09:58Still, he did seem to have the fossil record on his side.
0:09:58 > 0:10:00According to Darwin's theory,
0:10:00 > 0:10:04gradual change should be visible in the rocks.
0:10:04 > 0:10:07But it wasn't.
0:10:07 > 0:10:11Instead, species seemed to arrive and depart,
0:10:11 > 0:10:13leaving little in between.
0:10:15 > 0:10:19Such gaps in the fossil record would haunt Darwin's theory.
0:10:19 > 0:10:24The evidence for natural selection simply wasn't there.
0:10:25 > 0:10:28Even his friends had their doubts.
0:10:30 > 0:10:35Thomas Henry Huxley worked here at Imperial College London,
0:10:35 > 0:10:36where I now work.
0:10:36 > 0:10:42A firebrand and a populist, they called him Darwin's bulldog.
0:10:42 > 0:10:45Huxley also reviewed The Origin.
0:10:45 > 0:10:47"It's a magnificent work.
0:10:47 > 0:10:49"It makes the case for evolution."
0:10:49 > 0:10:53But then he turns to natural selection.
0:10:53 > 0:10:58"Yes, it's logical, yes, it's simple, but has Mr Darwin
0:10:58 > 0:11:01"actually seen a species originate by natural selection?
0:11:01 > 0:11:04"Can he even prove that it really exists?"
0:11:04 > 0:11:07" Well, no. It's a hypothesis,
0:11:07 > 0:11:12"perhaps even the best one going, but, and I say this as a friend,
0:11:12 > 0:11:17"you understand, Mr Darwin really hasn't proved his point."
0:11:20 > 0:11:24Huxley said natural selection can't be seen.
0:11:25 > 0:11:28Others said it doesn't work.
0:11:29 > 0:11:33They claimed that it was logically flawed.
0:11:35 > 0:11:40That it was inconsistent with Darwin's account of inheritance,
0:11:40 > 0:11:43of how species transmit their features
0:11:43 > 0:11:46from one generation to the next.
0:11:52 > 0:11:56This is how Darwin thought inheritance works.
0:11:56 > 0:12:00Suppose one parent has dark feathers, fur or skin,
0:12:00 > 0:12:04the colour of black coffee, while the other is milky white,
0:12:04 > 0:12:08their progeny would be a mix of the two.
0:12:10 > 0:12:12They would be a blend.
0:12:16 > 0:12:19It seems like an innocuous idea.
0:12:19 > 0:12:22Quite a reasonable one, too.
0:12:22 > 0:12:26After all, isn't this how human skin colour is inherited?
0:12:28 > 0:12:31But Darwin had walked into a theoretical trap,
0:12:31 > 0:12:36and a Scottish engineer called Fleeming Jenkin sprang it.
0:12:38 > 0:12:41This is how Jenkin phrased the argument.
0:12:44 > 0:12:48"Imagine that a white man arrives on an island of negroes.
0:12:48 > 0:12:52"He would, no doubt make himself king.
0:12:52 > 0:12:57"He would take many negro wives and father many mulatto children.
0:12:58 > 0:13:03"Yet no matter how successful our hero is, no matter how superior,
0:13:03 > 0:13:08"his coffee-coloured descendants would become progressively darker.
0:13:08 > 0:13:13"Within a few generations, all trace of his presence would disappear."
0:13:15 > 0:13:19Let's ignore, if we can, the casual racism.
0:13:19 > 0:13:25This is Scotland in the 1860s, and Jenkin had a point.
0:13:25 > 0:13:28Variation is the stuff of evolution,
0:13:28 > 0:13:31and if variation blends then it disappears.
0:13:31 > 0:13:36And as it disappears, so the power of natural selection ebbs away.
0:13:39 > 0:13:44Jenkin's challenge was serious, and Darwin had no response.
0:13:46 > 0:13:51How could he? His theory required some system, some law of inheritance
0:13:51 > 0:13:53in which variation did not blend,
0:13:53 > 0:13:56but remained stable over the generations.
0:13:59 > 0:14:04And yet no-one really knew how such a system could work.
0:14:04 > 0:14:08The thing is, Darwin knew all this.
0:14:08 > 0:14:12And we know that he knew because he told us so.
0:14:14 > 0:14:16Perhaps the most wonderful chapter
0:14:16 > 0:14:20of The Origin Of Species is Chapter VI.
0:14:20 > 0:14:23It's called Difficulties Of The Theory.
0:14:23 > 0:14:26Charles Darwin exposes, with unbearable candour,
0:14:26 > 0:14:31devastating honesty, all the weaknesses of his theory.
0:14:31 > 0:14:34He tells us all the reasons he may be wrong,
0:14:34 > 0:14:37the reasons that his critics pointed out, and more,
0:14:37 > 0:14:41but then, appeals to future generations of scientists
0:14:41 > 0:14:43to draw inspiration from his book,
0:14:43 > 0:14:49solve the difficulties with which his theory is riddled.
0:14:53 > 0:14:58It's easy to forget that Darwin was not the first evolutionist.
0:14:58 > 0:15:0150 years before The Origin Of Species,
0:15:01 > 0:15:05a Frenchman had proposed a theory of evolution,
0:15:05 > 0:15:10albeit less coherent and comprehensive than Darwin's.
0:15:12 > 0:15:15His name - Jean-Baptiste Lamarck.
0:15:17 > 0:15:22Lamarck was Professor of Zoology at the Museum d'Histoire Naturelle.
0:15:22 > 0:15:28He published his ideas on evolution in the year Darwin was born.
0:15:28 > 0:15:32They were, however, very different.
0:15:34 > 0:15:38When an animal, any animal, uses an organ, Lamarck argued,
0:15:38 > 0:15:41it becomes strengthened and enlarged.
0:15:43 > 0:15:45That's fairly obvious.
0:15:45 > 0:15:48We all know that exercise modifies the shape of our body.
0:15:48 > 0:15:52It's the same for other creatures as well.
0:15:52 > 0:15:56But Lamarck went further.
0:15:56 > 0:15:58He argued that these changes,
0:15:58 > 0:16:01acquired in one's lifetime, were passed on.
0:16:04 > 0:16:08And it is this, the inheritance of acquired characteristics,
0:16:08 > 0:16:12as it came to be known, that's the engine of evolution.
0:16:15 > 0:16:20The icon of Lamarckism is the giraffe.
0:16:20 > 0:16:25According to Lamarck, some ancestral giraffe had stretched its neck
0:16:25 > 0:16:28reaching for leaves on the highest branches.
0:16:28 > 0:16:33That stretched neck had been passed on to its offspring, who, in turn,
0:16:33 > 0:16:37had stretched their necks even further,
0:16:37 > 0:16:40so that now all giraffes have long necks.
0:16:43 > 0:16:48There is a seductive, intuitive, quality to Lamarck's logic,
0:16:48 > 0:16:52one that Darwin, confronting the inadequacies of his own theory,
0:16:52 > 0:16:55found increasingly hard to resist.
0:17:01 > 0:17:03The giraffe's neck is a cliche.
0:17:03 > 0:17:06It's in every textbook that explains the difference
0:17:06 > 0:17:10between Darwinian and Lamarckian evolution.
0:17:10 > 0:17:14But if you actually read what Darwin says about the giraffe
0:17:14 > 0:17:18in the sixth and final edition of The Origin,
0:17:18 > 0:17:22what you find is something rather different.
0:17:23 > 0:17:26First he talks about natural selection.
0:17:26 > 0:17:28That's what gives you the giraffe's neck.
0:17:28 > 0:17:30But then he adds another line
0:17:30 > 0:17:34about the inherited effects of the increased use of parts.
0:17:34 > 0:17:37Together, they give you the giraffe's neck.
0:17:37 > 0:17:40"Increased use of parts"?
0:17:40 > 0:17:41What's going on here?
0:17:41 > 0:17:43That's pure Lamarck.
0:17:43 > 0:17:49Can it be that Darwin, in his dotage, is becoming less Darwinian?
0:17:51 > 0:17:53Well, yes.
0:17:53 > 0:17:59Perhaps natural selection is not as powerful as once he had thought.
0:18:01 > 0:18:04A recantation? No.
0:18:04 > 0:18:07Just the candour of an old man
0:18:07 > 0:18:11who had spent his life trying to understand the world.
0:18:11 > 0:18:14Darwin died in April 1882.
0:18:18 > 0:18:22He had wanted to be buried quietly near his house in Kent,
0:18:22 > 0:18:28but his supporters arranged a funeral here at Westminster Abbey.
0:18:28 > 0:18:31They turned the agnostic into a saint
0:18:31 > 0:18:35of the new secular materialist age.
0:18:35 > 0:18:40It was the apotheosis of Charles Robert Darwin.
0:18:42 > 0:18:46He had become a great Briton.
0:18:47 > 0:18:51But the eulogies rang hollow.
0:18:51 > 0:18:56Darwin had shown that life on earth was the result of natural laws.
