The Greatest Show on Earth (32 page)

* You may be surprised to hear that horses evolved in North America, because it is widely known that when the European invaders first came to the Americas, the sight of them on horseback amazed the natives. The bulk of horse evolution did indeed take place in America. Then horses spread to the rest of the world, shortly (by geological standards) before going extinct in America. They are American animals that have been re-introduced to America by man.

* A single bone in mammals. The reptilian lower jaw is more complicated – and thereby hangs a fascinating tale that I reluctantly omitted from this book (you can’t have everything). In an amazing feat of evolutionary legerdemain, the smaller bones of the reptilian lower jaw were coopted into the mammalian ear, where they constitute an exquisitely delicate bridge, to transport sound from the eardrum to the inner ear.

* The Dutch ‘wildebeest’ is increasingly used in preference to ‘gnu’. I am trying to save ‘gnu’ because, if it dies out altogether, the witty song by Flanders and Swann won’t make sense any more. (‘Gnor am I in the least / Like that dreadful hartebeest / Oh gno gno gno, I’m a gnu!’)

* I presume my readers know better than the author(s) of Leviticus, who thought that bats were birds. In chapter 11, verses 13–19, is a long list of birds that are an abomination, beginning with the eagle and ending with ‘the stork, the heron after her kind, and the lapwing, and the bat’. It is a separate question why it was necessary to condemn any animals as abominations. It was a common practice in many religions.

* Biologists used to cite plant haemoglobin as a possible example of DNA borrowing by plants from the animal kingdom. Plants of the pea family (Leguminosae) have on their roots ‘nodules’, in which dwell bacteria that capture nitrogen from the atmosphere and make it available to the plants. This is why farmers often include a leguminous crop, such as clover or a vetch, in their rotation. It puts valuable nitrogen in the soil, especially if the clover crop is ploughed under. The nodules are a reddish colour because they contain a form of haemoglobin, similar to the oxygen-transporting molecule that makes our blood red. The genes for making haemoglobin are in the plant genome, not the bacterial genome. Haemoglobin is important to the bacteria, which need oxygen, and it can be regarded as part of the deal between bacteria and plants: the bacteria give the plants usable nitrogen, while the plants give the bacteria a house, and usable oxygen delivered via haemoglobin. Since we are accustomed to associating haemoglobin with blood, it was natural to wonder whether a gene for making it had somehow been ‘borrowed’ from an animal genome, perhaps ferried by a bacterium. It would, indeed, have been a very valuable idea to ‘borrow’. Unfortunately for this appealing idea – the ultimate blood transfusion – molecular biological evidence shows that haemoglobins are ancient denizens of plant genomes. They are not borrowed. They have been there from ancient times.

* It is a little-known fact that some dinosaurs had a ganglion in the pelvis, which was so large (at least relative to the brain in the head) as almost to deserve the title of second brain. This prompted the following delightfully witty verse by the American comic writer Bert Leston Taylor (1866–1921):
Behold the mighty dinosaur,

Famous in prehistoric lore,

Not only for his power and strength

But for his intellectual length.

You will observe by these remains

The creature had two sets of brains –

One in his head (the usual place),

The other at his spinal base,

Thus he could reason A priori

As well as A posteriori.

No problem bothered him a bit

He made both head and tail of it.

So wise was he, so wise and solemn,

Each thought filled just a spinal column.

If one brain found the pressure strong

It passed a few ideas along.

If something slipped his forward mind

’Twas rescued by the one behind.

And if in error he was caught

He had a saving afterthought.

As he thought twice before he spoke

He had no judgment to revoke.

Thus he could think without congestion

Upon both sides of every question.

Oh, gaze upon this model beast,

Defunct ten million years at least.

* D’Arcy Thompson was surely one of the most erudite scientists ever. Not only did he write famously beautiful English of a patrician character, not only was he a published mathematician and classical scholar as well as Professor of Natural History at Scotland’s oldest university, but his book is laced with quotations, which he assumed he had no need to translate (how times have changed), in Latin, Greek, Italian, German, French and even Provençal (the last he did deign to translate – into French!).

* Strictly speaking, two shapes are homeomorphic if you can distort one to become the other without breaking it, and without any new touchings.

