What forces are driving human evolution? Though scientists have witnessed natural selection of other species in nature and in the lab, our own changes have been a bit of a mystery. At the Harvard Museum of Natural History Thursday night, Pardis Sabeti of Harvard’s Center for Systems Biology explained how we have begun to scan our own genome to uncover evolution at work.
To find evidence of natural selection, researchers look for two main criteria: regions in the genome that are prevalent in a wide section of a population, and "young," or not ancestral. This may seem too easy, and it is quite straightforward, but scanning the genome to look for these areas would have been impossible even ten years ago. The completion of the Human Genome Project and the technology developed since then have enabled the searches Sabeti and others have carried out.
Through this research, scientists have confirmed and explained some of the evolutionary changes that were described in the late 20th century, such as red blood cell modifications that confer resistance to malaria. In many cases, they have described several different mutations affecting the same system and producing the same overall result. This confirms the presence of a pressure (such as malaria) driving the selection; any time a mutation arose which helped fight malaria, it quickly spread in later generations.
The most promising recent work has examined the human response to even more diseases. For example, Sabeti’s team is investigating mutations which have helped West African populations fight a particularly nasty disease called Lassa fever. In some cases, entire villages have died of the disease, with only a handful of exceptions. “We want to test to see if the individuals who survive are carrying something,” she explained. “We believe that like malaria, we’ll find all sorts of resistance variants spreading. When it’s that deadly, it’ll really start to affect evolution very quickly.”
However, this work represents one grain of sand in a Sahara of possibilities for future investigation. Early scans have tended to focus on genes, which are responsible for specific proteins with easily determined functions. However, genes make up less than 5% of the human genome. The remainder, once called “junk DNA,” now appears to contain regions with important—though poorly understood—regulatory roles.
Sabeti pointed out one huge area of 4 million base pairs. It contains no genes but has also undergone natural selection. In the future, such segments will undergo much closer scrutiny as their purpose is better described. One regulatory region she mentioned appears to be responsible for the difference in brain development between humans and other animals, so the vital character of some “junk DNA” cannot be understated.
While natural selection is sometimes relegated to talking about finches or describing lab experiments with single-celled critters, our evolutionary clocks are still ticking. What direction are we being pulled toward by today’s selective pressures? Only time will tell.
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