Chris Stringer

LONE SURVIVORS

He starts with a historical review of the advances in our knowledge of human evolution that have taken place since Darwin's time. In those days there was little evidence available apart from the Neanderthal fossils. A further difficulty was that although the relative ages of fossils could be deduced from the layers in which they were found, there was no way of dating them accurately. But this is now possible thanks to a range of laboratory techniques that can take us far back into the past. Stringer has a chapter describing these methods.

The fossil picture started to change in the first half of the twentieth century, and the number of discoveries has increased dramatically since then. And surprises continue to turn up—for example, the 'Hobbit' in Flores and the Denisovans, both of whom are difficult to accommodate in our present understanding. And detailed examination of fossil anatomy can reveal surprising information. For example, it is now possible to infer, from the thickness of their foot bones, whether individuals were wearing footwear or were barefoot. The Cro-Magnons had shoes, apparently, and so did early humans in China, but the Neanderthals did not.

Modern scanning techniques can tell us about the internal structure of bones without the need to damage or destroy the specimen. It appears that the middle ear bones of the Neanderthals were similar to our own, which suggests that their hearing was also similar. But the semi-circular canals in their inner ears, which are responsible for balance and motion detection, were different from ours. The reason for this is unclear; one idea is that it could be related to their thicker, more muscular, necks, which may have altered how they adjusted their head position and balance.

Teeth contain incremental lines similar to tree rings, which are visible microscopically and can tell us the rate at which the teeth grew. This, in turn, allows us to infer how long childhood may have lasted in our ancestors, by examining the unerupted teeth of children who died at different ages. The Neanderthals probably had shorter childhoods than modern humans. This, in turn, may indicate different social and cultural patterns in the two groups.

An important development in the study of human origins has come from modern genetics, and Stringer has a chapter on this. We can now infer a lot about our ancestors' migration patterns after their exit from Africa. It has also become possible to extract DNA from Neandethal bones, and this has brought about the dramatic recognition that people outside Africa have a small percentage of Neanderthal DNA, presumably owing to hybridisation. But the picture is more complicated than this implies, because there is also evidence of hybridisatio with earlier types of humans in Africans. So some of the admixture of ancient genes that we find in modern humans may have come with them when they left Africa. What effects, if any, these genes produce is unknown.

This brings us to the 'Out of Africa' theory. Stringer still thinks that this is broadly correct, but the situation is more complicated than at one time appeared, and he now prefers 'Recently Out of Africa', to reflect this. Earlier species, such as Homo erectus and the ancestors of the Neanderthals, left Africa long before modern humans had even evolved, and to make matters still more complicated, some of them may have migrated back to Africa later. But the ancestors of all modern humans outside Africa did leave quite recently, about 55,000 years ago, in one or perhaps two quite small groups.

There is still doubt about the exit route taken by these early explorers. The southern end of the Red Sea is a current favourite, but Stringer doesn't mention this, finding instead that the most likely course would be the 'common sense' one provided by the Sinai and the Levant. The eastern end of the Mediterranean, from North Africa to Gibraltar, is also a possibility, although there is no archaeological evidence to support this idea.

Stringer dislikes the term 'racial' to describe the visible differences in skin colour,hair, facial appearance, and build of modern humans, preferring 'regional' instead. The genetic differences underlying these appearances are very small. Some of them are probably adaptive, for example to different climates, but they may also be cultural, based on sexual preferences. They can also be due to genetic drift—in other words, the outcome of chance events. It is likely that at least the later Neanderthals had light skin and reddish hair, as an adaptation to prevent rickets caused by low levels of sunlight in the north, but this was produced by a different mutation from the one that causes white skin in modern people. Blue eyes are also a later development, favoured, Stringer thinks, by sexual selection.

Like the late Stephen J. Gould, Stringer does not find that there was anything inevitable about our own existence, whjch may in part be the outcome of chance. Given a slightly different course of events the world might be populated by Neanderthals, Denisovans, or any other of the numerous human species that have existed; or none of us might be here at all.

This is probably the best, most balanced, and most comprehensive book on human evolution for a general audience that is presently available. I hope Stringer will decide to bring out a new edition in a few years, to keep track of the rapidly changing face of research.

16 February 2013