* Perhaps ‘all living creatures’ calls for a note of caution. In an earlier section of this chapter we saw that while the principle of ‘No borrowing’ is almost completely appropriate for animals and plants, bacteria are different. Among bacteria (and archaea, which are superficially like bacteria but rather distantly related) there is plenty of sharing of genes. Whereas animals use sexual coupling to exchange DNA within species, bacteria use their own form of ‘Copy and Paste’ to pass DNA around, even between distantly related species. Although I was right to extol the ‘one true tree of life’ for animals and plants, the whole business gets messier when we turn to microorganisms. As my colleague the philosopher Dan Dennett has put it, where the tree of life for animals is a majestically spreading oak, that for bacteria is more like a banyan. Where bacteria are concerned, there is something to be said for compiling a ‘one true tree’ for each gene separately, regardless of which particular kinds of bacteria it happens to be travelling around in. What a fascinating prospect. How Darwin would have loved it.

* When I first read Calculus Made Easy by Silvanus P. Thompson, on the recommendation of my engineer grandfather, it gave me goosebumps when Thompson introduced e in italics as ‘a number never to be forgotten’. One consequence of using e rather than, say, 2, as the factor of choice, is that you can calculate the darwins directly by subtracting natural logarithms from one another. Other scientists have proposed, as a unit of evolutionary rate, the haldane.

* I have even been called an ‘ultra-Darwinist’, a gibe that I find less insulting than its coiners perhaps intended.

* ‘Degenerate’ is not the same (though the two terms are often confused) as ‘redundant’, another technical term of Information Theory. A redundant code is one in which the same message is conveyed more than once (e.g. ‘She is a female woman’ conveys the message of her sex three times). Redundancy is used by engineers to guard against transmission errors. A degenerate code is one in which more than one ‘word’ is used to mean the same thing. In the genetic code, for example, CUC and CUG both spell ‘Leucine’: a mutation from CUC to CUG therefore makes no difference. That’s ‘degenerate’.

