The broader theme of the book is inheritance and how we theorise it. In order to understand the ramifications of genetics, Kenneally looks into psychology, history, economics, neuroscience, linguistics, pop culture, what ordinary people think about things, and much more. She has a nice way with words, and combines first-person observations, interviews, and musings with expert rendering of complex scientific theory into ordinary language. Her goal seems to be for us to stop moving blindly into our new DNA-defined era, drop the cultural and intellectual baggage which has served us inadequately anyway, and really examine the ramifications of this mind-boggling frontier. Why are people, who are almost identical genetically, skin colour notwithstanding, so different culturally? Is race a purely social construct, and, if so, why did it become so psychologically and politically influential? Why is Ancestry.com so popular? What would you do if you were offered the chance, via DNA testing, to estimate when you would die and of what? The questions are wide-ranging.

Christine Kenneally (photograph by Nicole Cleary)

In a chapter titled ‘Ideas and Feelings’, for example, Kenneally examines the long-term effects of the slave trade on the economic and political concept of trust, increasingly important in a globalised world. She describes the history of wholesale abduction, and the trade across Arab lands and to America, and then probes further. Benin, for example, was a fertile source for slave trading and has very low levels of social trust today. Kenneally digs deep. ‘Did the slave trade give rise to a culture of mistrust that was passed down from the slave era even to individuals who live in the same places today?’ she asks. Or, ‘Perhaps the slave trade made people not less trusting but less trustworthy ... After all, chiefs turned on their own people, and families sent some of their own literally down the river.’ The echoes continue down the centuries. Leonard Wantchekon, a Benin-born politics professor at Princeton, worked with Harvard economist Nathan Nunn to examine Africa’s poorest countries. Sure enough, there is a correlation between a history of slavery and poverty today. ‘Modern Africans whose ancestors lost the most people to slavers distrusted not just their local government and other members of their ethnicity, but also relatives and neighbours,’ Kenneally writes, ‘much more than Africans whose ancestors were not as exposed to the slave trade.’ This, the economists surmise, profoundly affects the conduct of politics and business in different parts of Africa today.

In the same chapter, Kenneally examines anti-Semitism in Europe, from the scapegoating of Jews during the Black Death in the fourteenth century, to the death camps of World War II. Again, she found ways of thinking eerily enduring. Two German economists, Nico Voigtländer and Hans-Joachim Voth, compared records spanning six hundred years in Swiss and German towns. Again, they found a correlation between places exhibiting the most virulent anti-Semitism just before World War II and places that most enthusiastically pursued medieval pogroms.

Curiously, these social attitudes map onto genes – as correlations, not causes, but still in a way that is tremendously intriguing for our understanding of human history. The internal layering of the genome demonstrates even more than growth rings in trees do. By displaying genetic change over time, it can track not only the obvious results of interbreeding, but of population movements, biological catastrophes, accidents and coincidences. These, in turn, throw up fascinating historical questions. Are people who stay put within relatively closed communities more likely to transmit feelings and ideas, as well as DNA, than people who range widely across the world or who live in large, culturally and genetically mixed cosmopolitan cities? Do people take their original prejudices with them when they move, or do they adopt the mores of their new home? ‘It seems,’ Kenneally writes, ‘that both families and social institutions matter but that the former is more powerful ... Indeed, Nunn and Wantchekon found evidence that the inheritance of distrust within a family was twice as powerful as the distrust that is passed down in a community.’

Kenneally’s book is so wide-ranging that trying to précis its many pathways is impossible. Every chapter contains fascinating details that lodge in the mind. I did not know that eighty-five per cent of us carry DNA from Neanderthals, which shows that interbreeding occurred between ‘our’ species and theirs. In fact, the fifteen per cent of people who do not have it are Africans, which suggests that the interbreeding occurred when humans fanned out from Africa into Europe and Asia. Those travellers were, one might say, adventurous in more ways than one.

Another fascinating snippet concerns the difference in thinking between East and West, way beyond the broad-brush clichés that the mysterious East is more interdependent and holistic, the logical West more individualistic and rational.

Ester Boserup

The Danish economist Ester Boserup suggested in the 1970s that the invention of the plough might have affected the way sex differences are viewed. Ploughs require a great deal of upper-body strength to manoeuvre, while the handheld tools used for shifting cultivation previously did not. This led to the specialisation of men working in the fields and women tending the home, and, eventually, that fell into the naturalistic fallacy: because that is the way it is, that is the way it ought to be.

Nathan Nunn, looking into that research, found that people in traditional wheat-growing areas today have weaker beliefs regarding gender equality than people whose ancestors had grown sorghum or millet. He pushed the concept into more abstract ways of thinking, asking subjects within China to pair two of three pictures: of a dog, a rabbit, and a carrot. People from the Western wheat-growing area paired the dog and the rabbit, an analytic decision based on category. In the East, where people grow rice, they were more likely to pair the rabbit and the carrot, a relational judgement. These decisions had nothing to do with whether the subject still worked the land: they seem to have inherited their farming forefathers’ views.

There is a great more science in Kenneally’s book than these brief remarks suggest. She pushes C.P. Snow’s ‘two cultures’ discussion to the nth degree, dissolving barriers persuasively by showing, not telling. The result is shape-shifting, often dissonant, and thought-provoking. Even that uncrackable old philosophical nut, freedom versus determinism, becomes malleable in her hands.

Kenneally is an advocate for personal genome testing: not in order to find out who or what we are, but to sketch in parameters that may help us to make important decisions. We only have one life, after all, and should try to maximise its potential. Some people may not be able to handle the results: she talks to people who have and who have not chosen to test for Huntington’s disease, a deadly and distressing disease with strong genetic transmission. But she also reminds us that building blocks do not define the finished structure. Family, community, government, the food we eat, our personal idiosyncrasies, all change the calculation. We humans are, Kenneally writes, ‘creatures of changeless truths and of interesting possibilities’.