The tortoises did something similar,*4 evolving distinctive shell shapes on the different islands. The tortoises on the larger islands tend to have high domes. Those on the smaller islands have saddle-shaped shells with a high-lipped aperture for the head at the front. The reason for this seems to be that the large islands usually have enough water to grow grass, and the tortoises there are grazers. On the smaller islands there is often not enough water to grow grass, and the tortoises have to become browsers on cactuses. The high-lipped saddle shell allows the neck to reach up to the cactuses. The cactuses, for their part, grow taller and taller in an evolutionary arms race against the browsing tortoises.
The tortoise story adds to the finch model the further complication we have already noted: for them, volcanoes are islands within islands. They provide high, cool, damp, green oases, surrounded by dry lava fields at low altitude which, for a giant tortoise, constitute hostile desert. Most of the islands have but a single volcano and each has its own single species (or sub-species) of giant tortoise (some have none at all). The big island of Isabela has five major volcanoes, and each of them has its own species (or sub-species) of tortoise. Truly, Isabela is an archipelago within an archipelago. And the principle of archipelagoes as powerhouses of divergent evolution has never been more elegantly demonstrated than here in the islands of Darwin’s blest youth.
* * *
*1 The Ancestor’s Tale, now in its second edition jointly authored with Yan Wong, was first published shortly before I went on a memorable trip to the Galápagos Islands as the grateful guest of Victoria Getty. The book’s central motif is a ‘pilgrimage’ to the past. The hommage to Chaucer extends to ‘Tales’ told by particular animals, each tale conveying a general biological message. The momentum to tell such ‘tales’ persisted into my visit to the Galápagos, whose fauna moved me while on board ship to write three extra tales which were published in the Guardian. This one appeared on 19 February 2005.
*2 Confusingly, island dwarfism is also common. There were dwarf elephants on several Mediterranean islands and dwarf hominins, Homo floresiensis, on the small Indonesian island of Flores.
*3 There are also giant tortoises on the island of Aldabra in the Indian Ocean. And there were others, until nineteenth-century sailors drove them extinct along with the dodo and its cousins, on Mauritius and neighbouring islands. The Indian Ocean tortoises show the same evolutionary phenomenon of island gigantism as the Galápagos ones, but they evolved independently, in their case from smaller ancestors that drifted from Madagascar.
*4 But without the second stage of coming together again to share the same island, after diverging.
The sea turtle’s tale: there and back again (and again?)*1
IN ‘THE GIANT tortoise’s tale’ I described ancestral tortoises floating inadvertently across from South America, colonizing the Galápagos Islands by mistake, subsequently evolving local differences on each island and giant size on all of them. But why assume that the colonizer was a land tortoise? Wouldn’t it be simpler to guess that marine turtles, already at home in the sea, hauled up on the island beaches as if to lay their eggs, enjoyed what they saw, stayed on dry land and evolved into tortoises? No. Nothing like that happened on the Galápagos Islands, which have only been in existence a few million years.
Something very like it did happen, however, much longer ago in the ancestry of all tortoises. But that anticipates the climax to the turtle’s tale. (By the way, the word ‘turtle’ is a tiresome example of Bernard Shaw’s observation that England and America are two countries divided by a common language. In British usage, turtles live in water and tortoises on land. For Americans, tortoises are those turtles that live on land.)
There is good evidence that the most recent common ancestor of all today’s land tortoises, including those on the mainlands of America, Australia, Africa and Eurasia, as well as the giants of Galápagos, Aldabra, the Seychelles and other oceanic islands, was itself a land tortoise. In their less ancient ancestry, to misquote Stephen Hawking, it’s tortoises all the way down. The various giant tortoises of the Galápagos Islands are certainly descended from South American land tortoises.
If you go back far enough everything lived in the sea: watery alma mater of all life. At various points in evolutionary history, enterprising individuals within many different animal groups moved out onto the land, sometimes even to the most parched deserts, taking their own private sea water with them in blood and cellular fluids. In addition to the reptiles, birds, mammals and insects which we see all around us, other groups that have succeeded out of water include scorpions, snails, crustaceans such as woodlice and land crabs, millipedes and centipedes, spiders and their kin, and various worms. And we mustn’t forget the plants, without whose prior invasion of the land none of the other migrations could have happened.
This was an immense journey to undertake, not necessarily in terms of geographic distance but in terms of the upheaval in every aspect of life, from breathing to reproduction. Among the vertebrates, a particular group of lobe-finned fishes, related to today’s coelacanths and lungfishes, took to walking on land and developed lungs for breathing air. Their descendants the reptiles developed a large egg with a waterproof shell to retain the moisture that, from ancestral times in the sea, all vertebrate embryos need. Later descendants of the early reptiles included mammals and birds, which evolved a wide range of techniques for exploiting the land environment, including the habit of living in deserts, revolutionizing their way of life so that it became about as different from the ancestral life in the sea as can be imagined.
