by Dorothy H Crawford
The human genome sequence, published in 2003, is revealing a multitude of secrets. And when the chimpanzee genome followed in 2005, we discovered just how similar humans are to our closest living relative.
Now scientists have a draft genome sequence from the extinct Neanderthals (homo neanderthalensis) and are poised to clear up many long-debated issues, not least how like us our ancient cousins really were.
About 100 years ago, the anthropologist Sir Harry Johnston described Neanderthals as "gorilla-like monsters, with cunning brains, shambling gait, hairy bodies, strong teeth, and possible cannibalistic tendencies". And 50 years ago, in his novel The Inheritors, William Golding portrayed the Neanderthal hero, Lok, as a gentle giant. Can we now expect an accurate picture of our mysterious cousins?
Modern humans (homo sapiens) and Neanderthals split from a common ancestor in Africa some 500,000 years ago. Neanderthals colonised Europe, and, from around 130,000 to 28,000 years ago, roamed an area stretching from Spain to Siberia, including southern Britain. Modern humans arrived in Eurasia from Africa about 45,000 years ago, and the cousins co-existed in Europe for about 15,000 years before Neanderthals became extinct.
Neanderthals looked very similar to modern humans, although generally they had larger, stockier and more muscular bodies, shorter arms and legs, larger heads with prominent brows, large noses and reddish hair. They had bigger brains than modern humans, but exactly how intelligent they were is not entirely clear. They certainly made stone hunting tools, but there is no evidence of activities such as cave paintings or self-adornment until some 45,000 years ago. Then, they began to make bone tools and jewellery, but since this date coincides with modern humans' arrival in Europe, these may have merely been copied from the invaders.
With regard to brain size, humans develop massive brains while in utero and these continue to grow for several years after birth. But homo erectus, the direct ancestor of Neanderthals and humans from three million years ago, resembled chimps in doing most brain-growing in utero, and so grew up much faster than humans.
Humans have also evolved an extended skill-learning period during adolescence and, overall, take twice as long as chimps to reach adulthood. But what about Neanderthals? A recently discovered fossilised one to two week-old Neanderthal baby who died in Crimea about 70,000 years ago, as well as two infants aged 19 and 24 months from Syria, provide the answer.
It seems Neanderthal babies were more similar to humans than to homo erectus and chimps, with larger heads at birth. There is also some evidence Neanderthals had an extended childhood, indicating, if true, that prolongation of the nurturing period began during the evolution of the genus homo.
Another debate about Neanderthals is why they became extinct. Scientists previously thought they were killed off by humans, but as they co-existed for 15,000 years, and fossil records show no evidence of violent deaths, it seems more likely they were out-competed by humans, or were less able to survive the ice age that peaked 25,000 years ago.
Paleogeneticist Svante Pääbo and his team from the Max Planck Institute in Germany, may soon provide the answers, as they have undertaken the massive task of sequencing the Neanderthal genome. This is a daunting project, not just because of its scale, and the fact the DNA is old and decayed, but also because the material is contaminated by DNA from microbes and modern humans handling the specimens.
Despite these problems, Pääbo is confident he now has a draft DNA sequence derived entirely from 38,000 year-old bone fragments from two female Neanderthals found in Croatia. So far, comparison of three billion human and Neanderthal DNA bases has thrown up a mere 1,000 to 2,000 changes, compared with 50,000 between humans and chimps. Already, scientists are pretty sure Neanderthals and humans did not interbreed, and they ultimately hope to find out how intelligent Neanderthals were, and why they became extinct.
Dorothy H Crawford is a professor of medical microbiology at Edinburgh University.