The Greatest Show On Earth, The Evidence For Evolution Part 2

Acanthostega

Panderichthys So, we are left with a gap between Panderichthys Panderichthys, the amphibian-like fish, and Acanthostega Acanthostega, the fish-like amphibian. Where is the 'missing link' between them? A team of scientists from the University of Pennsylvania, including Neil Shubin and Edward Daeschler, set out to find it. Shubin made their quest the basis for a delightful series of reflections on human evolution in his book Your Inner Fish Your Inner Fish. They deliberately thought about where might be the best place to look, and carefully chose a rocky area of exactly the right late Devonian age in the Canadian Arctic. There they went and struck zoological gold. Tiktaalik Tiktaalik! A name never to be forgotten. It comes from an Inuit word for a large freshwater fish. As for the specific name, roseae roseae, let me tell a cautionary tale against myself. When I first heard the name, and saw photographs like the one reproduced on colour page 10 page 10, my mind immediately leapt to the Devonian, the 'Old Red Sandstone', the colour of the eponymous county of Devon, the colour of Petra ('A rose-red city, half as old as time'). Alas, I was quite wrong. The photograph exaggerates the rosy glow. The name was chosen in honour of a benefactor who helped finance the expedition to the Arctic Devonian. I was privileged to be shown Tiktaalik Tiktaalik roseae roseae by Dr Daeschler when I had lunch with him in Philadelphia, shortly after its discovery, and the lifelong zoologist in me or perhaps my inner fish was moved to speechlessness. Through rose-tinted spectacles I imagined I was gazing upon the face of my direct ancestor. Unrealistic as that was, this not-so-rose-red fossil was probably as close as I was going to get to meeting a real dead ancestor half as old as time. by Dr Daeschler when I had lunch with him in Philadelphia, shortly after its discovery, and the lifelong zoologist in me or perhaps my inner fish was moved to speechlessness. Through rose-tinted spectacles I imagined I was gazing upon the face of my direct ancestor. Unrealistic as that was, this not-so-rose-red fossil was probably as close as I was going to get to meeting a real dead ancestor half as old as time.If you were to meet a real live Tiktaalik Tiktaalik, snout to snout, you might start back as if threatened by a crocodile, for that is what its face resembled. A crocodile's head on a salamander's trunk, attached to a fish's rear end and tail. Unlike any fish, Tiktaalik Tiktaalik had a neck. It could turn its head. In almost every particular, had a neck. It could turn its head. In almost every particular, Tiktaalik Tiktaalik is the perfect missing link perfect, because it almost exactly splits the difference between fish and amphibian, and perfect because it is missing no longer. We have the fossil. You can see it, touch it, try to appreciate the age of it and fail. is the perfect missing link perfect, because it almost exactly splits the difference between fish and amphibian, and perfect because it is missing no longer. We have the fossil. You can see it, touch it, try to appreciate the age of it and fail.

I MUST GO DOWN TO THE SEA AGAIN * *

The move from water to land launched a major redesign of every aspect of life, from breathing to reproduction: it was a great trek through biological s.p.a.ce. Nevertheless, with what seems almost wanton perversity, a good number of thoroughgoing land animals later turned around, abandoned their hard-earned terrestrial retooling, and trooped back into the water again. Seals and sea lions have only gone part-way back. They show us what the intermediates might have been like, on the way to extreme cases such as whales and dugongs. Whales (including the small whales we call dolphins), and dugongs with their close cousins the manatees, ceased to be land creatures altogether and reverted to the full marine habits of their remote ancestors. They don't even come ash.o.r.e to breed. They do, however, still breathe air, having never developed anything equivalent to the gills of their earlier marine progenitors. Other animals that have returned from land to water, at least some of the time, are pond snails, water spiders, water beetles, crocodiles, otters, sea snakes, water shrews, Galapagos flightless cormorants, Galapagos marine iguanas, yapoks (aquatic marsupials from South America), platypuses, penguins and turtles.Whales were long an enigma, but recently our knowledge of whale evolution has become rather rich. Molecular genetic evidence (see Chapter 10 for the nature of this kind of evidence) shows that the closest living cousins of whales are hippos, then pigs, then ruminants. Even more surprisingly, the molecular evidence shows that hippos are more closely related to whales than they are to the cloven-hoofed animals (such as pigs and ruminants) which look much more like them. This is another example of the mismatch that can sometimes arise between closeness of cousins.h.i.+p and degree of physical resemblance. We noted it above in connection with fish that are closer cousins to us than they are to other fish. In that case, the anomaly arose because our lineage left the water for the land, and consequently surged away in evolution, leaving our close fish cousins, the lungfish and coelacanths, resembling our more distant fish cousins because they all stayed in the water. Now we meet the same phenomenon again, but in reverse. Hippos stayed, at least partly, on land, and so still resemble their more distant land-dwelling cousins, the ruminants, while their closer cousins, the whales, took off into the sea and changed so drastically that their affinities with hippos escaped all biologists except molecular geneticists. As when their remote fishy ancestors originally went in the other direction, it was a bit like taking off into s.p.a.ce, or at least like launching a balloon, as the ancestors of whales floated free of the constraining burden of gravity and severed their moorings to dry land.At the same time, the once rather scanty fossil record of whale evolution has been convincingly filled out, mostly by a new trove from Pakistan. However, the story of fossil whales has been so well treated in other recent books, for example Donald Prothero's Evolution: What the Fossils Say and Why it Matters Evolution: What the Fossils Say and Why it Matters, and, more recently, Jerry Coyne's Why Evolution is True Why Evolution is True, that I have decided not to cover the same details here. Instead, I have confined myself to one diagram (below), taken from Prothero's book, showing a sequence of fossils ordered in time. Note the careful way the picture is drawn. It is tempting and older books used to do this to draw sequences of fossils with arrows from older to younger ones. But n.o.body can say, for example, that Ambulocetus Ambulocetus was descended from was descended from Pakicetus Pakicetus. Or that Basilosaurus Basilosaurus was descended from was descended from Rodhocetus Rodhocetus. Instead, the diagram follows the more cautious policy of suggesting that, for example, whales are descended from a contemporary cousin of Ambulocetus Ambulocetus which was probably rather like which was probably rather like Ambulocetus Ambulocetus (and might even have been (and might even have been Ambulocetus Ambulocetus). The fossils shown are representative of various stages of whale evolution. The gradual disappearance of the hind limbs, the transformation of the front limbs from walking legs to swimming fins, and the flattening of the tail into flukes, are among the changes that emerged in elegant cascade.

