Just how alike chimps are to humans was underlined again last week when scientists found that the ape was the natural reservoir for the Aids virus. HIV has apparently spent tens of thousands of years infecting chimps, only to emerge as a human infection when the animals were hunted and killed for food by central African tribes. HIV could jump from chimp to man in essence because they share 98.5 per cent of their genetic material.
But the similarities do not end at the level of DNA. Many scientists who study chimpanzees remark how difficult it is to resist comparing the behaviour they witness in them with that of humans. The study of animal behaviour - ethology - is supposed to be a dispassionate science devoid of emotionally charged judgements. How can it be possible to equate the sounds of apparent distress coming from Trudy, a dumb beast, with the cries of an unhappy child without a toy in the world? Does Trudy's genetic similarity with a human toddler mean she suffers the same emotional trauma when someone leaves her bereft of the last substitute she has for a mother?
Jane Goodall, the veteran chimpanzee expert, recalls how she was criticised in the Sixties when she brazenly used words such as "childhood", "adolescence", "motivation", "mood" and "excitement" to describe the behaviour she observed in troupes of chimps living in the forests of central Africa. "Even worse was my crime of suggesting that chimpanzees had personalities. I was guilty of the worst of ethological sins - anthropomorphism," she wrote.
Goodall's observations have taught us a lot about what chimpanzees are capable of doing. They engage in social interactions which can be just as complex as any of those found in an office or factory. They can bully, cajole and form political liaisons, often ganging up on one another. They can fool around, have fun, lie and cheat each another, and form lifetime emotional bonds.
They can be taught sign language, they can invent and use new tools and can pass on this cultural knowledge to members of their own clan. Chimpanzees can display a rich array of facial expressions and are the only species other than humans to be able to recognise their own reflections in a mirror. They have a primitive ability to identify their own personal position within the society and environment in which they live. Some argue that they may even have the rudiments of consciousness - something that has hitherto been considered a unique preserve of the human mind.
Goodall has also demonstrated another all-too-human trait in chimps. They can wage war. She has documented many cases of rival bands of chimps infiltrating one another's territory to kill males, murder infants and steal females.
There are many physical similarities between chimpanzees and humans. We share flat fingernails and toenails, rather than claws; have thumbs that can be opposed to all four fingers and, in the case of males, have a penis that hangs down freely rather than being connected to the abdomen. But it was the revelation 20 years ago that chimps and humans share 98.5 per cent of their DNA that has emphasised the proximity of the chimp-human relationship.
Fifty years ago, textbooks on evolution suggested that apes and humans shared a common ancestry more than 30 million years ago. Humans, they said, were quite distinct and separate from our closest cousins - great apes such as chimps, gorillas, orang-utans and gibbons. When the first DNA analysis was done, showing that only 1.5 per cent of human DNA was different to that of chimps, it was evident that the common ancestor must have lived much sooner in history, probably no more than 7 million years ago.
Jared Diamond, the award-winning science writer and professor of physiology at the University of California, Los Angeles, has argued that humans and chimps are so similar that it would be more accurate to describe Homo sapiens as the "third chimpanzee" - third because there are already two species of chimpanzee, the common chimp and the pygmy chimp or bonobo. His argument stems from the finding that gorillas differ from both chimps and humans by more than 2.3 per cent.
"THE GORILLAS must have branched off from our family tree slightly before we separated from the common and pygmy chimpanzees. The chimpanzees, not the gorilla, are our closest relatives. Put another way, the chimpanzees' closest relative is not the gorilla but the human," Diamond says.
Goodall, Diamond and several other eminent scientists feel so strongly about the inclusion of chimps and gorillas into the family of man that they have signed a "Declaration on Great Apes", which hopes to extend the "community of equals", implicit in the international charter on human rights, to gorillas, chimps and orang-utans. For Diamond, the issue is this: "Somewhere along the scale from bacteria to humans, we have to decide where killing becomes murder, and where eating becomes cannibalism." Diamond, like Goodall and many other scientists, feels the genetic similarity between chimp and man is too strong to allow the animals to be used, for instance, in medical research.
Just how similar was best illustrated by Richard Dawkins, the eminent Oxford zoologist, who once asked people to imagine a line of mothers and daughters holding hands and standing on the Equator facing north. If the left hand of each female was held in the right hand of her own mother, who held the hand of her mother and so on going back though the generations, with one person occupying one yard of space, how long would the line be before it had gone back far enough in time to reach the common ancestor of humans and chimps? The answer is a mere 300 miles.
The point Dawkins makes is that nowhere along this chain would there be a sudden leap between adjacent generations. "Daughters would resemble mothers just as much (or as little) as they always do. Mothers would love daughters, and feel affinity with them, just as they always do," he says. Yet at one end of this line would be a human being and at the other an animal that would look very different although related through their common DNA.
SOME SCIENTISTS are investigating the functions of the DNA that represents the 1.5 per cent difference between chimps and humans. Little is known as yet, except that it is reasonable to suppose that much of it must be "junk" - meaning it is not responsible for encoding the genetic information of the genes - because a significant proportion of human DNA is known to be junk. The functionally significant differences must be confined to some small fraction of this 1.5 per cent.
As yet the only bit of DNA that has been identified as being functionally different in chimps and humans was isolated last year by scientists at the University of California. They found humans, unlike all other apes, lack a particular molecule on the surface of their cells. This seems to be the reason why humans are more prone to infections such as flu, cholera and malaria, compared to chimps. One American biotechnology company has already applied for patents on the discovery because of the potential medical benefits that it promises to provide.
Steve Jones, professor of genetics at University College London, points out that although one estimate of genetic relatedness has found only a 1.5 per cent difference between chimp and man, this belies even wider differences resulting from the rearrangement of human and chimp chromosomes during their separate evolutionary histories. "Man shares 98.5 per cent of his DNA with chimps, but so what? It knocks him off his pinnacle, but many would say that he was not on a pinnacle anyway - at least not a biological pinnacle," Jones says.
Like many geneticists, Jones agrees that, technically at least, it would be correct to describe Homo sapiens as a type of chimpanzee. "Anatomically we are the third chimpanzee, but mentally we are the second animal kingdom."
So although our DNA is 98.5 per cent similar to that of chimps, our brains and minds clearly harbour a much greater difference. Mary Chipperfield presumably knew this when she exercised her higher mental faculty to crush Trudy's spirit by removing her toy with the words "You can bloody cry". Trudy's problem was that, like any human baby, she could not answer back.