Many types of human evolved ‘to strut and fret their hour upon the stage’, but only Homo sapiens remains standing. How did we come to rule the roost? Were our forebears brighter or more industrious than their competitors? Did we cheat in the great survival exam by killing off the competition? Is it even possible that we slept with the enemy for our own advantage?
Michael Hammer, population geneticist at the University of Arizona, thinks that we did. Writing in the May edition of Scientific American, he argues that having sex with our rivals “played a key role in the triumph of our kind”.
The first humans evolved in Africa and spread throughout much of the Old World. Homo erectus arrived on the scene around 1.8m years ago. He was recognisably human. By 200,000 years ago, Homo sapiens had evolved. For the next 160,000 years, our forebears would not be alone; they shared the planet with other descendents of erectus, including Neanderthals, Flores Man and the mysterious Denisovans.
How we survived when all the other hominids perished is much debated. There are two schools of thought. ‘Out of Africa’ supporters argue that our ancestors left the dark continent in a series of migrations. Encountering Homo erectus descendants which had migrated previously, they exterminated them, or out-competed them for resources. Analysis of mitochondrial DNA from Neanderthals seems to support this ‘replacement model’; their genetic line differed from ours. In the 1980s, Alan Wilson, of Berkeley, identified Mitochondrial Eve, who lived around 200,000 years ago and from whom every person alive today is descended. Analysis of the Y-chromosome, handed down intact from father to son, seemed to show that a common male ancestor, Adam, lived around 100,000 years ago.
Supporters of the alternative ‘multi-regionalist’ theory claim that, on leaving Africa, our forebears didn’t exterminate the opposition; they bred with them. This might explain why people from various parts of the world appear so different from each other, after as little as 50,000 years of separation. The first DNA analyses didn’t support this ‘assimilation model’ but, in 2006, Hammer’s laboratory isolated sequences in modern human nuclear DNA which appeared to come from Neanderthals. By 2010, the Neanderthals’ genome had been largely reconstructed. It turns out that between one and four per cent of our genes come from them. Those archaic first Europeans did not become completely extinct; they are now part of us.
In 2008 Russian scientists, working in the Denisova Cave in the mountains of Siberia, found part of the fifth finger of a lady they named the ‘X Woman’. Carbon dating showed that she lived around 40,000 years ago. The ice age climate preserved her DNA. Mitochondrial analysis showed that X was not ‘one of us’. Nor was she a Neanderthal. Did her kind, the ‘Denisovans’, disappear or were they assimilated?
Subsequent analyses suggest that up to 6% of the genes of native Australians alive today are Denisovan. Neanderthal genes were also found.
Their ancestors, it seems, had ‘affairs’ with Neanderthals and subsequently with Denisovans.
The ancestral story, however, is far from complete. An American of Cameroonian descent, living in South Carolina, submitted his DNA recently to a genetic testing company. His Y chromosome lineage, according to Hammer, turned out to be much older than the 100,000 years recorded for everyone else. A paper published in March suggested that ‘Y-chromosome Adam’ lived between 231,000 and 581,000 years ago.