Fossil whales Figure 14.16. Evolution of whales from land creatures, showing the numerous transitional fossils now doc.u.mented from the Eocene beds of Africa and Pakistan. (Drawing by Carl Buell)That's all I'm going to say about the fossil history of whales, because it has been so well treated in the books I mentioned. The other, less numerous and diverse but just as thoroughly aquatic group of marine mammals, the sirenians dugongs and manatees are not so well doc.u.mented in the fossil record, but one outstandingly beautiful 'missing link' has recently been discovered. Roughly contemporary with Ambulocetus Ambulocetus, the Eocene 'walking whale', is Pezosiren Pezosiren, the 'walking manatee' fossil from Jamaica. It looks pretty much like a manatee or dugong, except that it has proper walking legs both front and rear, where they have flippers in the front and no limbs at all in the rear. The picture opposite shows a modern dugong skeleton above, Pezosiren Pezosiren below. below.Just as whales are related to hippos, so sirenians are related to elephants, as a great deal of evidence, including the all-important molecular evidence, attests. Pezosiren Pezosiren, however, probably lived like a hippo, spending much of its time in water and using its legs to walk on the bottom as well as swim. The skull is unmistakably sirenian. Pezosiren Pezosiren may or may not be the actual ancestor of modern manatees and dugongs, but it is certainly excellent casting for the role. may or may not be the actual ancestor of modern manatees and dugongs, but it is certainly excellent casting for the role.This book was about to go to the printer when exciting news came in, from the journal Nature Nature, of a new fossil from the Canadian Arctic, plugging a gap in the ancestry of modern seals, sea lions and walruses (collectively 'pinnipeds'). A single skeleton, about 65 per cent complete, Puijila darwini Puijila darwini dates from the early Miocene epoch (about 20 million years ago). That's recent enough that the map of the world was almost the same as today. So this early seal/sea lion (they had not diverged yet) was an Arctic animal, a denizen of cold water. Evidence suggests that it lived and fished in fresh water (like most otters except the famous sea otters of California), rather than in the sea (like most modern seals except the famous Lake Baikal seal). dates from the early Miocene epoch (about 20 million years ago). That's recent enough that the map of the world was almost the same as today. So this early seal/sea lion (they had not diverged yet) was an Arctic animal, a denizen of cold water. Evidence suggests that it lived and fished in fresh water (like most otters except the famous sea otters of California), rather than in the sea (like most modern seals except the famous Lake Baikal seal). Puijila Puijila did not have flippers, but webbed feet. It probably ran like a dog on land (very unlike a modern pinniped) but spent much of its time in water, where it swam like a dog, unlike either of the two styles adopted respectively by modern seals and sea lions. did not have flippers, but webbed feet. It probably ran like a dog on land (very unlike a modern pinniped) but spent much of its time in water, where it swam like a dog, unlike either of the two styles adopted respectively by modern seals and sea lions. Puijila Puijila neatly straddles the gap between land and water in the ancestry of pinnipeds. It is yet another delightful addition to our growing list of 'links' that are no longer missing. neatly straddles the gap between land and water in the ancestry of pinnipeds. It is yet another delightful addition to our growing list of 'links' that are no longer missing.

Modern dugong

Pezosiren ancient dugong ancient dugong I now want to turn to another group of animals that returned from the land to the water: a particularly intriguing example because some of them later reversed the process and returned to the land a second time! Sea turtles are, in one important respect, less fully given back to the water than whales or dugongs, for they still lay their eggs on beaches. Like all vertebrate returners to the water, turtles haven't given up breathing air, but in this department some of them go one better than whales. These turtles extract additional oxygen from the water through a pair of chambers at their rear end that are richly supplied with blood vessels. One Australian river turtle, indeed, gets the majority of its oxygen by breathing (as an Australian would not hesitate to say) through its a.r.s.e.Before going any further, I can't escape a tiresome point of terminology, and a regrettable vindication of George Bernard Shaw's observation that 'England and America are two countries divided by a common language.' In Britain, turtles live in the sea, tortoises live on land and terrapins live in fresh or brackish water. In America all these animals are 'turtles', whether they live on land or in water. 'Land turtle' sounds odd to me, but not to an American, for whom tortoises are the subset of turtles that live on land. Some Americans use 'tortoise' in a strict taxonomic sense to refer to the Testudinidae, which is the scientific name for modern land tortoises. In Britain, we'd be inclined to call any land-dwelling chelonian a tortoise, whether it is a member of the Testudinidae or not (as we shall see, there are fossil 'tortoises' that lived on land but are not members of the Testudinidae). In what follows, I'll try to avoid confusion, making allowance for readers in Britain and America (and Australia, where the usage is different again), but it's hard. The terminology is a mess, to put it mildly. Zoologists use 'chelonians' for all these animals, turtles, tortoises and terrapins, whichever version of English we speak.The most instantly noticeable feature of chelonians is their sh.e.l.l. How did it evolve, and what did the intermediates look like? Where are the missing links? What (a creationist zealot might ask) is the use of half a sh.e.l.l? Well, amazingly, a new fossil has just been described, which eloquently answers that question. It made its debut in the journal Nature Nature in the nick of time before I had to hand this book over to the publishers. It was an aquatic turtle, found in late Tria.s.sic sediments in China, and its age is estimated at 220 million years. Its name is in the nick of time before I had to hand this book over to the publishers. It was an aquatic turtle, found in late Tria.s.sic sediments in China, and its age is estimated at 220 million years. Its name is Odontochelys semitestacea Odontochelys semitestacea, from which you may deduce that, unlike a modern turtle or tortoise, it had teeth, and it did indeed have half a sh.e.l.l. It also had a much longer tail than a modern turtle or tortoise. All three of these features mark it out as prime 'missing link' material. The belly was covered by a sh.e.l.l, the so-called plastron, in pretty much the same way as that of a modern sea turtle. But it almost completely lacked the dorsal portion of the sh.e.l.l, known as the carapace. Its back was presumably soft, like a lizard's, although there were some hard, bony bits along the middle above the backbone, as in a crocodile, and the ribs were flattened, as though 'trying' to form the evolutionary beginnings of a carapace.And here we have an interesting controversy. The authors of the paper that introduced Odontochelys Odontochelys to the world, Li, Wu, Rieppel, w.a.n.g and Zhao (for brevity, I'll call them the Chinese authors, although Rieppel is not Chinese), think that their animal was indeed halfway towards acquiring a sh.e.l.l. Others dispute to the world, Li, Wu, Rieppel, w.a.n.g and Zhao (for brevity, I'll call them the Chinese authors, although Rieppel is not Chinese), think that their animal was indeed halfway towards acquiring a sh.e.l.l. Others dispute Odontochelys Odontochelys's claim to demonstrate that the sh.e.l.l evolved in water. Nature Nature has the admirable custom of commissioning experts other than the authors to write a commentary on the week's more interesting articles, which they publish in a section called 'News and Views'. The 'News and Views' commentary on the has the admirable custom of commissioning experts other than the authors to write a commentary on the week's more interesting articles, which they publish in a section called 'News and Views'. The 'News and Views' commentary on the Odontochelys Odontochelys paper is by two Canadian biologists, Robert Reisz and Jason Head, and they offer an alternative interpretation. Maybe the whole sh.e.l.l had already evolved on land, before paper is by two Canadian biologists, Robert Reisz and Jason Head, and they offer an alternative interpretation. Maybe the whole sh.e.l.l had already evolved on land, before Odontochelys Odontochelys's ancestors went back to the water. And maybe Odontochelys Odontochelys lost its carapace after returning to the water. Reisz and Head point out that some of today's sea turtles, for example the giant leatherback turtle, have lost or greatly reduced the carapace, so their theory is quite plausible. lost its carapace after returning to the water. Reisz and Head point out that some of today's sea turtles, for example the giant leatherback turtle, have lost or greatly reduced the carapace, so their theory is quite plausible.I need to digress for a brief aside on the question, 'What is the use of half a sh.e.l.l?' In particular, why would Odontochelys Odontochelys be armoured below but not above? Perhaps because danger threatened from below, which would suggest that these creatures spent a lot of their time swimming near the surface and of course they had to come to the surface to breathe, anyway. Sharks today often attack from below, sharks would have been a menacingly important part of the world of be armoured below but not above? Perhaps because danger threatened from below, which would suggest that these creatures spent a lot of their time swimming near the surface and of course they had to come to the surface to breathe, anyway. Sharks today often attack from below, sharks would have been a menacingly important part of the world of Odontochelys Odontochelys, and there's no reason to suppose that their hunting habits were different in those times. As a parallel example, one of the most surprising achievements of evolution, the extra pair of eyes in the fish Bathylychnops Bathylychnops (see over), is probably aimed at detecting predatory attacks from below. The main eye looks outwards, as in any ordinary fish. But each of the two main eyes has an extra little eye, complete with lens and retina, tucked into its lower side. If (see over), is probably aimed at detecting predatory attacks from below. The main eye looks outwards, as in any ordinary fish. But each of the two main eyes has an extra little eye, complete with lens and retina, tucked into its lower side. If Bathylychnops Bathylychnops can go to the trouble (you know what I mean, don't be pedantic) of growing a whole extra pair of eyes, presumably to look out for attacks coming from below, it seems quite plausible that can go to the trouble (you know what I mean, don't be pedantic) of growing a whole extra pair of eyes, presumably to look out for attacks coming from below, it seems quite plausible that Odontochelys Odontochelys might grow armour aimed at fending off attacks from the same direction. The plastron makes sense. And if you want to say, yes, but why not have a carapace on top as well, just to be extra safe, the reply is easy. Sh.e.l.ls are heavy and c.u.mbersome, they are costly to grow and costly to carry around. There are always trade-offs in evolution. For land tortoises, the trade-off ends up favouring stout, heavy armour above as well as below. For many sea turtles, the tradeoff favours a strong plastron underneath but lighter armour on top. And it is a plausible suggestion that might grow armour aimed at fending off attacks from the same direction. The plastron makes sense. And if you want to say, yes, but why not have a carapace on top as well, just to be extra safe, the reply is easy. Sh.e.l.ls are heavy and c.u.mbersome, they are costly to grow and costly to carry around. There are always trade-offs in evolution. For land tortoises, the trade-off ends up favouring stout, heavy armour above as well as below. For many sea turtles, the tradeoff favours a strong plastron underneath but lighter armour on top. And it is a plausible suggestion that Odontochelys Odontochelys just carried that trend a bit further. just carried that trend a bit further.

