Extended Heredity: A New Understanding of Inheritance and Evolution ed. by Russell Bonduriansky and Troy Day (review)

Nature or nurture? Is a person born knowing, or do they learn to know? Science today has gone far beyond such a 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. by 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, nongenetic transmissions (soft inheritance) of acquired traits from parent to child, often lasting for many generations. It potentially includes not only the embryological development of the child and its subsequent behavior but also 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 a 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, 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 a lactose tolerance gene in the DNA of 6,000-year-old human remains from Germany and Lithuania (and other findings) as evidence that it did not exist and therefore the gene for the digestion of milk had to be recent—as a result of an increased "mutation rate" in cattle farming societies. This argument does not indicate how the 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 yogurt 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 due to this lack of firm evidence that nongenetic 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...