Chromosome

Species

Man with all his noble qualities still bears in his bodily frame the indelible stamp of his lowly origin.

Charles Darwin

Sometimes the obvious can stare you in the face. Until 1955, it was agreed that human beings had twenty-four pairs of chromosomes. It was just one of those facts that everybody knew was right. They knew it was right because in 1921 a Texan named Theophilus Painter had sliced thin sections off the testicles of two black men and one white man castrated for insanity and 'self-abuse', fixed the slices in chemicals and examined them under the microscope. Painter tried to count the tangled mass of unpaired chromosomes he could see in the spermatocytes of the unfortunate men, and arrived at the figure of twenty-four. 'I feel confident that this is correct,' he said. Others later repeated his experiment in other ways. All agreed the number was twenty-four.

For thirty years, nobody disputed this 'fact'. One group ofscien-tists abandoned their experiments on human liver cells because they could only find twenty-three pairs of chromosomes in each cell.

Another researcher invented a method of separating the chromosomes, but still he thought he saw twenty-four pairs. It was not until 1955, when an Indonesian named Joe-Hin Tjio travelled from Spain to Sweden to work with Albert Levan, that the truth dawned. Tjio and Levan, using better techniques, plainly saw twenty-three pairs. They even went back and counted twenty-three pairs in photographs in books where the caption stated that there were twenty-four pairs. There are none so blind as do not wish to see.1

It is actually rather surprising that human beings do not have twenty-four pairs of chromosomes. Chimpanzees have twenty-four pairs of chromosomes; so do gorillas and orangutans. Among the apes we are the exception. Under the microscope, the most striking and obvious difference between ourselves and all the other great apes is that we have one pair less. The reason, it immediately becomes apparent, is not that a pair of ape chromosomes has gone missing in us, but that two ape chromosomes have fused together in us. Chromosome 2, the second biggest of the human chromosomes, is in fact formed from the fusion of two medium-sized ape chromosomes, as can be seen from the pattern of black bands on the respective chromosomes.

Pope John-Paul II, in his message to the Pontifical Academy of Sciences on 22 October 1996, argued that between ancestral apes and modern human beings, there was an 'ontological discontinuity' — a point at which God injected a human soul into an animal lineage. Thus can the Church be reconciled to evolutionary theory. Perhaps the ontological leap came at the moment when two ape chromosomes were fused, and the genes for the soul lie near the middle of chromosome 2.

The pope notwithstanding, the human species is by no means the pinnacle of evolution. Evolution has no pinnacle and there is no such thing as evolutionary progress. Natural selection is simply the process by which life-forms change to suit the myriad opportunities afforded by the physical environment and by other life-forms. The black-smoker bacterium, living in a sulphurous vent on the floor of the Atlantic ocean and descended from a stock of bacteria that parted company with our ancestors soon after Luca's day, is arguably more highly evolved than a bank clerk, at least at the genetic level. Given that it has a shorter generation time, it has had more time to perfect its genes.

This book's obsession with the condition of one species, the human species, says nothing about that species' importance. Human beings are of course unique. They have, perched between their ears, the most complicated biological machine on the planet. But complexity is not everything, and it is not the goal of evolution. Every species on the planet is unique. Uniqueness is a commodity in oversupply. None the less, I propose to try to probe this human uniqueness in this chapter, to uncover the causes of our idiosyncrasy as a species. Forgive my parochial concerns. The story of a briefly abundant hairless primate originating in Africa is but a footnote in the history of life, but in the history of the hairless primate it is central. What exactly is the unique selling point of our species?

Human beings are an ecological success. They are probably the most abundant large animal on the whole planet. There are nearly six billion of them, amounting collectively to something like 300 million tons of biomass. The only large animals that rival or exceed this quantity are ones we have domesticated - cows, chickens and sheep - or that depend on man-made habitats: sparrows and rats. By contrast, there are fewer than a thousand mountain gorillas in the world and even before we started slaughtering them and eroding their habitat there may not have been more than ten times that number. Moreover, the human species has shown a remarkable capacity for colonising different habitats, cold or hot, dry or wet, high or low, marine or desert. Ospreys, barn owls and roseate terns are the only other large species to thrive in every continent except Antarctica and they remain strictly confined to certain habitats. No doubt, this ecological success of the human being comes at a high price and we are doomed to catastrophe soon enough: for a successful species we are remarkably pessimistic about the future. But for now we are a success.

Yet the remarkable truth is that we come from a long line of failures. We are apes, a group that almost went extinct fifteen million years ago in competition with the better-designed monkeys. We are primates, a group of mammals that almost went extinct forty-five million years ago in competition with the better-designed rodents. We are synapsid tetrapods, a group of reptiles that almost went extinct 200 million years ago in competition with the better-designed dinosaurs. We are descended from limbed fishes, which almost went extinct 360 million years ago in competition with the better-designed ray-finned fishes. We are chordates, a phylum that survived the Cambrian era 500 million years ago by the skin of its teeth in competition with the brilliantly successful arthropods. Our ecological success came against humbling odds.