0:18:56 > 0:18:58But what were those laws?
0:18:58 > 0:19:02Everyone - everyone, that is, who mattered -
0:19:02 > 0:19:05agreed that evolution was a fact.
0:19:05 > 0:19:08But natural selection?
0:19:08 > 0:19:09No thanks.
0:19:15 > 0:19:18Even as Darwin lay in state,
0:19:18 > 0:19:21some Darwinians were breaking ranks.
0:19:24 > 0:19:27Chief among them, Dutch botanist, Hugo De Vries.
0:19:31 > 0:19:34Inspired by Darwin, he was searching for a suitable organism
0:19:34 > 0:19:38with which to investigate the workings of inheritance...
0:19:38 > 0:19:40and found one.
0:19:40 > 0:19:46He chose a plant called Oenothera lamarckiana.
0:19:48 > 0:19:52Gardeners will know it as the evening primrose
0:19:52 > 0:19:54for it blooms at dusk.
0:19:54 > 0:19:59It is found throughout the dunes that protect Holland from the sea.
0:19:59 > 0:20:01And it's really just a weed.
0:20:01 > 0:20:06Though lovely for all that.
0:20:08 > 0:20:12De Vries discovered that Oenothera lamarckiana
0:20:12 > 0:20:18occasionally produces progeny that looked very different from itself,
0:20:18 > 0:20:23that have different stems, leaves, flowers.
0:20:23 > 0:20:28These new variants he found did not blend
0:20:28 > 0:20:32but were stable, as stable as new species.
0:20:33 > 0:20:39Coining a term, De Vries called these dramatic variations mutations.
0:20:43 > 0:20:45Following his discovery,
0:20:45 > 0:20:49De Vries was made director of the botanical gardens in Amsterdam.
0:20:53 > 0:20:58He bred and crossbred more Oenotheras, 53,000 of them.
0:21:00 > 0:21:04It seems like a lot, but then again, he was Dutch.
0:21:08 > 0:21:11This is the palm house that Amsterdam built for De Vries.
0:21:11 > 0:21:16And these are some of his flowers pressed for posterity.
0:21:16 > 0:21:18They are ancient and desiccated
0:21:18 > 0:21:24but you can still see the differences in growth and form.
0:21:24 > 0:21:28Mutation, it seemed, could produce radically new plants.
0:21:28 > 0:21:33It could even, said De Vries, produce new species.
0:21:35 > 0:21:38This was all very unDarwinian.
0:21:42 > 0:21:48What of the vertiginous time scales, the infinity of incremental steps,
0:21:48 > 0:21:52the grandeur of Darwin's view of life?
0:21:52 > 0:21:55Irrelevant, said De Vries.
0:21:58 > 0:22:04The origin of species requires only one thing: mutation.
0:22:04 > 0:22:07He called it his Mutation Theory.
0:22:07 > 0:22:09It made him famous.
0:22:09 > 0:22:14For Lamarck, he said, the origin of species was a natural phenomenon.
0:22:14 > 0:22:18For Darwin, the object of scientific investigation,
0:22:18 > 0:22:21for De Vries, he liked to talk of himself in the third person,
0:22:21 > 0:22:24it was the object of experimental enquiry.
0:22:24 > 0:22:28Lamarck, Darwin, De Vries.
0:22:28 > 0:22:31No doubts about HIS place in the pantheon, then.
0:22:34 > 0:22:37What De Vries or no-one else realised at the time
0:22:37 > 0:22:42was that Oenothera lamarckiana was a genetic freak.
0:22:42 > 0:22:47Few other organisms mutate so spectacularly.
0:22:47 > 0:22:54He had based his entire theory on one, very peculiar, species.
0:22:54 > 0:22:59That, however, didn't stop the rise of mutationism.
0:23:02 > 0:23:05Others began to investigate the oddities of nature.
0:23:07 > 0:23:11In Britain, a Cambridge biologist, William Bateson,
0:23:11 > 0:23:14published materials for the study of variation.
0:23:14 > 0:23:17A collection of two-headed turtles,
0:23:17 > 0:23:21girls with four ears,
0:23:21 > 0:23:23and eight-fingered hands.
0:23:23 > 0:23:27It was a medieval monsters and marvels book,
0:23:27 > 0:23:30reworked for the evolutionary age.
0:23:33 > 0:23:37Mutation was the real creative force behind evolution
0:23:37 > 0:23:43and natural selection, said the mutationists, just wasn't needed.
0:23:49 > 0:23:51London 1909.
0:23:55 > 0:23:59Darwin is long dead, and his theory is 50 years old.
0:24:03 > 0:24:06The British Museum of Natural History celebrates
0:24:06 > 0:24:08with an exhibition of Darwiniana:
0:24:08 > 0:24:13specimens, letters, manuscripts.
0:24:13 > 0:24:15It's a magnificent celebration,
0:24:15 > 0:24:20a worthy commemoration of the man who gave us evolution.
0:24:21 > 0:24:23But something is missing,
0:24:23 > 0:24:27something upon which the organisers refuse to be drawn.
0:24:27 > 0:24:29Natural selection.
0:24:31 > 0:24:35You might have expected that the South Kensington museum,
0:24:35 > 0:24:37now a temple of evolutionism,
0:24:37 > 0:24:41would have wanted to tell the public about Darwin's theory.
0:24:41 > 0:24:45But no, that's all too controversial for the keepers and curators who'd
0:24:45 > 0:24:50much rather not commit themselves in the great evolution debate.
0:24:50 > 0:24:54Hardly courageous, but understandable.
0:24:54 > 0:25:00By 1909 scientific consensus had shifted against Darwin's theory.
0:25:02 > 0:25:06Evolution by natural selection was almost extinct.
0:25:18 > 0:25:24Just as Darwinism was at its nadir, a revival was underway.
0:25:24 > 0:25:27For Darwin's critics were, themselves, coming under attack.
0:25:27 > 0:25:34And leading the vanguard was a German scientist, August Weismann.
0:25:35 > 0:25:42Weismann was a doctor, a biologist and above all a great Darwinian.
0:25:42 > 0:25:46He would revive the case for natural selection.
0:25:46 > 0:25:51His key exhibit, an insect called Papilio dardanus.
0:25:54 > 0:25:59Papilo dardanus is a butterfly that lives throughout Africa,
0:25:59 > 0:26:05and the females of the species are mimics.
0:26:07 > 0:26:09This female Papilo dardanus here
0:26:09 > 0:26:13mimics this altogether unrelated species.
0:26:13 > 0:26:19And this one over here mimics something completely different.
0:26:19 > 0:26:23And it does so in every detail.
0:26:27 > 0:26:33Over many generations, Papilio dardanus females have evolved.
0:26:33 > 0:26:36The shapes and colours of their wings have transformed...
0:26:39 > 0:26:42..and the reason why is very obvious.
0:26:45 > 0:26:47Birds eat butterflies,
0:26:47 > 0:26:51so many butterflies have evolved offensive chemicals
0:26:51 > 0:26:53that make them taste repugnant.
0:26:54 > 0:26:57Papilio dardanus doesn't,
0:26:57 > 0:27:01but by mimicking those that do, they can fool the birds.
0:27:02 > 0:27:06They are the cardboard tanks in the battle of nature.
0:27:09 > 0:27:14Here, said Weismann, is evidence that Darwin was right.
0:27:14 > 0:27:18Only natural selection, that slow and subtle craftsman,
0:27:18 > 0:27:22working in infinitesimally small steps,
0:27:22 > 0:27:27could make two unrelated butterfly species so very much alike.
0:27:32 > 0:27:37Hugo De Vries and his fellow mutationists had argued that species
0:27:37 > 0:27:42originate instantaneously by single dramatic mutations.
0:27:47 > 0:27:51No, said Weismann, they evolve gradually
0:27:51 > 0:27:55by the accumulation of a great many tiny mutations.
0:27:55 > 0:27:58Natural selection is a subtle force -
0:27:58 > 0:28:03as subtle as the markings on the wings of a butterfly.
0:28:06 > 0:28:09But Weissman's real ire was reserved
0:28:09 > 0:28:14for that other great anti-Darwinian theory, Lamarckism.
0:28:16 > 0:28:22He knew that sperm and eggs carry the material of inheritance.
0:28:22 > 0:28:25But where, he asked, do they come from?
0:28:27 > 0:28:30By tracing the origin and fate of the cells in the embryo,
0:28:30 > 0:28:35Weismann realised that the cells that give rise to sperm and eggs
0:28:35 > 0:28:38were quickly isolated from the rest of the body's cells.
0:28:38 > 0:28:42That they formed a separate lineage.
0:28:45 > 0:28:48That, said Weissman, is why acquired characteristics
0:28:48 > 0:28:52could not be passed on to future generations,
0:28:52 > 0:28:59why Lamarck was wrong, and why the giraffe stretches its neck in vain.
