Hawks and other scientists quantified this in late 2007 by comparing more than 3 million genetic variants in the DNA of 269 people of African, European, Asian and Native American descent. They created sophisticated computer models to scour the As, Cs, Gs and Ts of the genome in search of telltale patterns that signal recent adoption of favorable genes.
Their methods rely on the observation that new mutations are not inherited alone, but are passed along in large DNA chunks. Over time, random changes make those chunks smaller. By comparing the length of those chunks in different people, scientists can estimate how long the beneficial mutation has been spreading through the gene pool.
The analysis turned up about 1,800 genes - 7 percent of the genome - that have been widely adopted in the past 40,000 years. Researchers using more conservative methods estimate the number of such genes at 300 to 500.
The function of most genes is unknown. The scientists identified 17 genes for hair cells that are also involved in hearing, and which may have been under selective pressure because they help people identify voices or speak tonal languages such as Mandarin.
And they found a number of genes involved in brain development, including a version of a dopamine receptor gene called DRD4 that is sweeping through the European population. Some think it is a novelty-seeking variant, others that it affects libido. What they do know is that having two copies increases the odds of attention deficit hyperactivity disorder.
Among the genes whose purpose is understood, the biggest category is devoted to fighting infectious diseases. For instance, the researchers found more than a dozen new genetic variants involved in fighting malaria spreading rapidly among Africans.
Scientists had previously identified several mutations that offered protection against the disease. Most were shared by people of African descent, since the scourge is most widespread on that continent. But malaria afflicts people throughout the tropics and subtropics, and additional mutations to combat the disease arose in Thailand and New Guinea, Hawks said.
One of the newly discovered mutations helps defend against a form of the disease in which malaria parasites congregate in blood cells in the placenta, causing a high rate of miscarriage.
Diet is another big force behind recent human evolution. As humans made the transition from being hunter-gatherers to farmers, their bodies had to adapt to new kinds of foods.
