How did humans evolve lactose tolerance?

CC-1923
A cow with an electronic milking machine attached to its udder. Needless to say, dairying was a lot less complicated in ancient times. Source: Wikimedia Commons, CC-1923

This article was originally published on NPR’s The Salt, 28 December 2012.

Got milk? Ancient European farmers who made cheese thousands of years ago certainly had it. But at that time, they lacked a genetic mutation that would have allowed them to digest raw milk’s dominant sugar, lactose, after childhood.

Today, however, 35 percent of the global population — mostly people with European ancestry — can digest lactose in adulthood without a hitch.

So, how did we transition from milk-a-phobics to milkaholics? “The first and most correct answer is, we don’t know,” says Mark Thomas, an evolutionary geneticist at University College London in the U.K.

Most babies can digest milk without getting an upset stomach thanks to an enzyme called lactase. Up until several thousand years ago, that enzyme turned off once a person grew into adulthood — meaning most adults were lactose intolerant (or “lactase nonpersistent,” as scientists call it).

But now that doesn’t happen for most people of Northern and Central European descent and in certain African and Middle Eastern populations. This development of lactose tolerance took only about 20,000 years — the evolutionary equivalent of a hot minute — but it would have required extremely strong selective pressure.

“Something happened when we started drinking milk that reduced mortality,” says Loren Cordain, an exercise physiologist at Colorado State University and an expert on Paleolithic nutrition. That something, though, is a bit of a mystery.

Read more…here.

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A chestnut revival

This article was originally published in Nature, 4 October 2012. 

A row of B3F3 chestnut hybrids. These trees are about a year old. They represent the final step in TACF's backcross breeding method. Once they come of age, they'll be tested for resistance through innoculation.
A row of B3F3 chestnut hybrids. These trees are about a year old. They represent the final step in TACF’s backcross breeding method. Once they come of age, they’ll be tested for resistance through innoculation.

“They’re hard to breed and easy to kill,” says plant pathologist Fred Hebard as he attacks a 2-metre-tall chestnut tree in southwest Virginia. Hebard bores a hole in the bark and squeezes a mash of orange fungus into the wood. The tree is a hybrid of the Chinese and American chestnut species, and Hebard hopes that it has enough resistance genes to keep the fungus — called chestnut blight — at bay. If so, the hybrid could help to resurrect a long-gone icon.

Until a century ago, the American chestnut (Castanea dentata) was the cornerstone tree species of eastern North America. With long, straight trunks and bushy crowns, it carpeted the forest floor each autumn with prickly brown nuts. But the arrival of chestnut blight (Cryphonectria parasitica) from Asia wiped out almost all the stately trees, leaving only a few, isolated stands. Since then, a faithful fan club of scientists and citizens has sought to tame the blight.

As chief scientist of the American Chestnut Foundation (ACF), a group of chestnut enthusiasts and scientists, Hebard has bred thousands of hybrids at the organization’s research farm in Meadowview, Virginia. He crosses descendants of the original American chestnut with the much smaller Chinese variety (Castanea mollissima), which has some natural immunity to the Asian fungus. And after decades of work, he is within reach of his goal, a tall American tree with enough Chinese traits to keep it healthy.

Other researchers are trying to attack the blight with viruses or are creating trees that are genetically modified (GM) to resist the fungus, and could be the first GM forest trees released in the wild in the United States. Progress with all three approaches is raising hopes that chestnuts will soon start to flourish again in the forests of the American east. “We’re starting to pull the American chestnut tree back from the brink of extinction,” says Hebard.

The work is also offering lessons that could help to save other trees, such as elm and ash, which face similar threats in America and abroad.

Read more…here

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