0:29:01 > 0:29:06A body could strive, suffer, stretch and sacrifice
0:29:06 > 0:29:08and none of it would matter.
0:29:08 > 0:29:13All bodies must die was Weismann's message,
0:29:13 > 0:29:17only your eggs and sperm have even a shot at immortality.
0:29:19 > 0:29:22But Weissman did more.
0:29:22 > 0:29:25He reasoned that the material of inheritance
0:29:25 > 0:29:29was something physical in the nucleus of each sperm and egg cell.
0:29:29 > 0:29:32He called this material germ plasm.
0:29:34 > 0:29:38Looking closer, his contemporaries saw distinct particles
0:29:38 > 0:29:41within the germ plasm - chromosomes.
0:29:44 > 0:29:49With reproduction, chromosomes mix, mingle, and recombine,
0:29:49 > 0:29:51but they never blend.
0:29:53 > 0:29:57They are always passed on intact.
0:29:58 > 0:30:00This dance of the chromosomes
0:30:00 > 0:30:07confirmed what an Augustinian monk has supposed three decades earlier.
0:30:09 > 0:30:14Gregor Mendel, he's the archetypal scientific hero:
0:30:14 > 0:30:17works away breeding peas in a Moravian monastery,
0:30:17 > 0:30:24publishes two luminous papers in an obscure journal that no-one reads,
0:30:24 > 0:30:27give up science and becomes,
0:30:27 > 0:30:30like so many great scientists, an administrator.
0:30:34 > 0:30:38Mendel was appointed abbot of Brno monastery.
0:30:38 > 0:30:43He abandoned his experiments; his publications were forgotten
0:30:43 > 0:30:48for 34 years, and when he died his papers were burned.
0:30:48 > 0:30:53When, in 1900, his experiments were rediscovered and republished,
0:30:53 > 0:30:57they became, however, the stuff of scientific legend.
0:31:03 > 0:31:06Think about it, you're breeding peas,
0:31:06 > 0:31:11green peas, yellow peas, wrinkly peas, smooth peas.
0:31:11 > 0:31:17You count the numbers of peas in each generation,
0:31:17 > 0:31:22calculate a few ratios, and you discover what everyone else,
0:31:22 > 0:31:25what Darwin himself had missed.
0:31:25 > 0:31:31The laws that rule the inheritance of nearly every living thing.
0:31:32 > 0:31:36Here, among Mendel's peas, were mathematical laws
0:31:36 > 0:31:41that explained how traits are passed down the generations.
0:31:41 > 0:31:48And, rather wonderfully, these laws, mere statistical abstractions,
0:31:48 > 0:31:53were the very system of inheritance that natural selection needed.
0:31:53 > 0:31:56They gave natural selection
0:31:56 > 0:32:00the supply of heritable variation that it needed to work.
0:32:10 > 0:32:13Rothamsted Agricultural Station in Hertfordshire
0:32:13 > 0:32:19is an unlikely landmark in the history of evolutionary biology.
0:32:19 > 0:32:23What it's really famous for is an experiment aimed at estimating
0:32:23 > 0:32:26the effects of fertilizers on crop yields.
0:32:28 > 0:32:31The experiment had begun in the 1840s.
0:32:31 > 0:32:34Every year, samples had been collected and stored
0:32:34 > 0:32:35and there they lay.
0:32:35 > 0:32:41Until 1919 when they hired a young Cambridge mathematician
0:32:41 > 0:32:42to analyse them.
0:32:42 > 0:32:47His name was Ronald Aylmer Fisher.
0:32:49 > 0:32:51Fisher was a prodigy.
0:32:51 > 0:32:53Profoundly myopic,
0:32:53 > 0:32:58he had learnt to visualize mathematical problems in his head.
0:33:02 > 0:33:05You wouldn't think that such a clever man
0:33:05 > 0:33:09would be happy calculating agricultural yields.
0:33:10 > 0:33:12But he liked it.
0:33:14 > 0:33:17There were lots of numbers to crunch,
0:33:17 > 0:33:19and no-one knew how to do it.
0:33:19 > 0:33:22No problem, said Fisher,
0:33:22 > 0:33:24and invented some new statistics.
0:33:29 > 0:33:30Some new statistics?
0:33:30 > 0:33:35Fisher invented just about every statistical test I've ever used.
0:33:35 > 0:33:39And not just the tests. When Fisher wanted to solve a problem,
0:33:39 > 0:33:43he would invent a whole new branch of mathematics.
0:33:45 > 0:33:50But Fisher was interested in more than crop yields.
0:33:50 > 0:33:54He was also rather keen on eugenics.
0:33:54 > 0:33:57NARRATOR: In institutions such as this all over the country,
0:33:57 > 0:33:59mental defectives are cared for.
0:33:59 > 0:34:02But it would have been better by far,
0:34:02 > 0:34:04for them and for the rest of the community,
0:34:04 > 0:34:06if they had never been born.
0:34:06 > 0:34:11Fisher was worried that the British were becoming thick.
0:34:11 > 0:34:14That the poor, feckless and stupid
0:34:14 > 0:34:18were outbreeding the rich, thrifty and smart.
0:34:18 > 0:34:21So you see, that in a mere matter of four generations
0:34:21 > 0:34:23individuals below the average
0:34:23 > 0:34:26have become more than five times as abundant as those above it.
0:34:27 > 0:34:31And so if we want to maintain the race at a high level,
0:34:31 > 0:34:34everybody sound in body and mind should marry
0:34:34 > 0:34:37and have enough children to perpetuate their stock
0:34:37 > 0:34:39and carry on the race.
0:34:39 > 0:34:42Fisher fathered eight children himself -
0:34:42 > 0:34:46his own personal eugenics programme.
0:34:48 > 0:34:50It was, of course, absurd.
0:34:50 > 0:34:54There's no evidence that the nation's collective IQ
0:34:54 > 0:34:56was in terminal decline.
0:34:56 > 0:35:00Yet out of Fisher's eugenical obsessions came something wonderful.
0:35:00 > 0:35:04For in the evenings between calculating correlation coefficients
0:35:04 > 0:35:09and fathering children, he thought about natural selection.
0:35:14 > 0:35:17For Fisher, natural selection was a force
0:35:17 > 0:35:20rather like the waves that beat against a beach.
0:35:22 > 0:35:27Just as they may at times pound the shore without relent
0:35:27 > 0:35:30and at other times lap gently,
0:35:30 > 0:35:35so too natural selection may gust or whisper, but it never disappears.
0:35:39 > 0:35:44And just as the waves winnow, sift and sort the pebbles on this beach,
0:35:44 > 0:35:47so natural selection winnows, sifts and sorts
0:35:47 > 0:35:49the variation within species.
0:35:51 > 0:35:55And it is this sorting that is evolution itself.
0:35:59 > 0:36:04All this Fisher described with a single equation.
0:36:04 > 0:36:10He called it his Fundamental Theorem of Natural Selection,
0:36:10 > 0:36:16and said that it was supreme among the laws of biological science.
0:36:16 > 0:36:21He even compared it in its scope and power and generality
0:36:21 > 0:36:24to the Second Law of Thermodynamics.
0:36:28 > 0:36:32Fisher turned natural selection into a formula.
0:36:32 > 0:36:37But a formula without data isn't much good.
0:36:37 > 0:36:39You have to show that it actually works -
0:36:39 > 0:36:41that it says something useful
0:36:41 > 0:36:45about living, breathing, copulating creatures.
0:36:47 > 0:36:51Which brings us to a rather dull-looking moth.
0:36:54 > 0:36:58Here is a story of natural selection in action.
0:36:58 > 0:37:03The story of Biston bestularia - the peppered moth.
0:37:05 > 0:37:09Once upon a time, it was the colour of speckled ivory.
0:37:09 > 0:37:12This colour was an adaptation,
0:37:12 > 0:37:16camouflaging the moth as it rested on woodland lichens,
0:37:16 > 0:37:19protecting it from the birds that would prey on it.
0:37:19 > 0:37:23But then came the Industrial Revolution.
0:37:23 > 0:37:27Soot killed the lichens and turned the trees black.
0:37:29 > 0:37:32The moths were no longer camouflaged,
0:37:32 > 0:37:35they were exposed and vulnerable to attack.
0:37:37 > 0:37:41So they began to evolve - fast.
0:37:41 > 0:37:44A new, dark, form of the moth spread.
0:37:44 > 0:37:48By the 1950s it was found across Britain.
0:37:51 > 0:37:56In the woods, biologist Bernard Kettlewell conducted experiments,
0:37:56 > 0:38:02calculated the rate of change and the strength of natural selection,
0:38:02 > 0:38:07and he found that the equations worked.
0:38:07 > 0:38:13Every character, in every species of insects, plant and man himself,
0:38:13 > 0:38:17is constantly under collective pressure.
