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 shell. 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 shell? Well, amazingly, a new fossil has just been described, which eloquently answers that question. It made its debut in the journal 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 Triassic sediments in China, and its age is estimated at 220 million years. Its name is Odontochelys semitestacea, from which you may deduce that, unlike a modern turtle or tortoise, it had teeth, and it did indeed have half a shell. 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 shell, 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 shell, 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 to the world, Li, Wu, Rieppel, Wang 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 shell. Others dispute Odontochelys’s claim to demonstrate that the shell evolved in water. 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 Odontochelys paper is by two Canadian biologists, Robert Reisz and Jason Head, and they offer an alternative interpretation. Maybe the whole shell had already evolved on land, before Odontochelys’s ancestors went back to the water. And maybe 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.
I need to digress for a brief aside on the question, ‘What is the use of half a shell?’ In particular, why would 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 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 (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 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 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. Shells are heavy and cumbersome, 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 just carried that trend a bit further.
Bathylychnops’ extra eye
If, on the other hand, the Chinese authors are right that Odontochelys was on its way to evolving a full shell, and that the shell evolved in water, it would seem to follow that modern land tortoises, which have well-developed shells, 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 migration from water to land. Nobody has ever claimed that any whales, or dugongs, 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 shell, 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
normal = aquatic
If 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 constitute 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 shell evolved in water, in a creature like 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 shelled animals called Proganochelys* and 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 was discovered, these two fossils were the oldest known chelonians. Like Odontochelys they lived in the late Triassic, but about 15 million years later than 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 anima
ls 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 believe, their half-shelled fossil shows that the shell evolved in water, how do we explain two genera of fully shelled ‘tortoises’ on land, 15 million years later? Until the discovery of Odontochelys, I would not have hesitated to say that Proganochelys and Palaeochersis were representative of the land-dwelling ancestral type before the return to water. The shell evolved on land. Some shelled 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. But Odontochelys throws speculation back into the melting pot. We now have three possibilities, all equally intriguing.
1 Proganochelys and Palaeochersis might be survivors of the land-dwelling animals that had earlier sent some representatives to sea, including the ancestors of Odontochelys. This hypothesis would suggest that the shell evolved on land early, and Odontochelys lost the carapace in the water, retaining the ventral plastron.
2 The shell 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 and Palaeochersis, who lived on land after Odontochelys lived, with its half shell, in water? Proganochelys and Palaeochersis might have evolved the shell independently. But there is another possibility:
3 Proganochelys and Palaeochersis 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! Not just to spawn the modern tortoises, but much longer ago, to give rise to Proganochelys and Palaeochersis in the Triassic.
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 constitute 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?
POSTSCRIPT
On 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. Named 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 undoubtedly is (see colour page 9), it comes trailing clouds of hype that obscure clear thinking. According to 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.
* 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, was to receive the greatest negative book review of all time, from the matchless Peter Medawar (reprinted in The Art of the Soluble and 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!
* ‘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 analytical 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.
† 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 class. 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 classified as a bird, I would have to revise my statement that the birds constitute a good class.
* 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 associated 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-assessment 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. 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.
* I’m advised that this doesn’t make a lot of sense in Greek. If it were 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 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, 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.
The Greatest Show on Earth Page 18