The ability to spoon down ice cream or chug a milkshake might not seem like an evolutionary advantage in our weight-conscious society. But scientists say that 5,000 to 10,000 years ago, around the time dairy farming began in Northern Europe, natural selection encouraged the spread of a genetic mutation that enabled adults to digest the sugars in milk. Those with the new gene-lactase-had a nutritional advantage over those who lacked it, so they proliferated, along with the mutation. This theory was first proposed in 1970, but scientists at Harvard Medical School demonstrated the genetic proof just last year.
Now geneticists are about to get a powerful new tool that should make it much easier to find such direct evidence of evolution in modern humans. This month, a three-year, $100-million venture called the Haplotype Mapping Project, or HapMap, will complete a massive public database of genetic variation in our species. The first HapMap-derived papers are due out soon, and scientists are abuzz about the prospects. â€It´s going to be very exciting,â€ says Christopher Wills, a biologist at the University of California at San Diego. â€It will allow us to understand ourselves at a far greater depth than we ever have before.â€
In recent years researchers have begun to uncover genetic evidence of mutations that conferred survival benefits in certain populations. In 2004, for instance, University of Utah geneticist Stephen Wooding and his colleagues showed that a mutation in the bitter-taste receptor, which popped up several hundred thousand years ago, helped humans migrating out of Africa avoid unfamiliar toxic plants. Wooding´s work and other discoveries like it stem from the completion of the Human Genome Project in 2003, which has provided a map that enables scientists to pinpoint key genes. But, Wooding notes, research like his focuses on single genes. Until now, geneticists hoping to study multiple genes in many different populations have been out of luck.
This is where HapMap comes in. If the sequenced genome is a massive
single-volume encyclopedia, HapMap breaks it up into hundreds of easy-to-read books. A strand of DNA comprises roughly three billion chemical units, known as A, T, C and G. Generally speaking, we all have the same letters in the same slots along that strand, which is why we resemble one another. Variations called single-nucleotide polymorphisms, or SNPs, however, pop up every 1,200 slots or so. Though tiny, these genetic disparities confer traits like hair, eye color and susceptibility to disease. HapMap has examined SNPs from the genomes of 270 individuals descended from four groups-Western Europeans, Japanese, the Yoruba of Africa, and the Han Chinese.
By comparing differences among those groups´ DNA, HapMap gives scientists a better shot at distinguishing the genetic factors involved in disease from the environmental ones. Ultimately, it will help them explain why, for instance, some people have a higher or lower risk of certain illnesses. And once scientists understand how deleterious genes affect various populations, they´ll be better equipped to develop more-effective, targeted drugs to combat them.