A Role for Junk DNA
In Chapter 1 we see that 97% of human DNA does not encode proteins and is referred to as "junk" DNA. At least some junk DNA is known to regulate the activity of genes, but demonstrating its effects in humans has proved difficult. When researchers inserted a stretch of this DNA called HACNS1 into mouse embryos it turned on a gene in the developing upper limbs and in the forearm area corresponding to the wrist and the thumb. DNA from chimps and monkeys affected only the upper limb area. HACNS1 has 16 "letter" variations that are unique to humans; the researchers think it contributed to human manual dexterity, including our unique opposable thumb, which has been credited for a number of distinguishing human accomplishments. It would also be possible to track the effects of junk DNA in the brain, but relating those effects to human cognitive abilities would be extremely difficult. Science, Vol 321, 1346-1350; click here for image.
Why We Differ from Chimps
Chapter 1 also indicates that one reason we differ from chimpanzees, with whom we share 95% to 98% of our DNA sequences, is because of which genes are turned on and which are turned off. Now another possible reason has emerged in the form of copy number variations (CNVs). An individual does not necessarily have the expected pair of a particular gene; with CNVs, the genes may be deleted or they may be duplicated, resulting in more than one pair. CNVs vary among individuals, but researchers who compared the genomes of 60 humans with those of 60 chimps found 92 variations that distinguished between the two species. Determining the impact of these differences will, of course, require further work. Genome Research, Vol 18, 1698-1710.
The Mind-Brain Problem Revisited
Most neuroscientists are material monists, believing that "mind" is no more than the term we use to refer to the brain's activities. This does not mean, however, that the issue is dead; dualists are capitalizing on neuroscience's "hard problem", the attempt to explain consciousness in terms of neural activity. They point to research such as a study from the 1990s in which people with obsessive-compulsive disorder were able to modify neural activity in a part of the brain implicated in the disorder by changing their thought patterns. Critics attribute the result to the material brain changing the material brain, but the dualists believe it indicates the influence of mind. This new effort is being mounted by creationists, who believe such results undermine both material neuroscience and the evolutionary explanation of complex human behavior and intelligence. New Scientist, October 24, 2008,
Heritability: A Blend of Heredity and Environment
This chapter's discussion of heredity makes the point that heritability is not an absolute measure, but rather the percentage of the variation in a characteristic that is attributable to genes; if environmental influence increases, the proportion of genetic influence decreases, and vice versa. A new study that found a genetic influence on age at first intercourse provides a particularly good example. The age at first intercourse was more highly correlated in pairs of identical twins reared apart (.34) than in pairs of fraternal twins reared apart (.22). What is more fascinating is that the correlation was .23 among identical female twins born before 1948, but increased to .54 for those born after 1948. Personality and Individual Differences (in press). Similarly, an earlier study of reared-together twins had estimated heritabilities of 32% among female twins over age 40 and 49% among female twins under 40. Psychological Science , Vol 8, 211-216. The genetic makeup could not have changed noticeably over a few decades, so the increased heritability must be due to a decrease in environmental influence—most likely in the form of greater sexual permissiveness. So what explains the hereditary effect? A third study traced it to a version of the gene for the D4 dopamine receptor that has been linked to impulsive, risk-taking behavior (Demography, Vol 43, 747-769).