In the four billion years since Luca, the word grew adept at building what Richard Dawkins has called 'survival machines': large, fleshy entities known as bodies that were good at locally reversing entropy the better to replicate the genes within them. They had done this by a venerable and massive process of trial and error, known as natural selection. Trillions of new bodies had been built, tested and enabled to breed only if they met increasingly stringent criteria for survival. At first, this had been a simple business of chemical efficiency: the best bodies were cells that found ways to convert other chemicals into DNA and protein. This phase lasted for about three billion years and it seemed as if life on earth, whatever it might do on other planets, consisted of a battle between competing strains of amoebae. Three billion years during which trillions of trillions of single-celled creatures lived, each one reproducing and dying every few days or so, amounts to a big heap of trial and error.

But it turned out that life was not finished. About a billion years ago, there came, quite suddenly, a new world order, with the invention of bigger, multicellular bodies, a sudden explosion of large creatures. Within the blink of a geological eye (the so-called Cambrian explosion may have lasted a mere ten or twenty million years), there were vast creatures of immense complexity: scuttling trilobites nearly a foot long; slimy worms even longer; waving algae half a yard across. Single-celled creatures still dominated, but these great unwieldy forms of giant survival machines were carving out a niche for themselves. And, strangely, these multicellular bodies had hit upon a sort of accidental progress. Although there were occasional setbacks caused by meteorites crashing into the earth from space, which had an unfortunate tendency to extirpate the larger and more complex forms, there was a trend of sorts discernible. The longer animals existed, the more complex some of them became. In particular, the brains of the brainiest animals were bigger and bigger in each successive age: the biggest brains in the Paleozoic were smaller than the biggest in the Mesozoic, which were smaller than the biggest in the Cenozoic, which were smaller than the biggest present now. The genes had found a way to delegate their ambitions, by building bodies capable not just of survival, but of intelligent behaviour as well. Now, if a gene found itself in an animal threatened by winter storms, it could rely on its body to do something clever like migrate south or build itself a shelter.

Our breathless journey from four billion years ago brings us to just ten million years ago. Past the first insects, fishes, dinosaurs and birds to the time when the biggest-brained creature on the planet (corrected for body size) was probably our ancestor, an ape. At that point, ten million years before the present, there probably lived at least two species of ape in Africa, though there may have been more. One was the ancestor of the gorilla, the other the common ancestor of the chimpanzee and the human being. The gorilla's ancestor had probably taken to the montane forests of a string of central African volcanoes, cutting itself off from the genes of other apes. Some time over the next five million years the other species gave rise to two different descendant species in the split that led to human beings and to chimpanzees.

The reason we know this is that the story is written in the genes. As recendy as 1950 the great anatomist J. Z. Young could write that it was still not certain whether human beings descended from a common ancestor with apes, or from an entirely different group of primates separated from the ape lineage more than sixty million years ago. Others still thought the orangutan might prove our closest cousin.2 Yet we now know not only that chimpanzees separated from the human line after gorillas did, but that the chimp—human split occurred not much more than ten, possibly even less than five, million years ago. The rate at which genes randomly accumulate spelling changes gives a firm indication of relationships between species. The spelling differences between gorilla and chimp are greater than the spelling differences between chimp and human being — in every gene, protein sequence or random stretch of DNA that you care to look at. At its most prosaic this means that a hybrid of human and chimpanzee DNA separates into its constituent strands at a higher temperature than do hybrids of chimp and gorilla DNA, or of gorilla and human DNA.

Calibrating the molecular clock to give an actual date in years is much more difficult. Because apes are long-lived and breed at a comparatively advanced age, their molecular clocks tick rather slowly (the spelling mistakes are picked up mostly at the moment of replication, at the creation of an egg or sperm). But it is not clear exactly how much to correct the clock for this factor; nor do all genes agree. Some stretches of DNA seem to imply an ancient split between chimps and human beings; others, such as the mitochondria, suggest a more recent date. The generally accepted range is five to ten million years.3

Apart from the fusion of chromosome 2, visible differences between chimp and human chromosomes are few and tiny. In thirteen chromosomes no visible differences of any kind exist. If you select at random any 'paragraph' in the chimp genome and compare it with the comparable 'paragraph' in the human genome, you will find very few 'letters' are different: on average, less than two in every hundred. We are, to a ninety-eight per cent approximation, chimpanzees,\and they are, with ninety-eight per cent confidence limits, human beings. If that does not dent your self-esteem, consider that chimpanzees are only ninety-seven per cent gorillas; and humans are also ninety-seven per cent gorillas. In other words we are more chimpanzee-like than gorillas are.