HISTORY WRITTEN ALL OVER US

I BEGAN this book by imagining a teacher of Latin forced to waste time and energy defending the proposition that the Romans and their language ever existed. Let’s return to that thought, and ask what actually is the evidence for the Roman Empire and the Latin language. I live in Britain where, as in the rest of Europe, Rome wrote her signature all over the map, carved her ways into the landscape, wove her language through ours and her history through our literature. Walk the length of Hadrian’s Wall, whose preferred local name is still ‘The Roman Wall’. Walk, as I walked Sunday after Sunday in crocodile formation from my boarding school in (relatively) new Salisbury, to the Roman flint fort of Old Sarum, and commune with the imagined ghosts of dead legions. Unfold an Ordnance Survey map of England. Wherever you see a long, dead straight country road, especially when there are green field gaps between stretches of road or cart track that you can exactly line up with a ruler, you’ll almost always find a Roman label beside it. Vestiges of the Roman Empire are all around us.
Living bodies, too, have their history written all over them. They bristle with the biological equivalent of Roman roads, walls, monuments, potsherds, even ancient inscriptions carved into the living DNA, ready to be deciphered by scholars.
Bristle? Yes, literally. When you are cold, or badly frightened, or haunted by the peerless craftsmanship of a Shakespeare sonnet, you get goosebumps. Why? Because your ancestors were normal mammals with hairs all over, and these were raised or lowered at the behest of sensitive bodily thermostats. Too cold, and the hairs were erected to plump up the layer of insulating trapped air. Too warm, and the coat was flattened to allow body heat to escape more easily. In later evolution, the hair-erection system was hijacked for social communication purposes, and became involved in The Expression of the Emotions, as Darwin was one of the first to appreciate in his book of that name. I can’t resist sharing with you some lines – vintage Darwin – from that book:Mr Sutton, the intelligent keeper in the Zoological Gardens, carefully observed for me the Chimpanzee and Orang; and he states that when they are suddenly frightened, as by a thunderstorm, or when they are made angry, as by being teased, their hair becomes erect. I saw a chimpanzee who was alarmed at the sight of a black coalheaver, and the hair rose all over his body . . . I took a stuffed snake into the monkey-house, and the hair on several of the species instantly became erect . . . When I showed a stuffed snake to a Peccary, the hair rose in a wonderful manner along its back; and so it does with a wild boar when enraged.
The hackles are raised in anger. In fear also, hairs stand on end to increase the body’s apparent size and scare off dangerous rivals or predators. Even we naked apes still have the machinery to raise non-existent (or barely-existent) hairs, and we call it goosebumps. The hair-erection machinery is a vestige, a non-functional relic of something that did a useful job in our long-dead ancestors. Vestigial hairs are among the many instances of history written all over us. They constitute persuasive evidence that evolution has occurred, and again it comes not from fossils but from modern animals.
As we saw in the previous chapter, where I compared it to a comparably sized fish such as a dorado, you don’t have to dig very deep inside a dolphin to uncover its history of life on dry land. Despite its streamlined, fish-like exterior, and despite the fact that it now makes its entire living in the sea and would soon die if beached, a dolphin, but not a dorado, has ‘land mammal’ woven through its very warp and woof. It has lungs not gills, and will drown like any land animal if prevented from coming up for air, although it can hold its breath for much longer than a land mammal. In all sorts of ways, its air-breathing apparatus is changed to fit its watery world. Instead of breathing through two little nostrils at the end of its nose like any normal land mammal, it has a single nostril in the top of its head, which enables it to breathe while only just breaking the surface. This ‘blowhole’ has a tight-sealing valve to keep water out, and a wide bore to minimize the time needed for breathing. In an 1845 communication to the Royal Society, which Darwin, as a Fellow, would quite likely have read, Francis Sibson Esq.* wrote: ‘The muscles that open and close the blow-hole, and that act upon the various sacs, form one of the most complicated yet most exquisitely adjusted pieces of machinery that either nature or art presents.’ The dolphin’s blowhole goes to great lengths to correct a problem that would never have arisen at all if only it breathed with gills, like a fish. And many of the details of the blowhole can be seen as corrections to secondary problems that arose when the air intake migrated from the nostrils to the top of the head. A real designer would have planned it in the top of the head in the first place – that’s if he hadn’t decided to abolish lungs and go for gills anyway. Throughout this chapter, we shall continually find examples of evolution correcting an initial ‘mistake’ or historical relic by post hoc compensation or tweaking, rather than by going back to the drawing board as a real designer would. In any case, the elaborate and complex gateway to the blowhole is eloquent testimony to the dolphin’s remote ancestry on dry land.
In countless other ways, dolphins and whales could be said to have their ancient history written all over and through them, like vestiges of Roman roads drawn out in dead straight cart tracks and bridleways across the map of England. Whales have no hind legs, but there are tiny bones, buried deep inside them, which are the remnants of the pelvic girdle and hind legs of their long-gone walking ancestors. The same is true of the sirenians or sea cows (I’ve already mentioned them several times: the manatees, dugongs and the 8-yard-long Steller’s sea cow, hunted to extinction by humans).* Sirenians are very different from whales and dolphins, but they are the only other group of wholly marine mammals that never come ashore. Where dolphins are fast, actively intelligent carnivores, manatees and dugongs are slow, dreamy herbivores. At the manatee aquarium that I visited in western Florida, for once I didn’t rage against the loudspeakers playing music. It was sleepy lagoon music and it seemed so languidly appropriate that all was forgiven. Manatees and dugongs float effortlessly in hydrostatic equilibrium, not by means of a swim bladder as fish do (see below), but through being equipped with heavy bones as a counterweight to the natural buoyancy of their blubber. Their specific gravity is therefore very close to that of water, and they can make fine adjustments to it by pulling in or expanding the rib cage. The precision of their buoyancy control is enhanced by the possession of a separate cavity for each lung: they have two independent diaphragms.
Dolphins and whales, dugongs and manatees give birth to live babies, like all mammals. That habit is not actually peculiar to mammals. Many fish are livebearers, but they do it in a very different way (actually a fascinating variety of very different ways, doubtless independently evolved). The dolphin’s placenta is unmistakably mammalian, and so is its habit of suckling the young with milk. Its brain is also beyond question the brain of a mammal, and a very advanced mammal at that. The cerebral cortex of a mammal is a sheet of grey matter, wrapped around the outside of the brain. Getting brainier partly consists in increasing the area of the sheet. This could be done by increasing the total size of the brain, and of the skull that houses it. But there are downsides to having a big skull. It makes it harder to be born, for one thing. As a result, brainy mammals contrive to increase the area of the sheet while staying within limits set by the skull, and they do it by throwing the whole sheet into deep folds and fissures. This is why the human brain looks like a wrinkled walnut; and the brains of dolphins and whales are the only ones to rival those of us apes for wrinkliness. Fish brains don’t have wrinkles at all. Indeed, they don’t have a cerebral cortex, and the whole brain is tiny compared to a dolphin’s or human’s. The dolphin’s mammalian history is deeply etched into the wrinkled surface of its brain. It’s a part of its mammalness, along with the placenta, milk, a four-chambered heart, a lower jaw having only a single bone, warm-bloodedness, and many other specifically mammalian features.