About 50 years ago, something happened that radically changed our ideas about what it meant to be human. A young secretary who had ventured into the African jungle witnessed a chimpanzee fashioning a tool out of a blade of grass, and using it to fish for termites. Jane Goodall had been sent to Tanzania by Dr Louis Leakey, who, on hearing her startling news, came up with an equally startling statement: "Now we must redefine 'tool', redefine 'man', or accept chimpanzees as humans."
During the Sixties, the then Dr Goodall, later founder of the Jane Goodall Institute, discovered more similarities between ourselves and chimpanzees: they can use stone tools; they have a rudimentary culture; their mothers teach their infants; they feel similar emotions to us, such as fear, sadness, happiness; and they grieve over lost loved ones.
Subsequently, genetic research started to shore up her theory that "the line between humans and other non-human beings, once thought so sharp, has become blurred". Movements sprang up such as the Great Ape Project, founded in 1993 by the bioethicist Peter Singer, which argued that apes should be awarded certain basic rights. And as genome mapping was developed, the genetic difference observed between humans and chimpanzees, our closest living ancestors, continued to shrink: it turned out that only 1.6 per cent of our genes were different.
This activity led to two basic conclusions: that humans and apes were not that different after all, and that if 98.4 per cent of our genes were shared with chimps, the remaining 1.6 per cent should explain why our development has differed so dramatically from that of our cousins.
Yet a new book, published last week, is attacking both assumptions. "Because we are virtually genetically identical, primatologists argue that in a logical sense, chimpanzees are very close to us cognitively," says Jeremy Taylor, author of Not a Chimp: The Hunt to Find the Genes that Make Us Human. "The way this idea has bled into popular culture enrages me."
Taylor is particularly scathing on the subject of primate rights. "I don't understand why conservation of the great apes has become synonymous with human rights and their similarity to us, whereas conservation of wetlands or a million other species doesn't carry any such conflations," he says. In this he echoes the Telegraph columnist Steve Jones, who has argued that it is a mistake to apply a human concept, such as rights, to an animal: "Chimpanzees share about 98 per cent of our DNA, but bananas share about 50 per cent, and we are not 98 per cent chimp or 50 per cent banana. We are entirely human and unique."
Goodall's work was followed by a spate of studies demonstrating how close chimps' mental capacities seem to be to ours. They can, it is thought, show self-awareness, as demonstrated by what has become a classic test. Researchers put a blob of paint on a chimp's face without the animal noticing or being able to see the spot, and then give it a mirror. Macaques and other monkeys will react aggressively to their mirror image, as if they are seeing another creature, whereas chimpanzees will calmly sit down and rub the paint off.
Thirty-one years ago, two scientists, David Premack and Guy Woodruff, published a seminal paper asking whether chimpanzees had what they termed "Theory of Mind": the ability to understand that another being has thoughts and beliefs, desires and feelings. Because we have this ability, we think about what other people are thinking: we don't treat our fellows as if they were objects or automatons following a set of rules.
The majority of scientists working in this field would argue that chimps do not have the same capacity as humans for thinking about how others think, but that, nevertheless, chimps still have some understanding of mental states. "It is time for humans to quit thinking that their nearest primate relatives only react to behaviour," says Dr Josep Call, from the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, who has been studying chimpanzees for many years. "All the evidence suggests that chimpanzees understand both the goals and intentions of others as well as the perception and knowledge of others."
Yet there are sceptics, chief among them Prof Daniel Povinelli of the University of Louisiana. In one of his experiments, chimps were made to beg for food from two researchers. One wore a bucket on his head, which prevented him from seeing the chimps, and the other did not. The chimps begged indiscriminately from both researchers, indicating that they could not understand what the researchers could see.
It was such problems that led Taylor to turn to genetics to understand the mental and genetic differences between ourselves and chimpanzees. For his new book, the television producer trawled through the latest research papers, and discovered that these differences could be far greater than previously thought.
Over the past five years, he points out, our understanding of genetics has become much more sophisticated. While there might only be a 1.6 per cent difference in the genome itself, the way it shapes our minds and bodies is radically different. "The key thing for me," says Taylor, "is that when you compare chimps and great apes with humans you notice how much more gene expression there is in humans."
Gene expression is when certain genes damp down or speed up chemical processes. A team from the Max Planck Institute for Evolutionary Anthropology showed that in human brains, there is a five-fold increase in the rate of gene expression. Other research has shown that more than 90 per cent of the genes in human brains have been "up-regulated" – that is, they have higher levels of gene expression. Most of these genes are associated with the speed of transmission of nerve impulses or energy production to fuel the brain. As Taylor says, "Bigger, faster, greedier, longer-living – that's the evolutionary story of the human brain."
Another genetic difference between us and chimps is "copy number variation". This is where a gene becomes copied, inserted into another part of the genome and yet still works. For instance, GLUD2 is a gene that governs an enzyme involved in nerve signalling in the brain. It is common to all the great apes, including humans – but with us, the gene has been copied, which makes the enzyme work faster. The resulting neurological intensity, says Taylor, "is like swapping a Lee-Enfield rifle for a machine gun".
Along with other genetic innovations, such as inversions, where whole chromosomes are flipped over, and gene splicing (in which one gene controls up to 50 proteins), the gulf between human and chimpanzee brains starts to widen dramatically. "If you add all this up," says Taylor, "the genetic similarity between humans and chimps drops to 87 per cent."
The differences between ourselves and chimpanzees are concentrated in our brains, our immune systems and our metabolisms, suggesting a level of uniqueness that marks us out from other creatures. Yet some do not agree. Professor Frans de Waal, a primatologist from Emory University in Atlanta, has written a number of books, such as Our Inner Ape, in which he argues for a continuum between us and chimpanzees.
"Evolutionary theory shows that there is a continuity between all life forms, including humans and other animals," says Prof de Waal. "Darwin was very clear on this, and modern neuroscience has yet to find any area in the human brain that is not also present in a chimpanzee's. If there is a qualitative jump between ape and human mental capacities, the challenge for Taylor will be to explain how we got there without major changes in the brain, apart from size.
"Certainly, the trend over the past few decades has been the opposite: those who have bet on similarities between humans and other animals have been proven right time after time. Claiming human uniqueness has been a losing battle."
Taylor agrees that chimpanzees "show many fundamentally human skills – to a degree that they have the ability to do maths, think abstractly, demonstrate altruism, make tools and imitate each other. There is nothing humans can do that apes can't do, however simplistically."
But, he adds, "we are talking about the difference between using a twig as a tool and using the internet. It is humans that have speech and language, humans that have culture, art, music, science and technology, humans who remember the past, plan for the future, fear death and pay taxes.
"Sometimes, amid all this scientific talk of genetic and cognitive similarity, we can lose sight of the most important facts." "
'Not a Chimp' by Jeremy Taylor is available from Telegraph Books for £14.99 plus £1.25 p&p