How can this be? The differences between me and a chimp are immense. It is hairier, it has a different shaped head, a different shaped body, different limbs, makes different noises. There is nothing about chimpanzees that looks ninety-eight per cent like me. Oh really? Compared with what? If you took two Plasticene models of a mouse and tried to turn one into a chimpanzee, the other into a human being, most of the changes you would make would be the same. If you took two Plasticene amoebae and turned one into a chimpanzee, the other into a human being, almost all the changes you would make would be the same. Both would need thirty-two teeth, five fingers, two eyes, four limbs and a liver. Both would need hair, dry skin, a spinal column and three little bones in the middle ear. From the perspective of an amoeba, or for that matter a fertilised egg, chimps and human beings are ninety-eight per cent the same. There is no bone in the chimpanzee body that I do not share. There is no known chemical in the chimpanzee brain that cannot be found in the human brain. There is no known part of the immune system, the digestive system, the vascular system, the lymph system or the nervous system that we have and chimpanzees do not, or vice versa.

There is not even a brain lobe in the chimpanzee brain that we do not share. In a last, desperate defence of his species against the theory of descent from the apes, the Victorian anatomist Sir Richard Owen once claimed that the hippocampus minor was a brain lobe unique to human brains, so it must be the seat of the soul and the proof of divine creation. He could not find the hippocampus minor in the freshly pickled brains of gorillas brought back from the Congo by the adventurer Paul du Chaillu. Thomas Henry Huxley furiously responded that the hippocampus minor was there in ape brains. 'No, it wasn't', said Owen. Was, too', said Huxley. Briefly, in 1861, the 'hippocampus question' was all the rage in Victorian London and found itself satirised in Punch and Charles Kingsley's novel The water babies. Huxley's point - of which there are loud modern echoes - was more than just anatomy:4 'It is not I who seek to base Man's dignity upon his great toe, or insinuate that we are lost if an Ape has a hippocampus minor. On the contrary, I have done my best to sweep away this vanity.' Huxley, by the way, was right.

After all, it is less than 300,000 human generations since the common ancestor of both species lived in central Africa. If you held hands with your mother, and she held hands with hers, and she with hers, the line would stretch only from New York to Washington before you were holding hands with the 'missing link' - the common ancestor with chimpanzees. Five million years is a long time, but evolution works not in years but in generations. Bacteria can pack in that many generations in just twenty-five years.

What did the missing link look like? By scratching back through the fossil record of direct human ancestors, scientists are getting remarkably close to knowing. The closest they have come is probably a little ape-man skeleton called Ardipithecus from just over four million years ago. Although a few scientists have speculated that Ardipithecus predates the missing link, it seems unlikely: the creature had a pelvis designed chiefly for upright walking; to modify that back to the gorilla-like pelvis design in the chimpanzee's lineage would have been drastically improbable. We need to find a fossil several million years older to be sure we are looking at a common ancestor of us and chimps. But we can guess, from Ardipithecus, what the missing link looked like: its brain was probably smaller than a modern chimp's. Its body was at least as agile on two legs as a modern chimp's. Its diet, too, was probably like a modern chimp's: mostly fruit and vegetation. Males were considerably bigger than females. It is hard, from the perspective of human beings, not to think of the missing link as more chimp-like than human-like. Chimps might disagree, of course, but none the less it seems as if our lineage has seen grosser changes than theirs.

Like every ape that had ever lived, the missing link was probably a forest creature: a model, modern, Pliocene ape at home among the trees. At some point, its population became split in half. We know this because the separation of two parts of a population is often the event that sparks speciation: the two daughter populations gradually diverge in genetic make-up. Perhaps it was a mountain range, or a river (the Congo river today divides the chimpanzee from its sister species, the bonobo), or the creation of the western Rift Valley itself about five million years ago that caused the split, leaving human ancestors on the dry, eastern side. The French paleontologist Yves Coppens has called this latter theory 'East Side Story'. Perhaps, and the theories are getting more far-fetched now, it was the newly formed Sahara desert that isolated our ancestor in North Africa, while the chimp's ancestor remained to the south. Perhaps the sudden flooding, five million years ago, of the then-dry Mediterranean basin by a gigantic marine cataract at Gibraltar, a cataract one thousand times the volume of Niagara, suddenly isolated a small population of missing links on some large Mediterranean island, where they took to a life of wading in the water after fish and shellfish. This 'aquatic hypothesis' has all sorts of things going for it except hard evidence.