0:38:17 > 0:38:23We have shown if the pressure is high enough within 50 generations
0:38:23 > 0:38:28one character can nearly entirely substitute another.
0:38:28 > 0:38:32It is due to such changes in many characters
0:38:32 > 0:38:36that new species are gradually evolving.
0:38:41 > 0:38:44And the moths have continued to evolve.
0:38:44 > 0:38:49In 1956 the Clean Air Act scrubbed the soot from the nation's skies,
0:38:49 > 0:38:51and from the bark of the nation's trees.
0:38:53 > 0:38:55The dark moths began to disappear,
0:38:55 > 0:38:57and the light moths returned.
0:38:59 > 0:39:01Evolution went into reverse.
0:39:04 > 0:39:09Natural selection not only existed, it was far more powerful,
0:39:09 > 0:39:14and evolution far swifter, than Darwin had ever imagined.
0:39:19 > 0:39:25But if there is a place of which Darwinians dream, it is Lake Malawi.
0:39:32 > 0:39:36One of the three great lakes of Africa,
0:39:36 > 0:39:38it was discovered by David Livingstone
0:39:38 > 0:39:41just as The Origin of Species was going to press.
0:39:47 > 0:39:51But it took the best part of a century before we realised
0:39:51 > 0:39:55just how remarkable are the fish in these waters.
0:39:57 > 0:39:59They are, quite simply,
0:39:59 > 0:40:03the most beautiful vindication of Darwin's theory.
0:40:04 > 0:40:07Dive among them, and the first thing you notice
0:40:07 > 0:40:12is how astonishingly various are these fish.
0:40:12 > 0:40:13They differ from each other
0:40:13 > 0:40:17in the shapes of their bodies, their mouth and teeth -
0:40:17 > 0:40:21in their colours and their breeding habits,
0:40:22 > 0:40:27and yet they are all members of the same family.
0:40:27 > 0:40:29They are all cichlids.
0:40:33 > 0:40:35Two million years ago
0:40:35 > 0:40:40a cichlid must have entered the lake and multiplied.
0:40:40 > 0:40:44Over time it seems the lake levels rose and fell,
0:40:44 > 0:40:47creating a universe of different habitats,
0:40:47 > 0:40:49each with its own resources to exploit,
0:40:49 > 0:40:53and each evolving its own set of cichlids.
0:41:01 > 0:41:07And now there are 400, 500, maybe 600 species here -
0:41:07 > 0:41:10more than all the species of fish
0:41:10 > 0:41:15in all the lakes and rivers of Europe or America.
0:41:27 > 0:41:31It's not just that this lake has so many species of cichlids,
0:41:31 > 0:41:33it's how diverse they,
0:41:33 > 0:41:36how many different ways in which they make a living.
0:41:36 > 0:41:40This fish over here is Pseudotropheus elongatus and
0:41:40 > 0:41:45it makes a living by scraping algae off rocks and combing through it.
0:41:45 > 0:41:50This one does the same except it has an even more elaborate mouth.
0:41:50 > 0:41:54It has its teeth arranged rather like a rasp
0:41:54 > 0:41:57with lots of little teeth with which it files away
0:41:57 > 0:41:59and scrapes the algae off the rocks.
0:41:59 > 0:42:05This...this fish here buries it head in the sand, opens its mouth
0:42:05 > 0:42:10and gobbles up little Chironomid midges that are living there.
0:42:10 > 0:42:14And this thing is the most remarkable cichlid of all,
0:42:14 > 0:42:15Rhamphochromis.
0:42:15 > 0:42:20It is the uber-predator, the Tyrannosaurus of the lake,
0:42:20 > 0:42:25gobbling anything that it can with those formidable sharp teeth.
0:42:28 > 0:42:33What's so most amazing about these fish is that they're all descended
0:42:33 > 0:42:38from one cichlid that entered this lake about two million years ago.
0:42:38 > 0:42:430 to 600 fish in about two million years.
0:42:43 > 0:42:46It is one of the most astonishing evolutionary events
0:42:46 > 0:42:49that has ever happened on this planet.
0:42:53 > 0:42:58By the end of the 1950s evolution had a new formula.
0:42:58 > 0:43:01A combination of natural selection,
0:43:01 > 0:43:06isolation, Mendelian inheritance, and mathematical theory.
0:43:08 > 0:43:11It was called the neo-Darwinian synthesis.
0:43:14 > 0:43:19The formula wasn't entirely Darwinian, but that didn't matter.
0:43:19 > 0:43:22On the centenary of The Origin of Species,
0:43:22 > 0:43:25everyone agreed that Darwin's vision had triumphed.
0:43:25 > 0:43:28Everyone was a Darwinian.
0:43:28 > 0:43:32You could almost say that they were more Darwinian than Darwin himself.
0:43:37 > 0:43:41And yet, as Darwinism entered its second century,
0:43:41 > 0:43:43some anomalies remained.
0:43:44 > 0:43:47Natural selection may have triumphed,
0:43:47 > 0:43:51yet some animal behaviours were still hard to explain
0:43:51 > 0:43:53in terms of natural selection.
0:43:58 > 0:44:01Such as altruism.
0:44:05 > 0:44:08In a world driven by competition,
0:44:08 > 0:44:12why are some animals altruistic?
0:44:13 > 0:44:15Termites, for instance,
0:44:15 > 0:44:20cooperate relentlessly building vast mounds on the African plain.
0:44:21 > 0:44:25A termite mound is filled with millions of altruists,
0:44:25 > 0:44:33the soldiers and workers
0:44:33 > 0:44:35whose lives
0:44:35 > 0:44:40are devoted to defending and feeding the queen.
0:44:43 > 0:44:47They are the eunuchs of the termite state
0:44:47 > 0:44:52and their existence is surprisingly hard to explain.
0:44:55 > 0:44:59According to Darwinian logic, creatures are engineered
0:44:59 > 0:45:03by natural selection to increase their chances of reproduction.
0:45:09 > 0:45:13Yet most termites are sterile, they don't reproduce at all.
0:45:13 > 0:45:17They work for the colony, apparently without reward.
0:45:19 > 0:45:21Why?
0:45:21 > 0:45:25In 1964 zoologist, Bill Hamilton,
0:45:25 > 0:45:28proposed a solution to the problem of altruism.
0:45:28 > 0:45:33One that explained the existence of social insects.
0:45:33 > 0:45:37This highly regimented move by hundreds of thousands of individuals
0:45:37 > 0:45:40is a typically impressive achievement of the social insects.
0:45:40 > 0:45:44Hamilton realised that the members of any colony
0:45:44 > 0:45:45are very closely related.
0:45:47 > 0:45:50And, as such, they share genes.
0:45:50 > 0:45:55The queen alone is replicating the genes of the colony on its behalf.
0:45:56 > 0:45:58That, said Hamilton, is the key.
0:46:01 > 0:46:06If your altruistic act benefits a relative then you may pay a cost.
0:46:06 > 0:46:10But at least some of your genes will reap the benefit.
0:46:14 > 0:46:19Natural selection does not count the fates of individuals,
0:46:19 > 0:46:21it counts the fates of genes.
0:46:27 > 0:46:31Termite soldiers sacrifice themselves for their queen
0:46:31 > 0:46:34because she shares many of their genes.
0:46:34 > 0:46:36And by devoting their lives to her,
0:46:36 > 0:46:39more of her genes, and hence their genes,
0:46:39 > 0:46:43are passed on than they could possibly achieve by themselves.
0:46:47 > 0:46:50This was a radical extension of Darwinism
0:46:50 > 0:46:52and it spawned a new science.
0:46:52 > 0:46:56At Harvard E O Wilson gave it a name - sociobiology.
0:46:56 > 0:47:02At Oxford, Richard Dawkins gave it a slogan - The Selfish Gene.
0:47:04 > 0:47:07Wherever sociobiologists looked, they explained
0:47:07 > 0:47:12all the strange things animals do as the product of natural selection,
0:47:12 > 0:47:16not on individuals, but on their genes.
0:47:16 > 0:47:19Each individual is just obeying its own genes.
0:47:19 > 0:47:23And humans, they said, are no different.
0:47:25 > 0:47:28Our behaviours too, can be explained by genetic programmes
0:47:28 > 0:47:31shaped by natural selection.
0:47:31 > 0:47:34This is the fundamental principle of sociobiology.
0:47:34 > 0:47:38The genes for particular social behaviour exist.
0:47:38 > 0:47:44Applied to humans, sociobiology seems excessively reductive.
0:47:45 > 0:47:49But there's no doubt that as an explanation of animal behaviour,
0:47:49 > 0:47:52it triumphed.
0:47:52 > 0:47:57And sociobiology's triumph was the triumph of natural selection.
0:48:02 > 0:48:07A force, that in Darwin's time, seemed weak and ephemeral,
0:48:07 > 0:48:10was now omnipotent and omnipresent.
0:48:15 > 0:48:18Yet Darwin gave us more than natural selection,
0:48:18 > 0:48:21more than a mechanism of evolution.