Among the wide range of specializations displayed by land creatures was one that seems wilfully perverse: a good number of thoroughgoing land animals later turned around, abandoned their hard-earned terrestrial re-tooling, and trooped back into the water again. Seals and sea lions (such as the astonishingly tame Galápagos sea lion) 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 ashore to breed. They do, however, still breathe air, having never developed anything equivalent to the gills of their earlier marine incarnation.
Other animals that have returned from land to water are pond snails, water spiders, water beetles, Galápagos flightless cormorants, penguins (Galápagos has the only penguins in the Northern Hemisphere*2), marine iguanas (found nowhere but Galápagos) and turtles (abundant in the surrounding waters).
Iguanas are adept at surviving accidental oceanic crossings on driftwood (well documented within the West Indies), and there can be no doubt that the marine iguanas of Galápagos trace back to just such a piece of living flotsam from South America. The oldest of the existing Galápagos Islands is no older than about four million years. Since the marine iguanas evolved here and nowhere else, you might think this sets a maximum limit on the date of their return to the water. The story is more complicated, however.
The Galápagos Islands were made, one after the other, as the Nazca tectonic plate moved, at a rate of a few centimetres per year, over a particular volcanic hotspot under the Pacific Ocean. As the plate moved east, from time to time the hotspot punched through, delivering another island along the production line. This is why the youngest islands are towards the west and the oldest to the east. But, at the same time as the Nazca plate continues to move east, it is also being subducted under the South American plate. The easternmost islands sink under the sea, at a rate of about one centimetre per year. It is now known that, although the oldest existing island is only four million years old, there has been an eastward-moving and sinking archipelago in this area for at least seventeen million years. Islands now submerged could have provided the initial haven for iguanas to colonize and evolve, at any time during that period. There would have been pl
enty of time for them to island-hop before their original ancestral island sank beneath the waves.
Turtles went back to the sea much longer ago. They are, in one respect, less fully given back to the water than whales or dugongs, for turtles still lay their eggs on beaches. Like all vertebrate returners to the water, they breathe air, but in this department they go one better than whales. Some turtles extract additional oxygen from the water through a pair of chambers at the rear end, 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 arse.
There is evidence that all modern turtles are descended from a terrestrial ancestor who lived before most of the dinosaurs. There are two key fossils called Proganochelys quenstedti and Palaeochersis talampayensis dating from early dinosaur times, which appear to be close to the ancestry of all modern turtles and tortoises. 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 it is a little less obvious. One neat way to tell is by measuring the bones of their forelimbs.
Walter Joyce and Jacques Gauthier, at Yale University, took three key measurements in the arm and hand bones of seventy-one species of living turtles and tortoises. They used triangular graph paper to plot the three measurements against one another. Lo and behold, all the land tortoise species formed a tight cluster of points in the upper part of the triangle; all the water turtles clustered in the lower part of the triangular graph. There was no overlap, except when they added some species that spend time in both water and land. Sure enough, these amphibious species show up, on the triangular graph, halfway between the ‘wet cluster’ and the ‘dry cluster’. Well then, to the obvious next step: where do the fossils fall? The hands of P. quenstedti and P. talampayensis leave us in no doubt. Their points on the graph are right in the thick of the dry cluster. Both these fossils were dry-land tortoises. They come from the era before our turtles returned to the water.
You might think, therefore, that modern land tortoises have probably stayed on land ever since those early terrestrial times, as most mammals did after a few of them went back to sea. But apparently not. If you draw out the family tree of all modern turtles and tortoises, nearly all the branches are aquatic. Today’s land tortoises constitute a single branch, deeply nested among branches consisting of aquatic turtles. This suggests that modern land tortoises have not stayed on land continuously since the time of P. quenstedti and P. talampayensis. Rather, their ancestors were among those who went back to the water, and they then re-emerged back onto the land in (relatively) more recent times.
Tortoises therefore represent a remarkable double return. In common with all mammals, reptiles and birds, their remote ancestors were marine fish and before that various more or less worm-like creatures stretching back, still in the sea, to the primeval bacteria. Later ancestors lived on land and stayed there for a very large number of generations. Even later ancestors evolved back into water-dwellers and became sea turtles. And finally they returned yet again to the land as tortoises, some of which, though not the Galápagos giants, now live in the driest of deserts.
