When humans left the Near East and invaded Europe around 40,000 years ago, they took with them bone and stone tools for hunting and engraving, early art in the form of beads and pendants, and social skills that were probably superior to those of Europe’s resident Neanderthals. It now seems they also took with them a mutant gene: the gene for the deadly disease cystic fibrosis. So robust is this gene that it spread all over Europe; cystic fibrosis is now the most common fatal genetic disorder among Caucasians. How could the disease have been passed from generation to generation for so long, when until the advent of modern medicine it generally killed people before they could have children? Recent experiments on mice offer an answer. They suggest that the 5 percent of all Caucasians who carry just one copy of the cystic fibrosis gene--and thus don’t suffer from the disease--are protected against another deadly scourge: diarrhea.
The gene for cystic fibrosis was discovered in 1989. It codes for a protein that forms channels in cell membranes, especially the cells lining the intestines and airways. Normally these channels funnel chloride ions out of a cell, thus making its surroundings saltier; that in turn draws water out of the cell by osmosis. In the lungs this fluid washes away bacteria and other unwanted debris. In the intestines it does the same and also brings digestive enzymes into contact with food. In sweat glands chloride channels have an additional function; they recycle salt out of the glands and back into the skin before it can be lost to the outside world.