0:48:23 > 0:48:28He also gave us a new narrative, or at least the promise of one.
0:48:28 > 0:48:35He told us that the history of life was a tale of epic forces and scales
0:48:35 > 0:48:39and that it was ours to discover.
0:48:48 > 0:48:52If there is an icon of Darwin's theory, it is this.
0:48:54 > 0:48:59A metaphor for all evolutionary history.
0:48:59 > 0:49:00A tree.
0:49:04 > 0:49:07The twigs, Darwin said, were species.
0:49:07 > 0:49:10And they were connected to their ancestors by branches,
0:49:10 > 0:49:15and those ancestors to theirs, reaching deep into the past.
0:49:15 > 0:49:17So that the whole history of life
0:49:17 > 0:49:21could be represented as a great tree.
0:49:24 > 0:49:29Darwin first conceived this image in 1837.
0:49:29 > 0:49:34He sketched a simple tree-like diagram to show how lineages
0:49:34 > 0:49:39could originate from a single source and then diverge and proliferate.
0:49:39 > 0:49:43Above it he scribbled the words, "I think".
0:49:46 > 0:49:4922 years later, in The Origin,
0:49:49 > 0:49:53he confidently asserts that just such a tree
0:49:53 > 0:49:57could be constructed for any group of creatures.
0:50:00 > 0:50:05Easy to say, hard to do, and Darwin didn't even try.
0:50:11 > 0:50:14Why did Darwin, so bold and so visionary,
0:50:14 > 0:50:18not give us the history of life that his theory implied?
0:50:20 > 0:50:24Perhaps because he was so acutely aware
0:50:24 > 0:50:28of the deficiencies of the fossil record.
0:50:28 > 0:50:33The rocks ought to bear mute testimony to titanic conflicts
0:50:33 > 0:50:36playing out over eons of time.
0:50:37 > 0:50:41But, as Richard Owen had so cruelly exposed,
0:50:41 > 0:50:44the reality was rather different.
0:50:46 > 0:50:50Animal fossils were abundant, but there were also huge gaps.
0:50:50 > 0:50:55And nowhere was the gap greater than at the base of the Cambrian.
0:50:57 > 0:51:02That's when an explosion of animal life seems to have occurred.
0:51:02 > 0:51:07New species, entire faunas emerged as if from nowhere,
0:51:07 > 0:51:10their ancestors absent.
0:51:14 > 0:51:17Those rocks over there are Cambrian.
0:51:17 > 0:51:21That makes them around 525 million years old.
0:51:21 > 0:51:25And they contain animal life, wonderful creatures such as
0:51:25 > 0:51:29brachiopods, ostracods and trilobites.
0:51:29 > 0:51:36These rocks are Precambrian, they're only about 30 million years older.
0:51:36 > 0:51:39And yet they are empty.
0:51:39 > 0:51:42There are no animal remains in them whatsoever.
0:51:44 > 0:51:46But how could this be?
0:51:46 > 0:51:50If these Precambrian rocks didn't have any fossils in them,
0:51:50 > 0:51:52where did the animals come from?
0:51:52 > 0:51:55Characteristically, Darwin did not shirk the problem.
0:51:55 > 0:52:00"During these vast, and unknown periods of time", he wrote,
0:52:00 > 0:52:04"the world must have swarmed with living creatures."
0:52:04 > 0:52:10That he couldn't produce them was, he admitted, a grave difficulty.
0:52:11 > 0:52:15Darwin despaired of being able to reconstruct the history of life.
0:52:15 > 0:52:21Yet he did not doubt that his successors would do just that.
0:52:21 > 0:52:23And so they have.
0:52:29 > 0:52:32Enter one of Darwin's most ardent disciples -
0:52:34 > 0:52:37a young German scientist, Ernst Haeckel.
0:52:41 > 0:52:44Of all the scientists who followed Darwin,
0:52:44 > 0:52:47Haeckel was the most protean.
0:52:49 > 0:52:52A gifted artist who could reveal nature's exquisite geometries
0:52:52 > 0:52:55with the stroke of a pen,
0:52:55 > 0:52:57he was also a brilliant anatomist,
0:52:57 > 0:53:02devoting months to the study of obscure sea creatures.
0:53:05 > 0:53:06And he was a romantic,
0:53:06 > 0:53:11of the sentimental, nature-loving Goethe-worshipping German kind.
0:53:14 > 0:53:17For Haeckel loved his cousin, Anna.
0:53:17 > 0:53:20She had golden hair and blue eyes.
0:53:20 > 0:53:25He described her as "a true German child of the forest."
0:53:28 > 0:53:32Haeckel was besotted with her and married her.
0:53:32 > 0:53:34What bliss!
0:53:34 > 0:53:37But for only a few months,
0:53:37 > 0:53:38and then she died.
0:53:44 > 0:53:46Anna's death left Haeckel unhinged.
0:53:46 > 0:53:49He contemplated suicide.
0:53:49 > 0:53:51But then he found religion.
0:53:51 > 0:53:54Not the false consolations of Christianity,
0:53:54 > 0:53:58but the harsh, godless clarity of Der Darwinismus.
0:53:58 > 0:54:01He would become its greatest apostle.
0:54:01 > 0:54:05He would take the good book to the German masses,
0:54:05 > 0:54:08he would preach the truth and he would do what Darwin had
0:54:08 > 0:54:15so conspicuously failed to do - he would re-write the history of life.
0:54:17 > 0:54:18But how?
0:54:18 > 0:54:23Haeckel needed a way of reconstructing the evolutionary past
0:54:23 > 0:54:25that did not rely on fossils.
0:54:25 > 0:54:29The answer, he said, was to look at the embryo.
0:54:31 > 0:54:34The embryo of an animal contains, is,
0:54:34 > 0:54:37a record of its evolutionary past.
0:54:37 > 0:54:40The earlier in development you look,
0:54:40 > 0:54:42the further back into their past you can see.
0:54:42 > 0:54:46The embryo, Haeckel said,
0:54:46 > 0:54:48is Ariadne's thread.
0:54:53 > 0:54:57He began by comparing vertebrate embryos.
0:54:58 > 0:55:04Just before birth they seem very different, as you'd expect.
0:55:04 > 0:55:07But follow the embryos back in time, to when they are younger,
0:55:07 > 0:55:11and less developed, they look remarkably alike.
0:55:11 > 0:55:18They have the same dorsal nerve cords, the same pharyngeal slits.
0:55:22 > 0:55:26But Haeckel looked further, deeper into the embryo,
0:55:26 > 0:55:28earlier into its development.
0:55:28 > 0:55:31Before the limbs appear,
0:55:31 > 0:55:35before there's a head or a tail, and further yet,
0:55:35 > 0:55:40to when it is but a ball of cells with the beginnings of a gut.
0:55:40 > 0:55:44This is a stage of development called "gastrulation".
0:55:44 > 0:55:48And here he thought he found something wonderful -
0:55:48 > 0:55:52the ancestor of us all.
0:55:55 > 0:55:59Here, said Haeckel, is a remembrance, a recollection,
0:55:59 > 0:56:03a recapitulation of the very first animal.
0:56:03 > 0:56:07A creature, no more than a ball of flagellated cells,
0:56:07 > 0:56:11that had once whirled through the Precambrian seas.
0:56:11 > 0:56:13He called it the gastrea
0:56:13 > 0:56:17and said that it was his most important discovery.
0:56:17 > 0:56:22Others said it was his most outrageous invention.
0:56:24 > 0:56:28Haeckel used embryos to produce evolutionary trees.
0:56:28 > 0:56:29Lots of them.
0:56:31 > 0:56:36They look a bit like Darwin's tree, but they are not abstract metaphors,
0:56:36 > 0:56:40they are the first attempt to put every living thing
0:56:40 > 0:56:42in its evolutionary place.
0:56:44 > 0:56:48All the animals are there, in more or less the right order,
0:56:48 > 0:56:51and somewhere near the base of the trunk
0:56:51 > 0:56:55leading to all the other animals is the gastrea -
0:56:55 > 0:56:58Haeckel's hypothetical ancestral beast.
0:57:01 > 0:57:05Haeckel's speculations were, no doubt, too bold.
0:57:05 > 0:57:09The embryo does not contain a simple picture of the history of life.
0:57:09 > 0:57:13And yet, there's no doubt that his guesses were,
0:57:13 > 0:57:16more often than not, inspired.
0:57:20 > 0:57:25Since the 1950s, a trickle of animal fossils
0:57:25 > 0:57:28has been emerging from Precambrian rocks.
0:57:29 > 0:57:34Some are little more than imprints, others are minute.
0:57:36 > 0:57:39But by using Computed Tomography Imaging,
0:57:39 > 0:57:41even individual cells can be seen.
0:57:42 > 0:57:46And, what's more, some of these proto-creatures
0:57:46 > 0:57:50are not so very different from Haeckel's gastrea.
