Genome: The Autobiography of a Species in 23 Chapters. Matt Ridley
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 recently 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 orang utan 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