Bathylychnops' extra eye extra eye If, on the other hand, the Chinese authors are right that Odontochelys Odontochelys was on its way to evolving a full sh.e.l.l, and that the sh.e.l.l evolved in water, it would seem to follow that modern land tortoises, which have well-developed sh.e.l.ls, are descended from water turtles. This, as we shall see, is probably true. But it is remarkable, because it means that today's land tortoises represent a was on its way to evolving a full sh.e.l.l, and that the sh.e.l.l evolved in water, it would seem to follow that modern land tortoises, which have well-developed sh.e.l.ls, are descended from water turtles. This, as we shall see, is probably true. But it is remarkable, because it means that today's land tortoises represent a second second migration from water to land. n.o.body has ever claimed that any whales, or dugongs, migration from water to land. n.o.body has ever claimed that any whales, or dugongs, returned returned to the land after invading the water. The alternative story for land tortoises is that they were on land all along and independently evolved the sh.e.l.l, in parallel to their aquatic cousins. This is by no means impossible; but, as it happens, we have good reason to believe that sea turtles did indeed return to the land for a second go at becoming land tortoises. to the land after invading the water. The alternative story for land tortoises is that they were on land all along and independently evolved the sh.e.l.l, in parallel to their aquatic cousins. This is by no means impossible; but, as it happens, we have good reason to believe that sea turtles did indeed return to the land for a second go at becoming land tortoises.

