Steve Olsen’s book is a history of the genetic marker mitochondria transmitted to both sexes though the mother. Mitochondrial DNA can be found in a number of fossilised human remains but most forms have become extinct along with the early forms of human in which it is found.

Olsen is primarily interested in the origin of modern mitochondrial DNA – that found in present day humans. Marshalling the arguments of a number of historical geneticists he traces the emergence of this back to Africa between 200,000 to 150,000 years ago. At this time, he argues, a genetic bottleneck occurred in the sense of the winnowing of the total mitochondrial DNA and the emergence from a small section of humanids of the ancestors of modern man. Why they survived and other human types did not, a problem to which Olsen gives some thought.

Olsen argues how, from its origins in Africa, the first modern humans migrated outwards to populate the Middle East, Europe and Asia. The human race overwhelmingly shares its genetic inheritance but migration, isolation and adaption produce changes in DNA over time and it is from their identification that a set of hypotheses about the history of the spread of humans and their divergence can be constructed. Each generational modification in DNA allows a roughly approximate time scale of migration to be constructed. 65,000 years ago modern humans had reached Australia, 40,000 years ago they moved into eastern Asia, 10,000 years ago into the Americas. However, interpretation differs among paleogeneticists. Other schools suggest the possibility of multi centre origins of the modern human and often a timescale for the appearance of modern human extended considerably backwards. But Olsen argues the hypothesis he puts forwards represents the existing consensus.

Interestingly enough Olsen, with the help of genetic analysis, treats what historians have typically seen as foundation myths with a great deal more respect than the average historian. For example he recounts the genetic evidence for the special status as sons of Aaron – Moses’s brother – claimed by the traditional priestly caste among the Jews. The Lemba people of Southern Africa can, as they maintain, trace their Jewish cultural practices back to real Jewish ancestors. Historians should start treating the ramblings of the elders of the tribe with greater respect. Also Olsen argues from a study of the Samaritans in Israel, whose numbers at one point were reduced to 150, that small inter-breeding populations, whilst they produce likeness and some genetic diseases specific to the group, also produce healthy human beings. Without that the spread of humankind might have been severely handicapped.

Cultural practices and genetic character are not identical but strong cultural identity together with geographical isolation can cause genetic differences. But this is measured by very small amounts of total DNA. Moreover Olsen points out that the strength of cultural practices themselves cannot, given migration and location, produce this effect. Further, cultures have lives of their own – many a cultural group in decline has strengthened itself by adoption or co-option often from peoples genetically different. This means the relation between genes and cultures is not amenable to over schematic generalisations.

Genetic variation among humans often has adaptive value. I remember one past Galton Institute Conference at which the possible adaptive value of the gene producing the disease cystic fibrosis – from which Europeans in particular suffer – was discussed though what this adaptive value is, or more likely was, was certainly not understood at that time. The greatest adaptive values of all are language and social organisation which spread faster and respond more flexibly to change than genes. For a slow breeding species like humans which requires quite a lot of food to sustain it these are important evolutionary developments though not it seems for other forms of life. However, before we get carried away with ourselves what also emerges from Olsen’s book is the very slow and conservative nature of human social evolution. Thousands and thousands of years of slow development, low population and simple technology punctuated by periods of rapid change about describes it.

Greta Jones