Whatever the mechanism, we can guess that our ancestors were a small, isolated band, while those of the chimpanzees were the main race. We can guess this because we know from the genes that human beings went through a much tighter genetic bottleneck (i.e., a small population size) than chimpanzees ever did: there is much less random variability in the human genome than the chimp genome.5

So let us picture this isolated group of animals on an island, real or virtual. Becoming inbred, flirting with extinction, exposed to the forces of the genetic founder effect (by which small populations can have large genetic changes thanks to chance), this little band of apes shares a large mutation: two of their chromosomes have become fused. Henceforth they can breed only with their own kind, even when the 'island' rejoins the 'mainland'. Hybrids between them and their mainland cousins are infertile. (I'm guessing again - but scientists show remarkably little curiosity about the reproductive isolation of our species: can we breed with chimps or not?)

By now other startling changes have begun to come about. The shape of the skeleton has changed to allow an upright posture and a bipedal method of walking, which is well suited to long distances in even terrain; the knuckle-walking of other apes is better suited to shorter distances over rougher terrain. The skin has changed, too. It is becoming less hairy and, unusually for an ape, it sweats profusely in the heat. These features, together with a mat of hair to shade the head and a radiator-shunt of veins in the scalp, suggest that our ancestors were no longer in a cloudy and shaded forest; they were walking in the open, in the hot equatorial sun.6

Speculate as much as you like about the ecology that selected such a dramatic change in our ancestral skeleton. Few suggestions can be ruled out or in. But by far the most plausible cause of these changes is the isolation of our ancestors in a relatively dry, open grassland environment. The habitat had come to us, not vice versa: in many parts of Africa the savannah replaced the forest about this time. Some time later, about 3.6 million years ago, on freshly wetted volcanic ash recently blown from the Sadiman volcano in what is now Tanzania, three hominids walked purposefully from south to north, the larger one in the lead, the middle-sized one stepping in the leader's footsteps and the small one, striding out to keep up, just a little to the left of the others. After a while, they paused and turned to the west briefly, then walked on, as upright as you or me. The Laetoli fossilised footprints tell as plain a tale of our ancestors' upright walking as we could wish for.

Yet we still know too little. Were the Laetoli ape-people a male, a female and a child or a male and two females? What did they eat? What habitat did they prefer? Eastern Africa was certainly growing drier as the Rift Valley interrupted the circulation of moist winds from the west, but that does not mean they sought dry places. Indeed, our need for water, our tendency to sweat, our peculiar adaptation to a diet rich in the oils and fats of fish and other factors (even our love of beaches and water sports) hint at something of an aquatic preference. We are really rather good at swimming. Were we at first to be found in riverine forests or at the edges of lakes?

In due time, human beings would turn dramatically carnivorous. A whole new species of ape-man, indeed several species, would appear before that, descendants of Laetoli-like creatures, but not ancestors of people, and probably dedicated vegetarians. They are called the robust australopithecines. The genes cannot help us here, because the robusts were dead ends. Just as we would never have known about our close cousinship with chimps if we could not read genes, so we would never have been aware of the existence of our many and closer australopithecine cousins if we had not found fossils (by 'we', I mean principally the Leakey family, Donald Johanson and others). Despite their robust name (which refers only to their heavy jaws), robust australopithecines were little creatures, smaller than chimps and stupider, but erect of posture and heavy of face: equipped with massive jaws supported by giant muscles. They were into chewing - probably grasses and other tough plants. They had lost their canine teeth the better to chew from side to side. Eventually, they became extinct, some time around a million years ago. We may never know much more about them. Perhaps we ate them.

After all, by then our ancestors were bigger animals, as big as modern people, maybe slightly bigger: strapping lads who would grow to nearly six foot, like the famous skeleton of the Nariokotome boy of 1.6 million years ago described by Alan Walker and Richard Leakey.7 They had begun to use stone tools as substitutes for tough teeth. Perfectly capable of killing and eating a defenceless robust australopithecine — in the animal world, cousins are not safe: lions kill leopards and wolves kill coyotes - these thugs had thick craniums and stone weapons (the two probably go together). Some competitive impulse was now marching the species towards its future explosive success, though nobody directed it - the brain just kept getting bigger and bigger. Some mathematical masochist has calculated that the brain was adding 150 million brain cells every hundred thousand years, the sort of useless statistic beloved of a tourist guide. Big brains, meat eating, slow development, the 'neotenised' retention into adulthood of childhood characters (bare skin, small jaws and a domed cranium) - all these went together. Without the meat, the protein-hungry brain was an expensive luxury. Without the neo-tenised skull, there was no cranial space for the brain. Without the slow development, there was no time for learning to maximise the advantages of big brains.

Driving the whole process, perhaps, was sexual selection. Besides the changes to brains, another remarkable change was going on. Females were getting big relative to males. Whereas in modern

0 0

Post a comment