Why do kids have problems with attention deficit disorder and hyperactivity? I've long believed that surely these behaviors must have had evolutionary value or else they wouldn't exist. In other words, use of Ritalin and similar drugs to tame kids amounts to trying to put a damper on the genetic nature of hyperactive humans. Our problem is that we now live in technologically created environments which we did not encounter in our evolutionary past. Therefore in many cases we are not adaptive to our new environments. Well, finally some really detailed evidence to support my hunch: Kenyan nomads do better with an ADHD gene whereas those who have converted to settled living do worse with this same version of the DRD4 gene.
A propensity for attention deficit hyperactivity disorder (ADHD) might be beneficial to a group of Kenyan nomads, according to new research published in the open access journal BMC Evolutionary Biology. Scientists have shown that an ADHD-associated version of the gene DRD4 is associated with better health in nomadic tribesmen, and yet may cause malnourishment in their settled cousins.
Different versions of a receptor for the neurotransmitter dopamine make people more or less hyperactive.
A study led by Dan Eisenberg, an anthropology graduate student from Northwestern University in the US, analyzed the correlates of body mass index (BMI) and height with two genetic polymorphisms in dopamine receptor genes, in particular the 48 base pair (bp) repeat polymorphism in the dopamine receptor D4 (DRD4) gene.
The DRD4 gene codes for a receptor for dopamine, one of the chemical messengers used in the brain. According to Eisenberg "this gene is likely to be involved in impulsivity, reward anticipation and addiction". One version of the DRD4 gene, the '7R allele', is believed to be associated with food craving as well as ADHD. By studying adult men of the Ariaal of Kenya, some of whom still live as nomads while others have recently settled, the research team investigated whether this association would have the same implications in different environments.
While those with the DRD4/7R allele were better nourished in the nomadic population, they were less well-nourished in the settled population. Although the effects of different versions of dopamine genes have already been studied in industrialized countries, very little research has been carried out in non-industrial, subsistence environments like the areas where the Ariaal live, despite the fact that such environments may be more similar to the environments where much of human genetic evolution took place.
Eisenberg explains, "The DRD4/7R allele has been linked to greater food and drug cravings, novelty-seeking, and ADHD symptoms. It is possible that in the nomadic setting, a boy with this allele might be able to more effectively defend livestock against raiders or locate food and water sources, but that the same tendencies might not be as beneficial in settled pursuits such as focusing in school, farming or selling goods".
These findings suggest that behavior differences previously associated with the DRD4 gene, such as ADHD, are more or less effective depending on the environment. Research into how this might occur in Ariaal children is planned in the near future.
Think about a nomad guarding his herd. If he can focus in on one thing he might not notice predators or raiders approaching. If he can't sit still he is more likely to spend his time looking around and will notice more of his environment. If he's more distractable by movement he sees out of the corner of his eye he's more likely to notice livestock wandering off or a threat.
Or look at geeky guys who are smart at math and physics but not strong. The development of computers and other complex technologies have made them far more adaptive than they used to be. Highly coordinated muscular guys have become relatively less valuable in the job market as robots and hydraulic heavy duty equipment do increasing portions of the heavy lifting and manipulation. Women can do a larger fraction of all jobs because hard manual labor has dwindled as a portion of all work.
The continued rapid decline in costs for DNA sequencing and DNA testing will lead to a torrent of studies such as the one reported above. We will learn the identities of many more genetic variations that affect behavior, intellectual abilities, and physical abilities along with the distribution of these genetic variations around the world. Our picture of humanity is on the verge of radical change. These discoveries will finally demonstrate how evolution is the most powerful force shaping humanity.
|Share |||Randall Parker, 2008 June 09 08:49 PM Brain Evolution|