SCIENCE
Notebook

Monday, December 11, 2006

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.

"By examining fossils and ancient sediment on the sea floor, we can see that something very unusual happened to the Earth's carbon cycle," said Ken Caldeira, a senior scientist at the Carnegie Institution's Department of Global Ecology. "Our carbon dioxide emissions are risking biological, chemical and climate changes of a magnitude that has not been seen in more than 50 million years."

-- Juliet Eilperin

Fish Can Team Up to Trap Prey

Can fish cooperate as they hunt for food, with one species flushing out prey that another then captures and eats?

This provocative possibility is raised by the unusual case of the grouper and the giant moray eel. The two live alongside each other in the Red Sea and, according to a study in the Public Library of Science/Biology, they sometimes act as a one-two punch as they hunt the coral reefs.

The moray eel hunts in the crevices of the reefs, since its long, thin body can slide into narrow spaces. The grouper hunts in open waters around the reef and can keep fleeing prey from escaping. The result is that the two together can flush and trap prey better than each could on its own.

Using snorkels to follow the fish for up to three hours at a time, Redouan Bshary of the University of Neuchatel in Switzerland found that groupers and eels spent more time together than would be expected by chance and did appear to hunt together. Bshary also found that groupers seemed to actively recruit moray eels through a curious head shake they made close to the eel's head, which resulted in the eel coming out of its crevice.

Those previously unknown head shakes especially intrigued the researchers. "The simplest explanation," they wrote, "is that these signals indicate only the motivation of the grouper to engage in hunting, which then becomes positively associated with hunting success for the moray eels."

-- Marc Kaufman