What a difference 2% makes

Since we're in the middle of an intense discussion, I should let everyone know that I'm heading out of town for the weekend. If you post a comment, and I don't respond until Monday, it isn't personal.

Meanwhile, I'm offering the following book excerpt for your reading pleasure. It isn't intended as a direct comment on our discussion of abortion, but it touches on some of the information I've used in defence of my position.

The excerpt discusses the correspondence between humans and chimpanzees. At root is a 98% correspondence of our respective genes, which gives rise to all the other physical correspondences:  the three little bones in the middle ear; all the chemicals present in the brain; various systems (immune, digestive, nervous), etc.

All these physical attributes are present, albeit in embryonic form, at conception. Nothing is added in the uterus of a chimpanzee or a human woman that changes the essential nature of what is already present.

I've been thinking about human genetics in recent days, because of our dialogue, so I dug this excerpt out of Matt Ridley's book. I'm confident you'll find it interesting for its own sake, even if you draw different conclusions from it.

Enjoy!
Q

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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 of scientists 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 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.

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 orang utans. 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....

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, by the way, was right.

A record of our past is etched into our genes. Some two per cent of the genome tells the story of our different ecological and social evolution from that of chimpanzees, and theirs from us. When the genome of a typical human being has been fully transcribed into our computers, when the same has been done for the average chimpanzee, when the active genes have been extracted from the noise, and when the differences come to be listed, we will have an extraordinary glimpse of the Pleistocene era on two different species derived from a common stock. The genes that will be the same will be the genes for basic biochemistry and body planning. Probably the only differences will be in genes for regulating growth and hormonal development. Somehow in their digital language, these genes will tell the foot of a human foetus to grow into a flat object with a heel and a big toe, whereas the same genes in a chimpanzee tell the foot of a chimp foetus to grow into a more curved object with less of a heel and longer, more prehensile toes.

It is mind-boggling even to try to imagine how that can be done — science still has only the vaguest clues about how growth and form are generated by genes — but that genes are responsible is not in doubt. The differences between human beings and chimpanzees are genetic differences and virtually nothing else. Even those who would stress the cultural side of the human condition and deny or doubt the importance of genetic differences between human individuals or races, accept that the differences between us and other species are primarily genetic. Suppose the nucleus of a chimpanzee cell were injected into an enucleated human egg and that egg were implanted into a human womb, and the resulting baby, if it survived to term, were reared in a human family. What would it look like? You do not even need to do the (highly unethical) experiment to know the answer:  a chimpanzee. Although it started with human cytoplasm, used a human placenta and had a human upbringing, it would not look even partly human.

Photography provides a helpful analogy. Imagine you take a photograph of a chimpanzee. To develop it you must put it in a bath of developer for the requisite time, but no matter how hard you try, you cannot develop a picture of a human being on the negative by changing the formula of the developer. The genes are the negative; the womb is the developer. Just as a photograph needs to be immersed in a bath of developer before the picture will appear, so the recipe for a chimpanzee, written in digital form in the genes of its egg, needs the correct milieu to become an adult — the nutrients, the fluids, the food and the care — but it already has the information to make a chimpanzee.

Genome:  the autobiography of a species in 23 chapters
"Chromosome 2, Species"
Matt Ridley