Bruce Hood - The Domesticated Brain - A Pelican Introduction (Pelican Books)

So the number of genes does not reflect the complexity of the animal. The reason we initially overestimated the number of genes for humans was because the role of epigenetics was not yet fully appreciated. Moreover, it turns out that there is more information encoded in the few genes we have than is ever actually used. Only 2 per cent of genes appear to be related to building proteins. This information is only activated when the gene becomes expressed and geneticists now understand that only a fraction of genes are expressed. In fact, gene expression is the exception and not the rule. The reason is that genes are sets of IF–THEN instructions that are activated by experiences. These experiences operate through a number of mechanisms, but genetic methylation is typically one that silences a gene and is believed to play a major role in long-term changes that shape our development. If you think about genes like books in a library and the library is the full genome, then each gene can be read to build proteins. Methylation acts a bit like moving a book out of reach so the information to build proteins cannot be read, or blocking access to it by placing some furniture in front of the book.

DNA may instruct cells how to form and organize themselves to build our bodies but these instructions unfold within environments that modulate their instructions. For example, the African butterfly bicyclus anyana comes in two different varieties, either colourful or drab, depending on whether the larvae hatch in the wet or the dry season. The genes do not know in advance, so are simply switched on by the environment.

Sometimes those switches are social in nature. For many fish, the social environment can play a fundamental role in shaping how genes operate, even to the extent of switching sex. Clownfish live in social groups that are headed up by the top female. What Pixar’s film Finding Nemo did not tell the audience is that clownfish have the potential for transsexuality. When the dominant female in a school of clownfish dies, the most dominant male changes into a female and takes over. Or consider the humble grasshopper. When the population of grasshoppers becomes overcrowded, they change colour, increase in size and become gregarious and socially sensitive to other locusts. This transformation from a solitary grasshopper within a swarm is triggered simply by the amount of physical contact they have with others. 40

Social environments can trigger a metamorphosis in a number of different species, but is there any evidence that social environments regulate human genes in a similar way? The example of the common cold helps to address the question. Social environments increase our susceptibility to the common cold but also influence how we fight it. Colds are more common in the winter months, not because of the lower temperatures (contrary to popular belief) but through the transmission of the virus between people. One reason why the virus may be more prevalent in the winter months is that we tend to congregate closer as the nights draw in, enabling the virus to transmit more readily from one to another. Viruses are small packets of DNA made up of about 10–100 genes that enter our cells and hijack the protein production to make copies of themselves. As this infection multiplies, the normal function of the cells and ultimately the whole of the body comes under attack. However, a virus’s ability to express and duplicate its own DNA is regulated by our own body’s reaction to social stress.

Social stress and isolation have long been known to affect viral infections, which is why we can all do with a little TLC along with our chicken soup when it comes to nursing a cold. 41All this sounds like common sense, but what this folk wisdom reflects is an increasing understanding of the role of social factors in illness. An analysis of the DNA in the white blood cells or leukocytes of lonely adults revealed different levels of gene expression in comparison to adults who were not lonely. 42Specifically, the genes responsible for producing antibodies to infection were downgraded, making their immune response less effective. This may explain why lonely adults are more vulnerable to diseases. What is remarkable is that the different gene expression is only found in those individuals who feel they are lonely and is not related to the actual number of social contacts they have. Even some of the most popular people can still be the loneliest in a crowd because it is how they feel that is more important, rather than the extent of their actual social circles.

If social factors can regulate the expression of viral genes, then our own complement of roughly 20,000 genes is likely to be regulated in biologically significant ways by social factors as well. 43It is not only our biology but also our psychology that affects how we cope with illness.

Lamarck’s daft idea

What is the evidence for epigenetic processes in humans? After all, humans do not spontaneously change sex when a dominant female leaves the group, but critical events can trigger changes in how our genes operate and sometimes the resulting changes in behaviour can be passed on to subsequent offspring. This is an astonishing idea but is not new. In the early nineteenth century a minor French noble, Jean-Baptiste Lamarck, proposed that characteristics acquired during a lifetime could be passed on to the next generation.

In support of this idea, he showed that the sons of blacksmiths had larger arm muscles than the sons of weavers before they ever took part in the family business, which he interpreted as an inherited characteristic. As another example, he suggested that giraffes’ necks became long through their constant reaching up to high branches to eat leaves – a physical trait that they then passed on to their young.

Contrast this Lamarckian notion to Darwinian natural selection. In Darwin’s theory there are two mechanisms that lead to change. The first is spontaneous mutation that generates variations among members of the group. Today, we now understand that this variation arises from genetic processes. Second, the environment operates to select those variations that endow the individual with a competitive advantage to breed and pass on the variation. With successive generations, the variant becomes stable in the population. In the case of giraffes, those born with a mutation that resulted in them having longer necks were more successful in breeding. It was not the experience of trying to reach leaves that was passed on to the offspring, but rather the genes that increased the length of the neck.

Darwin originally suggested that long necks would provide an advantage for reaching more leaves, but it turns out that there are a number of competing explanations. 44What is known is that the mechanism of inheritance is not Lamarckian. Rather, long necks originated as a genetic mutation that was passed on while giraffes with short necks did not get the same opportunity to reproduce for some reason. Lamarckian theory has been roundly denounced as daft in scientific circles but epigenetics is casting new light on his ideas. Maybe experiences during a lifetime can influence the biology of the next generation.

There are so many problems and errors with Lamarck’s evidence that it would be all too easy to consign the notion to the dung heap of bad ideas. Moreover, Darwin’s theory of evolution by natural selection is simply better at explaining and predicting the data. And yet aspects of Lamarck’s daft idea have been resurrected with the rise of epigenetics. Sometimes events during one’s lifetime can affect the next generation. Epigenetics explains how environmental signals change the activity of genes without altering the underlying sequence of the DNA. The process of natural selection will ultimately correct any epigenetic influences of the environment. Rather, the effects are more to do with the switches that are being flipped by epigenetic processes. So Lamarck may have gained a minor battle, but Darwin has won the war in explaining how we pass on characteristics from one generation to the next. Epigenetics may even explain why humans traumatized as infants grow up with an emotional legacy that can stay with them for the rest of their lives. Once again, studies of the rearing practices of generations of laboratory rats have shown how early experiences shape the bond between mother and daughters.