Review of Extended Heredity: A New Understanding of Inheritance and Evolution
Princeton University Press, Princeton and Oxford, 2018
280 pp., illus. 40 b/w,
Nature or nurture? Is a person born knowing, or learns to know? Science today has gone far beyond such simple dialectic. Extended Heredity explains how by asking another question: Could acquired experiences of an individual be passed on through the reproductive process, apart from her nuclear DNA?
The answer here is a nuanced yes. Nature and nurture are complementary, and a continuum: the DNA of a baby is only a ‘musical score’ to be read and interpreted according to the environment it enters. It starts at conception, when the fertilized egg’s new nucleus containing DNA from both parents is still surrounded by cytoplasm of the parental phenotype. Experiments have shown that male mice that have been traumatized (e.g. early separation from parent), transmit small non-coding RNAs in their sperm cytoplasm to the baby. These mice grow up with depression symptoms that are passed on to their own offspring. A similar situation occurs with obese fathers. This is epigenetics -- a young science that is rapidly changing our understanding of heredity.
Extended Heredity here comprises epigenetics as well as many heritable, non-genetic transmissions (soft inheritance) of acquired traits from parent to child, often lasting for many generations. It potentially includes not just the embryological development of the child and its subsequent behaviour, but cultural and educational impacts throughout life. Bonduriansky and Day cite many social customs that are the direct results of gene-culture interaction, such as lactose tolerance in hunter-gatherer societies. Most adult mammals lose their ability to digest milk sugar (lactose). But in some Northern Europeans, the lactose tolerance gene persists into adulthood. There is advantage in using milk as source of nutrient as its supply becomes more reliable with farming. The authors suggest a scenario where such tolerance could have started with soft inheritance, and only later being incorporated into the more permanent DNA – giving soft inheritance an evolutionary role. This is a neat theory but needs verification by molecular experiments showing the chemical pathways. Also, pinpointing the gene’s first occurrence and its evolutionary frequency in different cultures is difficult if not impossible. Ancient DNA is not reliably available for an entire population. For example, the authors cite the absence of lactose tolerance gene in the DNA of 6,000-year-old human remains from Germany and Lithuania (and other findings) as evidence it did not exist and therefore the gene for the digestion of milk had to be recent – as a result of increased ‘mutation rate’ in cattle farming societies. This argument does not indicate how mutation rate could have been increased by the environment. It also fails to consider the possibility of lactose bacteria in our body’s abundant microbiome, and the consumption of bacteria-fermented yoghurt in many cultures for millennia -- as these would have enabled lactose intolerant individuals to consume dairy products. We now know that the body’s microbiome can be passed through the vagina during birth and is a form of soft inheritance – but the authors do not discuss this in connection with lactose tolerance and evolution.
For now, it is for this lack of firm evidence that non-genetic inheritance is still fighting an uphill battle, I believe. The idea of ‘hard’ inheritance: that DNA, sequestered inside the nucleus of the cell is the master of an individual’s destiny, is still dominant in the public imagination – a frequent lament in the book. Here, perhaps, citing an example, or a survey of people’s attitudes could replace many repeated generalities. The story of soft inheritance is a convoluted one, littered with weak and biased experiments in its early days, many by experimenters aiming to prove a point, often political. And the opposing gene-centric view did no better. To its credit, the book supplies abundant examples on both sides.
One tragic consequence of the gene-centric view was the popularization of eugenics – that it was possible to produce a superior race by selective breeding, a sort of ‘purification’ of the genepool. Francis Galton, a cousin of Darwin, is held largely responsible for fostering the idea – leading to the Nazi atrocities and the many horrors of the twentieth century. The credibility of soft inheritance was also dealt a severe blow by the spectacular fraud embraced by Stalin in the Soviet Union: Lysenkoism. Lysenko was not a scientist but rejected genetics (therefore eugenics) as evidence of Western corruption in order to promote Communist ideas of social engineering – that people and plants (mostly food plants) can be transmuted to become more productive. This scandal hardened the dominance of genes. As a result, the gene-centric view of heredity prevails to this day, especially in the public eye.
Evolution biologists Bonduriansky and Day employ mathematical modelling to predict natural selection of certain behaviours and their competitive advantages. Such predictions become difficult to prove in actual populations, especially those long perished. But molecular biology today, being able to visualize gene expression in neurons while an animal is performing a behaviour, like parenting, could prove a promising tool in future research (the MERFISH Protocols, 2018). Thus, the tone of the book appears to me unnecessarily pessimistic about the recognition of soft inheritance. In addition to ample evidence from this book and other publications, it has a long history going back to Greek and Roman times: fetal alcohol spectrum disorders (FASD), now widely acknowledged. FASD concerns in 18th century London also prompted William Hogarth’s etching, the ‘hellish’ Gin Lane (1751, Tate Gallery) and the Gin Act by parliament the same year to reduce alcohol sale.
An expanded view of heredity can no longer be ignored.