The current New Scientist contains an article Our Asian Origins by Colin Barras which suggests that our ancestors might have moved from Africa to Asian and then moved back again and illustrates it with a modified classic human family tree – when evidence is rapidly accumulating that such a view is over-simplistic.
|A typical classic human family tree|
with no cousin links.
From The Quest for Human Origins
In order to understand human evolution we need to get a grip on the mechanisms – and to realise that we will never have more that a fraction of a millionth of the information we would need to get a complete picture, and the fragments of bones we find represent only some of the environments our early ancestors lived, with some environments being totally unrepresentative. Remains from different places with different features and from different dates are given different species names – but this does not automatically mean that the living creatures could not have interbred, had it been possible for them to meet.
We need to realise that every step in the human family tree will involve couples who are cousins (or closer) - although in some cases the cousin relationship could span many generations. Some years ago I did a calculation to show that every English person is virtually certain to be descended from everyone who was living in England in 1066 – so all the English are cousins if you go back about 40 generations. The current races of mankind go back perhaps 2000 generations and despite differences are definitely members of the same species. Our Neanderthal and Denisovan cousins represent perhaps 20-40 thousand generations. Our great ape common ancestors are perhaps 500 thousand generations back. We know that our ancestors interbred with the Neanderthals and Denisovans – which suggests that groups that became separated can interbreed successfully when separated by many 10s of thousands of generations.
But evolution can changes species quite faster than that in selected directions – for instance adult lactose tolerance in races which drink animal milk evolved in less than 200 generations. Other features such as the number of vertebra in the neck have remained the same over millions of generations. So what would happen if two groups separated for a long period meet and interbred. For instance during a 500,000 year separation a tool-making group could have evolved more dexterous hands, while the pursuit hunting group might be able to run upright faster. Some of their children would have the benefit of both more useful hands and greater mobility and hence have a big advantages in the survival stakes. The differences might be such that scientists later finding bone fragments assign a new species name, especially if the new remains were incomplete and not in the home territories of either of the parental groups.
I have earlier looked at the evolutionary pressures on the African Plains and it is likely that our early ancestors split into groups by developing cultures resulting to different types of environments and food sources. (A similar effect can be seen with animals with a complex social structure such as orca which uses very different hunting techniques in different parts of the world adapted to different prey.) Evolution will tend to select not only at the individual animal level, but also at the group level, favouring groups which are best at exploiting their environment, and changing when the environment changes. Some groups may have been, on occasions, more mobile, and some undoubtedly moved out of Africa into Asia and Europe, and many generations later moved back, and interbred with their distant cousins.
While it is clear that many groups will have evolved in such a way that they definitely represented different non-interbreeding species, there may soon be sufficient evidence to allow future evolutionary human family trees to show splits and reunions – with the different branches contributing in different ways to Home sapiens.