Matt Ridley

Others later repeated his experiment in other ways. All agreed the number was twenty-four.

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

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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 S P E C I E S 2 5

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 2 6 G E N O M E

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 D N A 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 S P E C I E S 2 7

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