0:57:54 > 0:57:57But there's another reason to think that animals lived
0:57:57 > 0:58:01long before the Cambrian and that's because DNA tells us so.
0:58:10 > 0:58:16When Watson and Crick elucidated the structure of DNA they unified life.
0:58:16 > 0:58:20The DNA story is without doubt one of the greatest success stories
0:58:20 > 0:58:22in the history of science.
0:58:22 > 0:58:25Because it can't be often that two newcomers to a field
0:58:25 > 0:58:28make such a major discovery so quickly.
0:58:30 > 0:58:35Nearly all living things use DNA as the stuff of inheritance.
0:58:35 > 0:58:37So they must all be related,
0:58:37 > 0:58:44and descend, much as Darwin had supposed, from a single ancestor.
0:58:47 > 0:58:49But now we can go further.
0:58:49 > 0:58:54We can sequence the genome of any living thing
0:58:54 > 0:58:58and read it as if we were reading a book.
0:59:00 > 0:59:05Genomes are documents written in billion of letters.
0:59:05 > 0:59:09They are palimpsests, endlessly augmented, erased,
0:59:09 > 0:59:13and rewritten by the hand of evolution.
0:59:13 > 0:59:15And if you can read them
0:59:15 > 0:59:19you can read the history of life.
0:59:20 > 0:59:23By sequencing genomes we can now date
0:59:23 > 0:59:27the origin of animals in the tree of life.
0:59:27 > 0:59:31Some of them turn out to be astonishingly ancient.
0:59:34 > 0:59:37Perhaps the simplest of all animals
0:59:37 > 0:59:40is a microscopic creature called Trichoplax.
0:59:40 > 0:59:46It doesn't have a gut, a mouth, a brain, or even sense organs.
0:59:46 > 0:59:49Its genome suggests that its ancestors departed
0:59:49 > 0:59:52from the main trunk of animal evolution,
0:59:52 > 0:59:54perhaps a billion years old.
0:59:56 > 0:59:58Just as Darwin had supposed,
0:59:58 > 1:00:02there must have been animals in the Precambrian seas.
1:00:04 > 1:00:07The sequencing machines are revealing new branches
1:00:07 > 1:00:08on the tree of life.
1:00:08 > 1:00:13They are giving us a new historical narrative.
1:00:14 > 1:00:17But, we are also discovering new fossils.
1:00:17 > 1:00:22And often, the story they tell is the same.
1:00:24 > 1:00:27Consider the whale.
1:00:27 > 1:00:31Whales obviously evolved from some land mammal but, if so,
1:00:31 > 1:00:34where were the fossil half-whales?
1:00:34 > 1:00:36Where were the whales with legs?
1:00:36 > 1:00:41By Darwin's logic, they must have existed and they must have been big.
1:00:41 > 1:00:43So where were they?
1:00:46 > 1:00:51For years, the origin of whales was shrouded in obscurity.
1:00:51 > 1:00:53Not any more.
1:00:54 > 1:00:59In recent decades, the fossil record has become wonderfully complete.
1:01:02 > 1:01:06Just as Darwin had predicted, just as Darwin had hoped,
1:01:06 > 1:01:10we now have an astonishing array of fossils that show how a land mammal
1:01:10 > 1:01:13makes a transition to one that lives in the sea.
1:01:13 > 1:01:18They show how front limbs evolved into flippers,
1:01:18 > 1:01:22and how hind limbs just wither away,
1:01:22 > 1:01:24and how a whale comes to breathe
1:01:24 > 1:01:27not through their nostrils in the tips of its snout
1:01:27 > 1:01:30but rather a blowhole in the back of its head.
1:01:30 > 1:01:33And they show us one more thing.
1:01:33 > 1:01:39They tell us about the place of whales in the tree of life.
1:01:41 > 1:01:45The evidence hinges, literally, on an ankle bone.
1:01:45 > 1:01:49For primitive whales, it turns out, have ankle bones
1:01:49 > 1:01:52that are remarkably similar to those of modern ungulates,
1:01:52 > 1:01:55such as cows, sheep and pigs.
1:01:56 > 1:01:59Yet we don't have to rely on fossilised bones
1:01:59 > 1:02:01to tell us about the ancestry of whales.
1:02:04 > 1:02:07We can use DNA too.
1:02:09 > 1:02:13And to do that we have to go to Africa.
1:02:17 > 1:02:21Compare the DNA of a cetacean to that of any other mammal
1:02:21 > 1:02:24and something surprising emerges.
1:02:24 > 1:02:29Their closest living relation is this, a hippo.
1:02:38 > 1:02:41It's not that whales evolved from hippos,
1:02:41 > 1:02:45or that hippos evolved from whales.
1:02:45 > 1:02:51Rather, it is simply that hippos and cetaceans are, as it were, cousins.
1:02:51 > 1:02:55They are descended from a common ancestor that lived, perhaps,
1:02:55 > 1:02:5755 million years ago.
1:03:05 > 1:03:08It is precisely this sort of agreement
1:03:08 > 1:03:11between DNA and fossil evidence
1:03:11 > 1:03:15that makes the case for evolution so utterly compelling.
1:03:19 > 1:03:23And so, to the evolution of the one species
1:03:23 > 1:03:26we care about more than any other.
1:03:29 > 1:03:34One might have expected Darwin to say something about human evolution.
1:03:36 > 1:03:40And, in The Origin Of Species, he does.
1:03:44 > 1:03:50After 400 pages of ants and agoutis, bats and barnacles,
1:03:50 > 1:03:53the whole bestiary, in fact, right through to zebras,
1:03:53 > 1:03:57he settles down to consider humanity.
1:03:57 > 1:04:00And what he says is this.
1:04:04 > 1:04:10"Light will be thrown on the origin of Man and his history."
1:04:19 > 1:04:21And that's it.
1:04:21 > 1:04:24Well, thanks for that, Charles.
1:04:27 > 1:04:30Darwin, of course, knew we were descended from apes,
1:04:30 > 1:04:33but left others to spell it out.
1:04:35 > 1:04:38Among them, Ernst Haeckel.
1:04:42 > 1:04:44With a characteristic flourish,
1:04:44 > 1:04:48he imagined humanity like Botticelli's Venus,
1:04:48 > 1:04:52rising gloriously from the brutes that surround her.
1:04:55 > 1:04:59At the time, there were no fossils linking man to apes.
1:05:01 > 1:05:05And so he set to imagining what lay between.
1:05:07 > 1:05:11No human fossils, no problem.
1:05:11 > 1:05:13Let's just invent one.
1:05:13 > 1:05:16Something between an ape and a man.
1:05:16 > 1:05:20Let's give it a name. Ape man.
1:05:20 > 1:05:25Let's give it a real, proper, Latin name, Pithecanthropus.
1:05:28 > 1:05:33Like the gastrea, Pithecanthropus was an invention,
1:05:33 > 1:05:36a hypothetical ancestor.
1:05:36 > 1:05:38Yet Haeckel's reasoning was sound.
1:05:40 > 1:05:43If we were descended from apes,
1:05:43 > 1:05:47then sooner or later intermediates would be found.
1:05:50 > 1:05:54In 1891, a Dutch physician, Eugene Dubois,
1:05:54 > 1:05:57digging in the banks of the Solo River in Java
1:05:57 > 1:05:59discovered this skullcap.
1:05:59 > 1:06:04It wasn't human, it wasn't ape, it was an ape man.
1:06:04 > 1:06:10In homage to Haeckel, Dubois called it Pithecanthropus.
1:06:16 > 1:06:18In the centuries since,
1:06:18 > 1:06:23Pithecanthropus has acquired a new name, Homo erectus,
1:06:23 > 1:06:26and has been joined by a collection of other fossils,
1:06:26 > 1:06:30some, apish humans,
1:06:30 > 1:06:33others, human apes.
1:06:35 > 1:06:40The family tree of humanity can now be richly filled with species
1:06:40 > 1:06:43and there's a clear and unambiguous line
1:06:43 > 1:06:49between the earliest apes and us, Homo sapiens.
1:06:54 > 1:06:58But which of the great apes now alive is our closest relation?
1:07:00 > 1:07:04That question, endlessly debated since Darwin's time,
1:07:04 > 1:07:07hasn't been answered by fossils.
1:07:07 > 1:07:09It required DNA.
1:07:13 > 1:07:16By comparing DNA sequences from each of the great apes,
1:07:16 > 1:07:20the order of evolutionary descent has become clear.
1:07:23 > 1:07:27We're genetically closest to chimpanzees and bonobos.
1:07:27 > 1:07:33Five to six million years ago our ancestor was theirs.
1:07:36 > 1:07:40Seven million years ago, we shared an ancestor with the gorilla.
1:07:43 > 1:07:4612 million years ago, with an orang-utan.
1:07:48 > 1:07:53And so on back to the very beginning of life.