I have described DNA as ‘the genetic book of the dead’ (see also this page). 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 humans and most other 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, turtles and, remarkably, tortoises, 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 tortoises, perhaps uniquely, 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 whom the genetic book of the dead is such a palimpsest of multiple evolutionary U-turns?
* * *
*1 This was the second of my additional tales, written on board ship in Galápagos and published in the Guardian, 26 February 2005.
*2 On my most recent visit to Galápagos, our senior Ecuadorian guide told an amusing story. A previous guest on the ship had raved about the experience – the scenery, the natural history, the food, the boat. He had only one complaint: Galápagos penguins are too small.
Farewell to a digerati dreamer
MY OWN LAST chance to see*1 Douglas Adams in action as a public speaker was at the Digital Biota conference in Cambridge in September 1998. As it happens, I dreamed last night of a similar event: a small conference of like-minded people, Douglas’s kind of people, denizens of the wild ‘Here be Digerati’ badlands between zoology and computer technology, one of Douglas’s favourite habitats. He was there of course, holding court (as I saw it, although his large and generously jocular modesty would have mocked the phrase). I had that familiar dreaming sense of knowing that he was dead, but of finding it not the least bit odd that he should be among us anyway, talking about science and making us laugh with his uniquely scientific wit. He was eagerly telling us over lunch about a remarkable adaptation in a fish, and he informed us that it would need only twenty-seven mutations to evolve it from a trout. I wish I could remember what the remarkable adaptation was, for it was exactly the kind of thing Douglas would have read about somewhere, and ‘twenty-seven mutations’ is exactly the kind of detail he would have relished.
From Cambridge to Komodo (from digerati to dragons) is no big step for a dreamer, so perhaps Douglas’s fish was the mudskipper that prompted his ancestral reflections at the end of the Komodo dragon chapter. His use of mudskippers and their 350-million-year-old forerunners – and ours – to tie up the dragon chapter and assuage his nagging guilt at not having spoken up for the hapless goat is a literary tour de force. Even the unfortunate chicken comes back as a metaphor, to reprise its tragi-comic role as the uneasy starter before the main course of pathetically bleating goat.
It is an uncomfortable experience to share a long ride on a small boat with four live chickens who are eyeing you with a deep and dreadful suspicion which you are in no position to allay.
Nobody’s written like that since P. G. Wodehouse. Or like this:
a benign man with the air of a vicar apologizing for something.
Or like this, on a rhinoceros grazing:
It was like watching a JCB excavator quietly getting on with a little weeding…The animal measured about six feet high at its shoulders, and sloped down gradually towards its hindquarters and its rear legs, which were chubby with muscle. The sheer immensity of every part of it exercised a fearful magnetism on the mind. When the rhino moved a leg, just slightly, huge muscles moved easily under its heavy skin like Volkswagens parking…The rhino snapped to attention, turned away from us, and hurtled off across the plain like a nimble young tank.
That last phrase is pure PGW, but Douglas had the advantage of an additional, scientific dimension to his humour. Wodehouse could never have achieved this:
It felt as if we were participating in a problem of three-body physics, swinging round in the gravitational pull of the rhinos.
Or this, of the Philippines monkey-eating eagle:
a wildly improbable-looking piece of flying hardware that you would more readily expect to see coming in to land on an aircraft carrier than nesting in a tree.
Chapter One’s reverie on ‘Twig Technology’ is original enough to provoke a scientist to serious thought, as is Douglas’s meditation on the rhinoceros as an animal whose world is dominated by smell, instead of vision. Douglas was not just knowledgeable about science. He didn’t just make jokes about science. He had the mind of a scientist, h
e mined science deeply and brought to the surface…humour, and a style of wit that was simultaneously literary and scientific, and uniquely his own.
There is probably no page in this book that doesn’t set me laughing out loud whenever I reread it – which is even more often than I read his fiction. In addition to the witty language, there are wonderful passages of sustained set-piece comedy, as in the epic quest for a condom in Shanghai (to sheathe an underwater microphone for listening to Yangtze River dolphins). Or there’s the legless taxi driver who kept diving under the dashboard to operate the clutch with his hands. Or there’s the wry comedy of the bureaucrats of Mobuto’s Zaire, whose corrupt nastiness exposes in Douglas and his comrade Mark Carwardine a benign innocence that recalls the kakapo, out of its depth in a harsh and uncaring world:
The kakapo is a bird out of time. If you look one in its large, round, greeny-brown face, it has a look of serenely innocent incomprehension that makes you want to hug it and tell it that everything will be all right, though you know that it probably will not be.
Science in the Soul Page 35