Family tree of tortoises and turtles KEY bold = land = land normal = aquaticIf you draw out the family tree of all modern turtles and tortoises, based on molecular and other comparisons, nearly all the branches are aquatic (normal type). Land tortoises are represented by bold type, and you can see that today's land tortoises const.i.tute a single branch, the Testudinidae, deeply nested within rich branchings of otherwise aquatic chelonians. All their close cousins are aquatic. Modern land tortoises are a single twig on the bush of otherwise aquatic turtles. Their aquatic ancestors turned turtle and trooped back on to the land. This fact is compatible with the hypothesis that the sh.e.l.l evolved in water, in a creature like Odontochelys Odontochelys. But now we have another difficulty. If you look at the family tree, you'll notice that, in addition to the Testudinidae (all modern land tortoises) there are two fossil genera of fully sh.e.l.led animals called Proganochelys Proganochelys* and and Palaeochersis Palaeochersis. These are drawn as land-dwellers, for reasons we shall come to in the next paragraph. They lie right outside the branches representing the water turtles. It would seem that these two genera are anciently terrestrial.Before Odontochelys Odontochelys was discovered, these two fossils were the oldest known chelonians. Like was discovered, these two fossils were the oldest known chelonians. Like Odontochelys Odontochelys they lived in the late Tria.s.sic, but about 15 million years later than they lived in the late Tria.s.sic, but about 15 million years later than Odontochelys Odontochelys. Some authorities have reconstructed them as living in fresh water, but recent evidence does indeed place them on land, as indicated by bold type on the diagram. You might wonder how we tell whether fossil animals, especially if only fragments are found, lived on land or in water. Sometimes it's pretty obvious. Ichthyosaurs were reptilian contemporaries of the dinosaurs, with fins and streamlined bodies. The fossils look like dolphins and they surely lived like dolphins, in the water. With turtles and tortoises it is a little less obvious. As you might expect, the biggest giveaway is their limbs. Paddles really are rather different from walking legs. Walter Joyce and Jacques Gauthier, of Yale University, took this common-sense intuition and provided the numbers to support it. They took three key measurements in the arm and hand bones of seventy-one species of living chelonians. I'll resist the temptation to explain their elegant calculations, but their conclusion was clear. These animals had had walking legs, not paddles. In British English, they were 'tortoises', not 'turtles'. They lived on land. They were only distant cousins, however, of modern land tortoises.Now we seem to have a problem. If, as the authors of the paper describing Odontochelys Odontochelys believe, their half-sh.e.l.led fossil shows that the sh.e.l.l evolved in water, how do we explain two genera of fully sh.e.l.led 'tortoises' on land, 15 million years later? Until the discovery of believe, their half-sh.e.l.led fossil shows that the sh.e.l.l evolved in water, how do we explain two genera of fully sh.e.l.led 'tortoises' on land, 15 million years later? Until the discovery of Odontochelys Odontochelys, I would not have hesitated to say that Proganochelys Proganochelys and and Palaeochersis Palaeochersis were representative of the land-dwelling ancestral type were representative of the land-dwelling ancestral type before before the return to water. The sh.e.l.l evolved on land. Some sh.e.l.led tortoises returned to the sea, as seals, whales and dugongs were later to do. Others stayed on land, but went extinct. And then some sea turtles returned to the land, to give rise to all modern land tortoises. That's what I would have said indeed what I did say in the earlier draft of this chapter that preceded the announcement of the return to water. The sh.e.l.l evolved on land. Some sh.e.l.led tortoises returned to the sea, as seals, whales and dugongs were later to do. Others stayed on land, but went extinct. And then some sea turtles returned to the land, to give rise to all modern land tortoises. That's what I would have said indeed what I did say in the earlier draft of this chapter that preceded the announcement of Odontochelys Odontochelys. But Odontochelys Odontochelys throws speculation back into the melting pot. We now have three possibilities, all equally intriguing. throws speculation back into the melting pot. We now have three possibilities, all equally intriguing.1 Proganochelys Proganochelys and and Palaeochersis Palaeochersis might be survivors of the land-dwelling animals that had earlier sent some representatives to sea, including the ancestors of might be survivors of the land-dwelling animals that had earlier sent some representatives to sea, including the ancestors of Odontochelys Odontochelys. This hypothesis would suggest that the sh.e.l.l evolved on land early, and Odontochelys Odontochelys lost the carapace in the water, retaining the ventral plastron. lost the carapace in the water, retaining the ventral plastron.2 The sh.e.l.l might have evolved in water, as the Chinese authors suggest, with the plastron over the belly evolving first, and the carapace over the back evolving later. In this case, what do we make of Proganochelys Proganochelys and and Palaeochersis Palaeochersis, who lived on land after Odontochelys after Odontochelys lived, with its half sh.e.l.l, in water? lived, with its half sh.e.l.l, in water? Proganochelys Proganochelys and and Palaeochersis Palaeochersis might have evolved the sh.e.l.l independently. But there is another possibility: might have evolved the sh.e.l.l independently. But there is another possibility:3 Proganochelys Proganochelys and and Palaeochersis Palaeochersis might represent an earlier return from the water to the land. Isn't that a startlingly exciting thought? might represent an earlier return from the water to the land. Isn't that a startlingly exciting thought?We are already pretty confident of the remarkable fact that the turtles accomplished an evolutionary doubling back to the land: an early marque of land 'tortoises' went back to the watery environment of their even earlier fish ancestors, became sea turtles, then returned to the land yet again, as a new incarnation of land tortoises, the Testudinidae. That we know, or are nearly certain of. But now we are facing up to the additional suggestion that this doubling back happened twice! twice! Not just to sp.a.w.n the modern tortoises, but much longer ago, to give rise to Not just to sp.a.w.n the modern tortoises, but much longer ago, to give rise to Proganochelys Proganochelys and and Palaeochersis Palaeochersis in the Tria.s.sic. in the Tria.s.sic.In another book I described DNA as 'the Genetic Book of the Dead'. Because of the way natural selection works, there is a sense in which the DNA of an animal is a textual description of the worlds in which its ancestors were naturally selected. For a fish, the genetic book of the dead describes ancestral seas. For us and most mammals, the early chapters of the book are all set in the sea and the later ones all out on land. For whales, dugongs, marine iguanas, penguins, seals, sea lions and turtles, there is a third section of the book which recounts their epic return to the proving grounds of their remote past, the sea. But for the land tortoises, perhaps twice independently on two widely separated occasions, there is yet a fourth section of the book devoted to a final or is it? re-emergence, yet again to the land. Can there be another animal for which the genetic book of the dead is such a palimpsest of multiple evolutionary U-turns? As a parting shot, I cannot help wondering about those freshwater and brackish water forms ('terrapins'), which are close cousins of the land tortoises. Did their ancestors move directly from the sea into brackish and then fresh water? Do they represent an intermediate stage on the way from the sea back to the land? Or is it possible that they const.i.tute yet another doubling-back to the water from ancestors that were modern land tortoises? Have the chelonians been shuttling back and forth in evolutionary time between water and land? Could the palimpsest be even more densely over-written than I have so far suggested?POSTSCRIPTOn 19 May 2009, as I was correcting the proofs of this book, a 'missing link' between lemur-like and monkey-like primates was announced in the online scientific journal PLOS One PLOS One. Named Darwinius masillae Darwinius masillae, it lived 47 million years ago in rain forest in what is now Germany. It is claimed by the authors to be the most complete fossil primate ever found: not just bones but skin, hair, some internal organs and its last meal. Beautiful as Darwinius masillae Darwinius masillae undoubtedly is (see colour undoubtedly is (see colour page 9 page 9), it comes trailing clouds of hype that obscure clear thinking. According to Sky News Sky News it is 'the eighth wonder of the world' which 'finally confirms Charles Darwin's theory of evolution'. Goodness me! The more-or-less nonsensical mystique of the 'missing link' seems to have lost none of its power. it is 'the eighth wonder of the world' which 'finally confirms Charles Darwin's theory of evolution'. Goodness me! The more-or-less nonsensical mystique of the 'missing link' seems to have lost none of its power.

* Majority opinion suspects the amateur palaeontologist Charles Dawson, but Stephen Jay Gould intriguingly floated the alternative theory that it might have been Pierre Teilhard de Chardin. You may recognize Teilhard's name as the Jesuit theologian whose later book, Majority opinion suspects the amateur palaeontologist Charles Dawson, but Stephen Jay Gould intriguingly floated the alternative theory that it might have been Pierre Teilhard de Chardin. You may recognize Teilhard's name as the Jesuit theologian whose later book, The Phenomenon of Man The Phenomenon of Man, was to receive the greatest negative book review of all time, from the matchless Peter Medawar (reprinted in The Art of the Soluble The Art of the Soluble and and Pluto's Republic Pluto's Republic).

* I'm using 'infinite' in the common, often abused, rhetorical sense of very very large. The actual number is the number of pairwise combinations of every species with every other and that's as near infinite as makes no practical difference! I'm using 'infinite' in the common, often abused, rhetorical sense of very very large. The actual number is the number of pairwise combinations of every species with every other and that's as near infinite as makes no practical difference!