1:07:58 > 1:08:01"Who do you think you are?" asked Haeckel.
1:08:01 > 1:08:02And answered,
1:08:02 > 1:08:08"You are an ape, a mammal, a reptile, a fish,
1:08:08 > 1:08:13"a worm, a ball of cells and finally a single cell
1:08:13 > 1:08:18"floating in the saline womb of the primordial seas."
1:08:19 > 1:08:23150 years ago, Darwin spoke of a time
1:08:23 > 1:08:26when the tree of life would be more than a metaphor,
1:08:26 > 1:08:31when it would be an accurate historical record.
1:08:31 > 1:08:33That time has come.
1:08:33 > 1:08:36The tree of life stands before us,
1:08:36 > 1:08:42its branches becoming clearer with every fossil and DNA sequence,
1:08:42 > 1:08:46and our species is but a leaf on a twig,
1:08:46 > 1:08:51buried within in its vast and ramifying canopy.
1:09:04 > 1:09:10When we look at living things it is the differences that we first see,
1:09:10 > 1:09:16the astonishing variety of form, colour and behaviour.
1:09:16 > 1:09:23And yet, beneath this diversity runs a deep, unifying plan.
1:09:23 > 1:09:27For most animals have much the same geometry,
1:09:27 > 1:09:30the same basic body plan.
1:09:32 > 1:09:37For Darwin, this paradox of unity within diversity
1:09:37 > 1:09:39was the gift of the tree of life,
1:09:39 > 1:09:44the consequence of species giving rise to species,
1:09:44 > 1:09:49endlessly adapting over countless generations.
1:09:56 > 1:09:58But what Darwin could never have imagined
1:09:58 > 1:10:04is that such unity within diversity can be found in every cell,
1:10:04 > 1:10:09every molecule and every gene of every living thing.
1:10:09 > 1:10:11Well done, where's that?
1:10:11 > 1:10:14Can you look at the light? Where's my light?
1:10:14 > 1:10:16Where's that?
1:10:16 > 1:10:20Ellie is a patient at Moorfields Hospital in London.
1:10:20 > 1:10:23She was born without irises.
1:10:23 > 1:10:27Her pupils are enormous for they cannot contract,
1:10:27 > 1:10:29and she can barely see.
1:10:29 > 1:10:32Where's it gone now? Where's it gone now?
1:10:32 > 1:10:35The disorder is called Aniridia
1:10:35 > 1:10:39and it's caused by a mutation that she inherited from her mother.
1:10:39 > 1:10:42Well done, oh, you get a lovely view of your eye.
1:10:42 > 1:10:43She's such a good girl.
1:10:43 > 1:10:45Good, well done.
1:10:45 > 1:10:49In 1992, geneticists identified the mutant gene.
1:10:50 > 1:10:53Now that one... Good, good, good, good.
1:10:53 > 1:10:57Located on chromosome 11, it's called PAX6.
1:10:57 > 1:11:00Well done, well done. You're such a good girl.
1:11:00 > 1:11:04PAX6 is a very special kind of gene.
1:11:04 > 1:11:09It's a molecular switch, a gene that turns other genes on and off,
1:11:09 > 1:11:12and this particular molecular switch
1:11:12 > 1:11:16is needed in the construction of the human eye.
1:11:16 > 1:11:21The really interesting bit, however, is what happens next.
1:11:27 > 1:11:31In 1994, geneticists were studying fruit flies,
1:11:31 > 1:11:35searching for the genes that make its different organs.
1:11:37 > 1:11:44They screened thousands of flies for mutations that cause abnormality.
1:11:47 > 1:11:49Some of the mutant flies were blind.
1:11:51 > 1:11:54They had a mutation called eyeless.
1:11:57 > 1:12:01Normal flies have large, red, compound eyes.
1:12:01 > 1:12:05Eyeless flies have none.
1:12:06 > 1:12:13Analysing the DNA of eyeless flies, they identified the mutant gene.
1:12:13 > 1:12:15It was PAX6 -
1:12:15 > 1:12:19the same gene, or at least its fly version,
1:12:19 > 1:12:22that causes aniridia in humans.
1:12:25 > 1:12:29I remember how amazed we all were when we heard about this result.
1:12:29 > 1:12:33Everyone just knew that human eyes and fly eyes
1:12:33 > 1:12:35had evolved independently.
1:12:35 > 1:12:40How could it be otherwise - they just looked so very different?
1:12:40 > 1:12:43But the discovery that they shared a gene
1:12:43 > 1:12:47told us that they shared an evolutionary history.
1:12:54 > 1:12:57Geneticists started looking into the eyes of other animals
1:12:58 > 1:13:04and wherever they looked, they found the same thing.
1:13:04 > 1:13:06The molecular circuit of which PAX6 is a part
1:13:06 > 1:13:08is universal.
1:13:10 > 1:13:16Eyes are so ubiquitous, so useful and so various in their design,
1:13:16 > 1:13:17that for most of the 20th century,
1:13:17 > 1:13:21zoologists had argued that they must have evolved many times.
1:13:23 > 1:13:25This, however, must be wrong.
1:13:26 > 1:13:29Eyes have evolved only once.
1:13:31 > 1:13:36All the eyes, belonging to all the animals on earth,
1:13:36 > 1:13:40can trace their origin to one very simple eye
1:13:40 > 1:13:44that belonged to one, doubtless, very simple creature that lived,
1:13:44 > 1:13:47perhaps, one billion years ago.
1:13:50 > 1:13:53Here is another mutant fruit fly.
1:13:55 > 1:13:57Its eyes are quite normal,
1:13:57 > 1:14:02but it has an extra pair of legs where its antennae should be.
1:14:05 > 1:14:08And here's a different mutant.
1:14:10 > 1:14:14Instead of the usual two wings, it has four.
1:14:14 > 1:14:16True, they're a little deformed,
1:14:16 > 1:14:19but they're definitely an extra set of wings.
1:14:21 > 1:14:25These flies have mutations in their Hox genes.
1:14:29 > 1:14:32Like PAX6, Hox genes are molecular switches
1:14:32 > 1:14:35that turn other genes on and off.
1:14:35 > 1:14:40They determine a fly's basic geometry, the number and position
1:14:40 > 1:14:43of its segments, limbs and wings.
1:14:44 > 1:14:48And like PAX6, Hox genes are universal.
1:14:48 > 1:14:52They can be found in all animals, including us.
1:14:55 > 1:14:59Animal bodies seem so very different from each other.
1:14:59 > 1:15:02And yet they are not.
1:15:02 > 1:15:09A fly has wings and segments, we have arms and vertebrae.
1:15:09 > 1:15:15A facile comparison, you may think, until you look at the embryo.
1:15:18 > 1:15:21This is a fruit fly embryo.
1:15:21 > 1:15:26And here, one by one, the Hox genes are being expressed.
1:15:26 > 1:15:30They ensure that every segment, head to tail,
1:15:30 > 1:15:32knows what structure to make.
1:15:32 > 1:15:36They work with the exquisite Boolean logic of a computer programme,
1:15:36 > 1:15:39directing cells to their fates.
1:15:42 > 1:15:45And this is a human embryo.
1:15:45 > 1:15:49Again the Hox genes are activated along the head-to-tail axis
1:15:49 > 1:15:53ensuring that our parts are arranged in the right order
1:15:53 > 1:15:55and end up in the right place.
1:15:57 > 1:16:00The circuitry is more complex than a fruit fly's,
1:16:00 > 1:16:02but the logic is the same.
1:16:07 > 1:16:10The discovery that many important genes
1:16:10 > 1:16:14are shared across the animal kingdom was thrilling.
1:16:16 > 1:16:18A new science was born -
1:16:18 > 1:16:21Evolutionary Developmental Biology,
1:16:21 > 1:16:24Evo-Devo, for short.
1:16:29 > 1:16:30Like Haeckel,
1:16:30 > 1:16:35we would delve into the embryo to unravel our evolutionary past.
1:16:36 > 1:16:41But now, instead of working out what our ancestors looked like,
1:16:41 > 1:16:44we'd work out the genetic programmes that made them.
1:16:47 > 1:16:51Evo-Devo would explain how organic structures can be, at once,
1:16:51 > 1:16:58so similar and conservative and, yet, so promiscuously different.
1:17:01 > 1:17:04There is a paradoxical quality to genes.
1:17:04 > 1:17:07Look at any animal and you can see many of the same genes
1:17:07 > 1:17:09doing much the same thing.
1:17:09 > 1:17:15And yet, look at them closely, look at them working in the embryo,
1:17:15 > 1:17:19and you can see the origins of all diversity.
1:17:22 > 1:17:25This is the embryo of a tetrapod.
1:17:25 > 1:17:31As it grows, Hox genes switch on and off in a kaleidoscopic pattern.
1:17:31 > 1:17:34Molecular signals sweep across the limb buds, telling each cell
1:17:34 > 1:17:37what it is and what it must become.
