DRD4, a theory
This week’s New Scientist magazine (11th March 2006) has an article entitled “Evolution and Us” Unfortunately the online article is subscriber-only.
There are other, more puzzling examples [of recent human evolution]. One form of the dopamine receptor gene DRD4 has become much more common over the past few thousand years. The rate of increase suggests the gene has been positively selected for, though it’s not clear why: the variant is associated with attention deficit hyperactivity disorder. (p.32)
Running a search on the Genetic Association Database confirms that variations of the DRD4 gene are not only associated with ADHD, but might also (in unreplicated studies) be associated with obesity, dyslexia, schizophrenia, personality traits, temperament, migraine, obsessive compulsive disorder, novelty seeking, mood disorders, and depression. DRD4 mutations have been linked with addictions such as alcoholism and smoking.
UCI says this version of the gene, called 7R (it is a variable number tandem repeat that in this case is repeated seven times) occurred recently, between 10,000 and 40,000 years ago.
This was a period in human history that spanned the ice ages and the dawn of agriculture. During this time humans in the Northern hemisphere lived almost exclusively on animal foods, from cattle to woolly mammoth. Plant foods would have been almost absent in the diet, though much meat would have been preserved by freezing, smoking, and burying in ash. I imagine a tribe of thirty people could get through cows and sheep within day or two of death. I calculate it would take a minimum of 133 days to get through an entire frozen woolly mammoth, which explains why so many were only partially eaten! Human beings have a strong biological instinct to vomit when they smell rotten flesh. As the ice ages retreated, plant foods became available again and humans lived on fertile green savannahs. Fat grains began to grow in abundance in the ashes of human-made fires, and these were gathered, pounded, cooked, and eaten.
UCI theorise that ADHD characteristic novelty-seeking behaviour had some sort of survival or selection advantage at the dawn of civilisation. Yet anyone who has seen a ‘high’ ADHD child knows full well how dangerous such ‘novelty-seeking’ is to their life. It leaves them at great risk of self-injury, and indeed risk of punishment injury from other members of their group subjected to their bad behaviour. Therefore any genetic advantage must have outweighed these factors, either that or the diet was much lower in food chemicals, so the negative effects on behaviour were negated – in other words, there was no eating of leafy greens and fruits.
This particular breed of human must have been behaving in a particularly special and inventive, innovative way for the variant to spread so rapidly through the gene pool.
Food chemical intolerances could even provide part of the explanation. Avoiding potentially poisonous plants and rotten meats due to them making you feel and behave badly would give you a survival advantage over other members of your tribe who carried on eating them and died of food poisoning. Feeling ill with food chemical hangovers would force you to avoid food chemicals. Reactions would have been much clearer in a ‘clean’ Palaeolithic/Neolithic diet.
Being particularly innovative and inventive, this gene may even have pushed you towards eating grains, and corralling and herding animals on the plain for a ready supply of fresh meat and milk, instead of preserving meat for periods of famine.
Agriculture, whilst it was associated with an increase in degenerative diseases, was also associated with a survival advantage in times of under abundance, and an exponential increase in human fertility. Much as grains and dairy are maligned, we would not be where we are today without them. They actually provide useful alternatives to poisonous plants and rotten stuff.