* 'Well-educated' reminds me of Peter Medawar's wickedly astute observation that 'the spread of secondary and latterly of tertiary education has created a large population of people, often with well-developed literary and scholarly tastes, who have been educated far beyond their capacity to undertake a.n.a.lytical thought'. Isn't that priceless? It is the kind of writing that makes me want to rush out into the street to share with somebody anybody because it is too good to keep to oneself. 'Well-educated' reminds me of Peter Medawar's wickedly astute observation that 'the spread of secondary and latterly of tertiary education has created a large population of people, often with well-developed literary and scholarly tastes, who have been educated far beyond their capacity to undertake a.n.a.lytical thought'. Isn't that priceless? It is the kind of writing that makes me want to rush out into the street to share with somebody anybody because it is too good to keep to oneself.

* From the term 'clade', meaning a group of organisms believed to comprise all the evolutionary descendants of a common ancestor. From the term 'clade', meaning a group of organisms believed to comprise all the evolutionary descendants of a common ancestor. At least according to a consensus of zoologists, and I shall continue to use the birds, for the sake of argument, as an example of a good cla.s.s. Recent fossil research is showing up a number of feathered dinosaurs, and it is open to somebody to claim that some of the modern animals we call birds are descended from a different group of feathered dinosaurs than others. If the most recent common ancestor of all modern birds turns out to be an animal that would not be cla.s.sified as a bird, I would have to revise my statement that the birds const.i.tute a good cla.s.s.

* It's been suggested, by the way, that this gigantism was made possible by the higher oxygen content in the atmosphere at that time. Insects lack lungs, and they breathe by means of tiny air tubes that pipe air throughout the body. Air tubes can't mount such an intricately comprehensive distribution system as blood tubes can, and it is plausible that this limits body size. That limit would have been higher in an atmosphere with 35% oxygen, instead of the mere 21% that we breathe today. This provides a satisfying explanation for the giant dragonflies, but it may not necessarily be the right one. Incidentally, I'm puzzled why, with so much oxygen about, things didn't burst into flames all the time. Perhaps they did. Forest fires must have been more common than today, and the fossils indicate a high incidence of fire-resistant plant species. It is not certain why the oxygen content of the atmosphere peaked during the Carboniferous and Permian. It may be a.s.sociated with the sequestering of so much carbon under the ground, as coal. It's been suggested, by the way, that this gigantism was made possible by the higher oxygen content in the atmosphere at that time. Insects lack lungs, and they breathe by means of tiny air tubes that pipe air throughout the body. Air tubes can't mount such an intricately comprehensive distribution system as blood tubes can, and it is plausible that this limits body size. That limit would have been higher in an atmosphere with 35% oxygen, instead of the mere 21% that we breathe today. This provides a satisfying explanation for the giant dragonflies, but it may not necessarily be the right one. Incidentally, I'm puzzled why, with so much oxygen about, things didn't burst into flames all the time. Perhaps they did. Forest fires must have been more common than today, and the fossils indicate a high incidence of fire-resistant plant species. It is not certain why the oxygen content of the atmosphere peaked during the Carboniferous and Permian. It may be a.s.sociated with the sequestering of so much carbon under the ground, as coal.

* An Oxford don of the old school, who believed he was there to teach undergraduates, he would not have survived in today's research-a.s.sessment culture. With scarcely a single published article to his name, his legacy rests in the generations of grateful pupils to whom he imparted his wisdom and at least some of his immense learning. An Oxford don of the old school, who believed he was there to teach undergraduates, he would not have survived in today's research-a.s.sessment culture. With scarcely a single published article to his name, his legacy rests in the generations of grateful pupils to whom he imparted his wisdom and at least some of his immense learning.

* This seems to be correct. This seems to be correct. The Oxford Dictionary of Quotations The Oxford Dictionary of Quotations suggests that the commonly quoted 'seas' stems from a misprint in Masefield's original 1902 edition: a nice example of a successful mutant meme. suggests that the commonly quoted 'seas' stems from a misprint in Masefield's original 1902 edition: a nice example of a successful mutant meme.

* I'm advised that this doesn't make a lot of sense in Greek. If it were I'm advised that this doesn't make a lot of sense in Greek. If it were Progonochelys Progonochelys it would make perfect sense. It would mean something like 'ancestral tortoise' or 'primeval tortoise', and I can't help feeling that that may be what the original authors intended when they named it. Unfortunately, the rules of zoological nomenclature are strict, and even obvious mistakes can't be changed, once they are enshrined in a naming publication. The taxonomy is littered with such fossilized mistakes. My favourite is it would make perfect sense. It would mean something like 'ancestral tortoise' or 'primeval tortoise', and I can't help feeling that that may be what the original authors intended when they named it. Unfortunately, the rules of zoological nomenclature are strict, and even obvious mistakes can't be changed, once they are enshrined in a naming publication. The taxonomy is littered with such fossilized mistakes. My favourite is Khaya Khaya, African mahogany. Legend (which I long to believe) has it that in a local language it means 'I don't know', with the presumed subtext, 'And I don't care and why don't you stop asking stupid questions about plant names.'

