SCIENCE
Notebook
Monday, December 11, 2006; Page A08
The Key to Lactose Tolerance
Through most of human history, there was no need to be able to digest milk after the age of weaning. Cows had not been domesticated. And Mom was busy with your little brother and sister. So humans evolved as creatures that, as adults, lose the ability to digest lactose, the major sugar in milk.
One well-known exception is people of Northern European descent, most of whom have inherited mutations that grant lifelong lactose-digesting skills; less well known is that several African populations share this capacity. Now, researchers have evidence that the domestication of cattle thousands of years ago was the key to lactose tolerance emerging independently in Europe and Africa.
Sarah Tishkoff and Floyd Reed of the University of Maryland at College Park, and co-workers, took blood from 470 individuals from 43 ethnic groups in Tanzania, Kenya and Sudan. Those groups are mostly cattle herders and can digest lactose, but they lack the European mutations. Blood tests showed that these people harbor one or another of three previously unknown mutations that -- as with the European ones -- keep the lactose-digestion enzyme active throughout life.
Tests that can estimate when a genetic trait first appeared showed that the three mutations became common about 5,000 years ago -- the same time that archaeological evidence says these groups switched from hunting and gathering to herding. As in Northern Europe, the team concludes, the availability of milk that came with herding made these otherwise useless mutations invaluable and led to their rapid rise in Eastern Africa.
"Considering the symptoms of lactose intolerance, which includes water loss from diarrhea, individuals who . . . could tolerate milk could have had a very strong selective advantage," the team wrote in yesterday's online edition of the journal Nature Genetics.
-- Rick Weiss
Carbon Cycle Linked to Climate
Earth's climate may be more sensitive to the long-term release of carbon than was previously thought, according to a study published in the journal Science.
Four climate researchers examining the Paleocene-Eocene Thermal Maximum, a period 55 million years ago when Earth warmed dramatically, found that the amount of carbon released into the atmosphere at that time was roughly the same amount available in current resources of coal, oil and gas. During that period, the North Pole became nearly as warm as Miami is now, and rising temperatures changed rainfall patterns and made oceans more acidic.
Scientists have known that this hotter period -- during which the planet's average temperature rose by roughly 9 percent in less than 10,000 years -- resulted from a massive release of carbon, though they are unclear how that happened. The researchers -- from Yale University, the Carnegie Institution, the University of Chicago and the University of California at Santa Cruz -- made new calculations based on carbon found in the fossils of ancient terrestrial plants and marine plankton.