1:17:38 > 1:17:42The cells of the limb condense into the outline of bones
1:17:42 > 1:17:46and so a mole grows a paw.
1:17:48 > 1:17:51Another embryo and the same thing happens...
1:17:51 > 1:17:53..at least to begin with.
1:17:53 > 1:17:56But there's far more bone growth signal in this embryo.
1:17:56 > 1:18:00The bones of the limb grow longer and the cells between the fingers
1:18:00 > 1:18:03do not die, they become webbing.
1:18:03 > 1:18:06And so a bat grows a wing.
1:18:08 > 1:18:12Two embryos that start their lives in much the way, but with just
1:18:12 > 1:18:18a subtle shift in gene activity, become two very different creatures.
1:18:21 > 1:18:28I could tell many such stories about the evolution of beaks and feathers,
1:18:28 > 1:18:31scales and spots and snouts.
1:18:31 > 1:18:33But is it enough?
1:18:33 > 1:18:37Surely the point of science is not merely to tell stories,
1:18:37 > 1:18:42but rather to reveal the laws of nature.
1:18:44 > 1:18:48Haeckel believed that the symmetries shown by living things
1:18:48 > 1:18:50were caused by simple laws,
1:18:50 > 1:18:54not very different from those that explained
1:18:54 > 1:18:56the symmetries of crystals.
1:18:58 > 1:19:02Living things are, of course, much more complex than crystals,
1:19:02 > 1:19:06yet they do have a geometry.
1:19:06 > 1:19:12An internal one composed of vast and intricate networks
1:19:12 > 1:19:18of genes, proteins and molecules, all working together to give life.
1:19:20 > 1:19:24Perhaps then, the war of nature is not simply
1:19:24 > 1:19:28a struggle among individuals, or even genes,
1:19:28 > 1:19:33but a struggle among different ways of organising life.
1:19:33 > 1:19:36A struggle among systems.
1:19:38 > 1:19:42In the short run, success depends on being robust.
1:19:42 > 1:19:46On being able to weather the vicissitudes of existence
1:19:46 > 1:19:47and then reproduce.
1:19:48 > 1:19:52In the long run, however, in the evolutionary long run,
1:19:52 > 1:19:56success needs something else, it needs flexibility.
1:19:56 > 1:20:02On being able to respond to a mutable and contingent world.
1:20:02 > 1:20:04Let me put it another way -
1:20:04 > 1:20:07in the short run, creatures evolve.
1:20:07 > 1:20:12In the long run, they evolve evolvability.
1:20:14 > 1:20:17About a billion years ago, a creature,
1:20:17 > 1:20:21something like Trichoplax, crawled out of the pre-Cambrian ooze.
1:20:24 > 1:20:28Superficially, it may have been unremarkable.
1:20:28 > 1:20:32But it seems that there was something new and rather special
1:20:32 > 1:20:34about its genetic network.
1:20:35 > 1:20:39It had a structure that was robust enough to be strong,
1:20:39 > 1:20:42yet flexible enough to change.
1:20:45 > 1:20:49Over time, this way of organising life was copied
1:20:49 > 1:20:51and modified countless times.
1:20:56 > 1:20:58Parts could added, even rearranged,
1:20:58 > 1:21:03and yet the system as a whole would continue to work.
1:21:03 > 1:21:08It was an engine of innovation
1:21:08 > 1:21:10and it went on to conquer the world.
1:21:40 > 1:21:43The theory of evolution by natural selection
1:21:43 > 1:21:47is one of the most beautiful products of science.
1:21:47 > 1:21:50It provided an answer, the only rational one we have,
1:21:50 > 1:21:55as to why living things bear the hallmarks of design.
1:21:58 > 1:22:05It is so beguilingly simple, so powerful and so manifestly true,
1:22:05 > 1:22:10that it is easy to forget just how inadequate it is.
1:22:10 > 1:22:14There are so many questions that we can't answer...
1:22:14 > 1:22:21Why are there 600 species of cichlid in this lake and not 60 or 6,000?
1:22:21 > 1:22:25Why are there no anglerfish cichlids, or flying fish cichlids?
1:22:25 > 1:22:33Or, cichlid sharks? What, in short, are the limits to evolution?
1:22:35 > 1:22:39The theory is inadequate because it is not predictive.
1:22:39 > 1:22:44It explains what has evolved, but not what will.
1:22:46 > 1:22:50There are simply too many possible streams
1:22:50 > 1:22:53and there's no saying which one will be chosen.
1:22:57 > 1:23:01We can only follow the journey and reconstruct the route
1:23:01 > 1:23:06once it is done. But must it always be so?
1:23:09 > 1:23:12Compare Darwin's theory of evolution
1:23:12 > 1:23:15to Newton's account of the planetary motions.
1:23:19 > 1:23:24Newton, after all, gave us celestial mechanics -
1:23:24 > 1:23:27a mathematical theory that enables us
1:23:27 > 1:23:30to make predictions far into the future.
1:23:32 > 1:23:35Darwin did nothing of the kind,
1:23:35 > 1:23:39but can we, perhaps, make a predictive theory of evolution?
1:23:39 > 1:23:41Can we compute
1:23:41 > 1:23:43the future of life?
1:23:45 > 1:23:48Many would say no.
1:23:48 > 1:23:53The evolution of life, they say, is ruled by chaos and contingency.
1:23:57 > 1:24:00Perhaps evolution is like the weather.
1:24:03 > 1:24:06Given enough data and computing power,
1:24:06 > 1:24:09forecasters can tell us whether or not it will rain at the weekend.
1:24:11 > 1:24:14But ask them about a month of Sundays and their predictions
1:24:14 > 1:24:20fall apart - defeated by imperfect data
1:24:20 > 1:24:23and the chaos concealed in their own equations.
1:24:37 > 1:24:39And yet, I still think
1:24:39 > 1:24:42a predictive theory of evolution might be possible.
1:24:42 > 1:24:48And the reason I do is because of the fish in Lake Malawi.
1:24:49 > 1:24:54Two million years of evolution have produced 600 species of cichlid here
1:24:54 > 1:24:57and an astonishing diversity of form.
1:24:59 > 1:25:02But what's even more remarkable
1:25:02 > 1:25:06is that this great evolutionary experiment is not unique.
1:25:13 > 1:25:17Follow the African rift north-west for just a few hundred miles
1:25:17 > 1:25:19and you come to Lake Tanganyika.
1:25:22 > 1:25:26There, too, a cichlid arrived a few million years ago
1:25:26 > 1:25:29and there, too, it multiplied and evolved
1:25:29 > 1:25:32into hundreds of different species.
1:25:34 > 1:25:37The fish in both lakes are only remotely related,
1:25:37 > 1:25:44but they have similar colours, fins, teeth, diets, habits and habitats.
1:25:47 > 1:25:51You can hardly tell some of them apart without DNA.
1:25:55 > 1:26:00In other words, the tape of cichlid evolution has been run twice
1:26:00 > 1:26:05and both times the outcome has been much the same.
1:26:05 > 1:26:09It's this repeatability that makes me think
1:26:09 > 1:26:12that much of evolution is indeed predictable.
1:26:16 > 1:26:19Perhaps then evolution does not so much resemble the weather
1:26:19 > 1:26:21as it does our climate.
1:26:23 > 1:26:28At the grandest scales of space and time, the atmosphere is not chaotic.
1:26:28 > 1:26:32The physics of our planet imposes order and, thus,
1:26:32 > 1:26:35predictability upon it.
1:26:37 > 1:26:40So, although we can scarcely tell what the weather will be
1:26:40 > 1:26:43three weeks from now, we can predict, at least
1:26:43 > 1:26:48probabilistically, what the climate will be three centuries hence.
1:26:52 > 1:26:56I think evolution is like that.
1:26:56 > 1:26:59As we explore the internal geometry of living things,
1:26:59 > 1:27:03I think we'll discover that there are deterministic laws
1:27:03 > 1:27:07that impose order upon the capricious designs
1:27:07 > 1:27:12of natural selection and that limit the paths of evolution.
1:27:14 > 1:27:18And when we discover those laws, we'll be able to construct
1:27:18 > 1:27:21a predictive theory of evolution.
1:27:22 > 1:27:26We'll be able to complete Darwin's great project.
1:27:27 > 1:27:31We'll explain life itself.
1:27:36 > 1:27:41"There is grandeur in this view of life, with its several powers
1:27:41 > 1:27:46"having been originally breathed into a few forms, or into one.
1:27:46 > 1:27:51"From so simple a beginning, endless forms most beautiful
1:27:51 > 1:27:57"and most wonderful have and are being evolved."
1:27:57 > 1:28:02That is how The Origin ends.
1:28:02 > 1:28:05And it is how our science begins.
1:28:13 > 1:28:16Subtitles by Red Bee Media Ltd
1:28:16 > 1:28:19E-mail subtitling@bbc.co.uk