CHAPTER 7

MISSING PERSONS? MISSING NO LONGER

DARWIN'S treatment of human evolution in his most famous work, treatment of human evolution in his most famous work, On the Origin of Species On the Origin of Species, is limited to twelve portentous words: 'Light will be thrown on the origin of man and his history.' That is the wording in the first edition, which is the edition I always cite unless otherwise stated. By the sixth (and last) edition, Darwin allowed himself to stretch a point, and the sentence became 'Much light will be thrown on the origin of man and his history.' I like to think of his pen, poised over the fifth edition, while the great man judiciously pondered whether to indulge himself in the luxury of 'Much'. Even with it, the sentence is a calculated understatement.Darwin deliberately deferred his treatment of human evolution to another book, The Descent of Man The Descent of Man. Perhaps it is not surprising that the two volumes of that later work devote more s.p.a.ce to the topic of its subt.i.tle, Selection in Relation to s.e.x Selection in Relation to s.e.x (investigated largely in birds), than to human evolution. Not surprising because, at the time of Darwin's writing, there were no fossils at all linking us to our closest relatives among the apes. Darwin had only living apes to look at, and he used them well, arguing correctly (and almost alone) that our closest living relatives were all African (gorillas and chimpanzees bon.o.bos were not recognized as separate from chimpanzees in those days, but they are African too), and therefore predicting that, if ancestral human fossils were ever to be found, Africa was the place to search. Darwin regretted the paucity of fossils, but he maintained a robustly bullish att.i.tude to it. Citing Lyell, his mentor and the great geologist of the time, he pointed out that 'in all the vertebrate cla.s.ses the discovery of fossil remains has been an extremely slow and fortuitous process' and added, 'Nor should it be forgotten that those regions which are the most likely to afford remains connecting man with some extinct ape-like creature, have not as yet been searched by geologists.' He meant Africa, and the quest was not helped by the fact that his immediate successors largely ignored his advice and searched Asia instead. (investigated largely in birds), than to human evolution. Not surprising because, at the time of Darwin's writing, there were no fossils at all linking us to our closest relatives among the apes. Darwin had only living apes to look at, and he used them well, arguing correctly (and almost alone) that our closest living relatives were all African (gorillas and chimpanzees bon.o.bos were not recognized as separate from chimpanzees in those days, but they are African too), and therefore predicting that, if ancestral human fossils were ever to be found, Africa was the place to search. Darwin regretted the paucity of fossils, but he maintained a robustly bullish att.i.tude to it. Citing Lyell, his mentor and the great geologist of the time, he pointed out that 'in all the vertebrate cla.s.ses the discovery of fossil remains has been an extremely slow and fortuitous process' and added, 'Nor should it be forgotten that those regions which are the most likely to afford remains connecting man with some extinct ape-like creature, have not as yet been searched by geologists.' He meant Africa, and the quest was not helped by the fact that his immediate successors largely ignored his advice and searched Asia instead.It was indeed in Asia that the 'missing links' first began to become less missing. But those first fossils to be discovered were relatively recent, less than a million years old, dating from a time when hominids were pretty close to modern humans and had migrated out of Africa and reached the Far East. They were called 'Java Man' and 'Peking Man' after their discovery sites.* Java Man was discovered by the Dutch anthropologist Eugene Dubois in 1891. He named it Java Man was discovered by the Dutch anthropologist Eugene Dubois in 1891. He named it Pithecanthropus erectus Pithecanthropus erectus, signifying his belief that he had realized his life's ambition and found 'the missing link'. Disagreement came from two opposite sources, which rather proved his point: some said his fossil was purely human, others that it was a giant gibbon. Later in his rather embittered and cantankerous life, Dubois resented the suggestion that the more recently discovered Peking fossils were similar to his Java Man. Fiercely possessive about, not to say protective of, his fossil, Dubois believed that only Java Man was the true missing link. To emphasize the distinction from the various Peking Man fossils, he described them as far closer to modern man, and his own Java Man of Trinil as intermediate between man and ape.Pithecanthropus [Java Man] was not a man, but a gigantic genus allied to the gibbons, however superior to the gibbons on account of its exceedingly large brain volume and distinguished at the same time by its faculty of a.s.suming an erect att.i.tude and gait. It had the double cephalization [ratio of brain size to body size] of the anthropoid apes in general and half that of man . . . [Java Man] was not a man, but a gigantic genus allied to the gibbons, however superior to the gibbons on account of its exceedingly large brain volume and distinguished at the same time by its faculty of a.s.suming an erect att.i.tude and gait. It had the double cephalization [ratio of brain size to body size] of the anthropoid apes in general and half that of man . . .It was the surprising volume of the brain which is very much too large for an anthropoid ape, and which is small compared with the average, though not smaller than the smallest human brain that led to the now almost general view that the 'Ape Man' of Trinil, Java was really a primitive Man. Morphologically, however, the calvaria [skullcap] closely resembles that of anthropoid apes, especially the gibbon . . .It can't have improved Dubois' temper that others took him to be saying that Pithecanthropus Pithecanthropus was just a giant gibbon, not intermediate between them and humans at all, and he was at pains to rea.s.sert his earlier stand: 'I still believe, now more firmly than ever, that the was just a giant gibbon, not intermediate between them and humans at all, and he was at pains to rea.s.sert his earlier stand: 'I still believe, now more firmly than ever, that the Pithecanthropus Pithecanthropus of Trinil is the real "missing link".' of Trinil is the real "missing link".'Creationists have from time to time used as a political weapon the allegation that Dubois backed off from his claim that Pithecanthropus Pithecanthropus was an intermediate ape-man. The creationist organization Answers in Genesis has, however, added it to their list of discredited arguments which they now say should not be used. It is to their credit that they maintain such a list at all. As I said, both the Java and Peking specimens of was an intermediate ape-man. The creationist organization Answers in Genesis has, however, added it to their list of discredited arguments which they now say should not be used. It is to their credit that they maintain such a list at all. As I said, both the Java and Peking specimens of Pithecanthropus Pithecanthropus have now been shown to be quite young, less than a million years old. They are now cla.s.sified along with us in the genus have now been shown to be quite young, less than a million years old. They are now cla.s.sified along with us in the genus h.o.m.o h.o.m.o, retaining Dubois' specific name erectus erectus: h.o.m.o erectus h.o.m.o erectus.Dubois chose the wrong part of the world for his single-minded quest for the 'missing link'. It was natural for a Dutchman to head first for the Dutch East Indies, but a man of his dedication should have followed Darwin's advice and gone on to Africa: for Africa is where our ancestors evolved, as we shall see. So what were these h.o.m.o erectus h.o.m.o erectus specimens doing out of Africa? The phrase 'out of Africa' has been borrowed from Karen Blixen specimens doing out of Africa? The phrase 'out of Africa' has been borrowed from Karen Blixen* to refer to the great exodus of our ancestors from Africa. But there were two exoduses and it is important not to confuse them. Relatively recently, maybe less than 100,000 years ago, roving bands of to refer to the great exodus of our ancestors from Africa. But there were two exoduses and it is important not to confuse them. Relatively recently, maybe less than 100,000 years ago, roving bands of h.o.m.o sapiens h.o.m.o sapiens looking pretty much like us left Africa and diversified into all the races that we see around the world today: Inuit, native Americans, native Australians, Chinese, and so on. It is to this recent exodus that the phrase 'out of Africa' is normally applied. But there was an earlier exodus from Africa, and these looking pretty much like us left Africa and diversified into all the races that we see around the world today: Inuit, native Americans, native Australians, Chinese, and so on. It is to this recent exodus that the phrase 'out of Africa' is normally applied. But there was an earlier exodus from Africa, and these erectus erectus pioneers left fossils in Asia and Europe, including the Java and Peking specimens. The oldest fossil known outside Africa was found in the central Asian country of Georgia and dubbed 'Georgian Man': a diminutive creature whose (rather well-preserved) skull is dated, by modern methods, to about 1.8 million years ago. It has been called pioneers left fossils in Asia and Europe, including the Java and Peking specimens. The oldest fossil known outside Africa was found in the central Asian country of Georgia and dubbed 'Georgian Man': a diminutive creature whose (rather well-preserved) skull is dated, by modern methods, to about 1.8 million years ago. It has been called h.o.m.o georgicus h.o.m.o georgicus (by some taxonomists, although others don't recognize it as a separate species) to indicate that it seems rather more primitive than the rest of the early refugees from Africa, who are all cla.s.sified as (by some taxonomists, although others don't recognize it as a separate species) to indicate that it seems rather more primitive than the rest of the early refugees from Africa, who are all cla.s.sified as h.o.m.o erectus h.o.m.o erectus. Some stone tools slightly older than Georgian Man have just been discovered in Malaysia, sparking a new search for fossil bones in that peninsula. But in any case, all these early Asian fossils are pretty close to modern humans and all are nowadays cla.s.sified in the genus h.o.m.o h.o.m.o; for our earlier antecedents we must go to Africa. First, though, let's pause to ask what we should expect of a 'missing link'.

h.o.m.o georgicus

Chimpanzee Suppose, for the sake of argument, we take seriously the original confused meaning of the term 'missing link', and seek an intermediate between chimpanzees (see right) and ourselves. We are not descended from chimpanzees, but it is a fair bet that the common ancestor that we share with them was more like a chimp than like us. In particular, it didn't have a huge brain like ours, it probably didn't walk upright as we do, it probably was a lot hairier than we are, and it surely didn't have such advanced human features as language. So, even though we must remain adamant, in the face of common misunderstanding, that we are not descended from chimpanzees, there's still no harm in asking what an intermediate between something like a chimpanzee and us would look like.Well, hair and language don't fossilize well, but we can get good clues about brain size from the skull, and good clues about gait from the whole skeleton (including the skull, for the foramen magnum foramen magnum, the hole for the spinal cord, points downwards in bipeds, more backwards in quadrupeds). Possible candidates for missing links might have any of the following attributes:1 Intermediate brain size and intermediate gait: perhaps a sort of stooping shamble rather than the proudly erect bearing favoured by sergeant majors and deportment mistresses.2 Chimpanzee-sized brain with human upright gait.3 Large, more human-like brain, walking on all fours like a chimp.So, bearing these possibilities in mind, let's examine some of the many African fossils that are now available to us, but unfortunately were not available to Darwin.

I'M STILL MISCHIEVOUSLY HOPING . . .

Molecular evidence (which I shall come on to in Chapter 10) shows that the common ancestor we share with chimpanzees lived about six million years ago or a bit earlier, so let's split the difference and look at some three-million-year-old fossils. The most famous fossil of this vintage is 'Lucy', cla.s.sified by her discoverer in Ethiopia, Donald Johanson, as Australopithecus afarensis Australopithecus afarensis. Unfortunately we have only fragments of Lucy's cranium, but her lower jaw is unusually well preserved. She was small by modern standards, although not as small as h.o.m.o floresiensis h.o.m.o floresiensis, the tiny creature the newspapers have irritatingly dubbed 'the Hobbit', which died out tantalizingly recently on the island of Flores in Indonesia. Lucy's skeleton is complete enough to suggest that she walked upright on the ground, but probably also sought refuge in trees, where she was an agile climber. There is good evidence that the bones attributed to Lucy really did all come from a single individual. The same is not true of the so-called 'First Family', a collection of bones from at least thirteen individuals, similar to Lucy and of approximately the same vintage, who somehow became buried together, also in Ethiopia. The fragments from Lucy and from the First Family give a good idea of what Australopithecus afarensis Australopithecus afarensis looked like, but it is hard to make an authentically complete reconstruction from pieces of many different individuals. Fortunately, a rather complete skull known as AL 444-2 (right) was discovered in 1992 in the same area of Ethiopia, and this confirmed the tentative reconstructions that had previously been made. looked like, but it is hard to make an authentically complete reconstruction from pieces of many different individuals. Fortunately, a rather complete skull known as AL 444-2 (right) was discovered in 1992 in the same area of Ethiopia, and this confirmed the tentative reconstructions that had previously been made.The conclusion from studies of Lucy and her kind is that they had brains about the same size as chimpanzees' but, unlike chimpanzees, they walked upright on their hind legs, as we do the second of our three hypothetical scenarios. 'Lucys' were a bit like upright-walking chimps. Their bipedality is dramatically confirmed by the poignantly evocative set of footprints discovered by Mary Leakey in fossilized volcanic ash. These are further south, at Laetoli in Tanzania, and they are older than Lucy and AL 444-2: about 3.6 million years. They are usually attributed to a pair of Australopithecus afarensis Australopithecus afarensis walking together (hand in hand?) but what matters is that, by 3.6 million years ago, an erect ape walked the Earth, on two feet which were pretty much like ours although its brain was the size of a chimpanzee's. walking together (hand in hand?) but what matters is that, by 3.6 million years ago, an erect ape walked the Earth, on two feet which were pretty much like ours although its brain was the size of a chimpanzee's.

AL 444-2 It seems quite likely that the species we call Australopithecus Australopithecus afarensis afarensis Lucy's species included our ancestors of three million years ago. Other fossils have been placed in different species of the same genus, and it is virtually certain that our ancestors were members of that genus. The first Australopithecine to be discovered, and the type specimen of the genus, was the so-called Taung Child. At the age of three and a half the Taung Child was eaten by an eagle. The evidence is that damage marks to the eye sockets of the fossil are identical to marks made by modern eagles on modern monkeys as they rip out their eyes. Poor little Taung Child, shrieking on the wind as you were borne aloft by the aquiline fury, you would have found no comfort in your destined fame, two and a half million years on, as the type specimen of Australopithecus africa.n.u.s Australopithecus africa.n.u.s. Poor Taung mother, weeping in the Pliocene.The type specimen is the first individual of a new species to be named and officially given the virgin label in a museum. Theoretically, later finds are compared against the type specimen to see if they match. The Taung Child was discovered and given brand new genus and species names by the South African anthropologist Raymond Dart in 1924.What's the difference between 'species' and 'genus'? Let's get the question swiftly out of the way, before proceeding. Genus is the more inclusive division. A species belongs within a genus, and often it shares the genus with other species. h.o.m.o sapiens h.o.m.o sapiens and and h.o.m.o erectus h.o.m.o erectus are two species within the genus are two species within the genus h.o.m.o. Australopithecus africa.n.u.s h.o.m.o. Australopithecus africa.n.u.s and and Australopithecus afarensis Australopithecus afarensis are two species within the genus are two species within the genus Australopithecus Australopithecus. The Latin name of an animal or plant always includes a generic name (with an initial capital letter) followed by a specific name (without a capital letter). Both names are written in italics. Sometimes there is an additional sub-specific name, which follows the specific name, as in, for example, h.o.m.o sapiens neanderthalensis h.o.m.o sapiens neanderthalensis. Taxonomists often dispute names. Many, for example, would speak of h.o.m.o neanderthalensis h.o.m.o neanderthalensis not not h.o.m.o sapiens neanderthalensis h.o.m.o sapiens neanderthalensis, elevating Neanderthal man from sub-species to species status. Generic names and specific names are also often disputed, and often change with successive revisions in the scientific literature. Paranthropus boisei Paranthropus boisei has been, in its time, has been, in its time, Zinjanthropus boisei Zinjanthropus boisei and and Australopithecus boisei Australopithecus boisei,* and is still often referred to, informally, as a robust Australopithecine as opposed to the two 'gracile' (slender) species of and is still often referred to, informally, as a robust Australopithecine as opposed to the two 'gracile' (slender) species of Australopithecus Australopithecus mentioned above. One of the main messages of this chapter concerns the somewhat arbitrary nature of zoological cla.s.sification. mentioned above. One of the main messages of this chapter concerns the somewhat arbitrary nature of zoological cla.s.sification.Raymond Dart, then, gave the name Australopithecus Australopithecus to the Taung Child, the type specimen of the genus, and we have been stuck with this depressingly unimaginative name for our ancestor ever since. It simply means 'southern ape'. Nothing to do with Australia, which just means 'southern country'. You'd think Dart might have thought of a more imaginative name for such an important genus. He might even have guessed that other members of the genus would later be discovered north of the equator. to the Taung Child, the type specimen of the genus, and we have been stuck with this depressingly unimaginative name for our ancestor ever since. It simply means 'southern ape'. Nothing to do with Australia, which just means 'southern country'. You'd think Dart might have thought of a more imaginative name for such an important genus. He might even have guessed that other members of the genus would later be discovered north of the equator.Slightly older than the Taung Child, one of the most beautifully preserved skulls we have, although lacking a lower jaw, is called 'Mrs Ples'. Mrs Ples, who may actually have been a small male rather than a large female, obtained 'her' nickname because she was originally cla.s.sified in the genus Plesianthropus Plesianthropus. This means 'nearly human', which is a better name than 'southern ape'. One might have hoped that, when later taxonomists decided that Mrs Ples and her kind were really of the same genus as the Taung Child, Plesianthropus Plesianthropus would have become the name for all of them. Unfortunately, the rules of zoological nomenclature are strict to the point of pedantry. Priority of naming takes precedence over sense and suitability. 'Southern ape' might be a lousy name but no matter: it predates the much more sensible would have become the name for all of them. Unfortunately, the rules of zoological nomenclature are strict to the point of pedantry. Priority of naming takes precedence over sense and suitability. 'Southern ape' might be a lousy name but no matter: it predates the much more sensible Plesianthropus Plesianthropus and we seem to be stuck with it, unless . . . I'm still mischievously hoping somebody will uncover, in a dusty drawer in a South African museum, a long-forgotten fossil, clearly the same kind as Mrs Ples and the Taung Child, but bearing the scrawled label, ' and we seem to be stuck with it, unless . . . I'm still mischievously hoping somebody will uncover, in a dusty drawer in a South African museum, a long-forgotten fossil, clearly the same kind as Mrs Ples and the Taung Child, but bearing the scrawled label, 'Hemianthropus type specimen, 1920'. At a stroke, all the museums in the world would immediately have to relabel their type specimen, 1920'. At a stroke, all the museums in the world would immediately have to relabel their Australopithecus Australopithecus specimens and casts, and all books and articles on hominid prehistory would have to follow suit. Wordprocessing programs across the world would work overtime sniffing out any occurrences of specimens and casts, and all books and articles on hominid prehistory would have to follow suit. Wordprocessing programs across the world would work overtime sniffing out any occurrences of Australopithecus Australopithecus and replacing them with and replacing them with Hemianthropus Hemianthropus. I can't think of any other case where international rules are potent enough to dictate a worldwide and backdated change of language overnight.

'Mrs Ples'

Now for my next important point about allegedly missing links and the arbitrariness of names. Obviously, when Mrs Ples's name was changed from Plesianthropus Plesianthropus to to Australopithecus Australopithecus, nothing changed in the real world at all. Presumably n.o.body would be tempted to think anything else. But consider a similar case where a fossil is re-examined and moved, for anatomical reasons, from one genus to another. Or where its generic status is disputed and this very frequently happens by rival anthropologists. It is, after all, essential to the logic of evolution that there must have existed individuals sitting exactly on the borderline between two genera, say Australopithecus Australopithecus and and h.o.m.o h.o.m.o. It is easy to look at Mrs Ples and a modern h.o.m.o sapiens h.o.m.o sapiens skull and say, yes, there is no doubt these two skulls belong in different genera. If we a.s.sume, as almost every anthropologist today accepts, that all members of the genus skull and say, yes, there is no doubt these two skulls belong in different genera. If we a.s.sume, as almost every anthropologist today accepts, that all members of the genus h.o.m.o h.o.m.o are descended from ancestors belonging to the genus we call are descended from ancestors belonging to the genus we call Australopithecus Australopithecus, it necessarily follows that, somewhere along the chain of descent from one species to the other, there must have been at least one individual who sat exactly on the borderline. This is an important point, so let me stay with it a little longer.

KNM ER 1813

KNM ER 1470 Bearing in mind the shape of Mrs Ples's skull as a representative of Australopithecus africa.n.u.s Australopithecus africa.n.u.s 2.6 million years ago, have a look at the top skull opposite, called KNM ER 1813. Then look at the one underneath it, called KNM ER 1470. Both are dated at approximately 1.9 million years ago, and both are placed by most authorities in the genus 2.6 million years ago, have a look at the top skull opposite, called KNM ER 1813. Then look at the one underneath it, called KNM ER 1470. Both are dated at approximately 1.9 million years ago, and both are placed by most authorities in the genus h.o.m.o h.o.m.o. Today, 1813 is cla.s.sified as h.o.m.o habilis h.o.m.o habilis, but it wasn't always. Until recently, 1470 was too, but there is now a move afoot to recla.s.sify it as h.o.m.o rudolfensis h.o.m.o rudolfensis. Once again, see how fickle and transitory our names are. But no matter: both have an apparently agreed foothold in the genus h.o.m.o h.o.m.o. The obvious difference from Mrs Ples and her kind is that she had a more forward-protruding face and a smaller brain-case. In both respects, 1813 and 1470 seem more human, Mrs Ples more 'ape-like'.Now look at the skull below, called 'Twiggy'. Twiggy is also normally cla.s.sified nowadays as h.o.m.o habilis h.o.m.o habilis. But her forward-pointing muzzle has more of a suggestion of Mrs Ples about it than of 1470 or 1813. You will perhaps not be surprised to be told that Twiggy has been placed by some anthropologists in the genus Australopithecus Australopithecus and by other anthropologists in and by other anthropologists in h.o.m.o h.o.m.o. In fact, each of these three fossils has been, at various times, cla.s.sified as h.o.m.o habilis h.o.m.o habilis and as and as Australopithecus habilis Australopithecus habilis. As I have already noted, some authorities at some times have given 1470 a different specific name, changing habilis habilis to to rudolfensis rudolfensis. And, to cap it all, the specific name rudolfensis rudolfensis has been fastened to both generic names, has been fastened to both generic names, Australopithecus Australopithecus and and h.o.m.o h.o.m.o. In summary, these three fossils have been variously called, by different authorities at different times, the following range of names:

'Twiggy'

KNM ER 1813: Australopithecus habilis, h.o.m.o habilis h.o.m.o habilis KNM ER 1470: Australopithecus habilis, h.o.m.o habilis h.o.m.o habilis, Australopithecus rudolfensis, h.o.m.o