SarahTishkoff_Comments - NEWS AND VIEWS Following the herd...

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NEWS AND VIEWS Following the herd Stephen P Wooding The ability to digest lactose into adulthood is a recently evolved trait that has risen to high frequency in some human populations, coincident with the introduction of cattle domestication. A new study shows that variants responsible for this trait arose independently in Europeans and Africans, providing a striking example of convergent evolution. Stephen P. Wooding is at the McDermott Center for Human Growth and Development, University of Texas Southwestern Medical Center, 6000 Harry Hines Boulevard, Dallas, Texas 75390, USA. e-mail: stephen.wooding@utsouthwestern.edu The emergence of agriculture was a transfor- mative event in human history. By providing abundant, stable food supplies and relief from the energy expenditure and danger of hunting and gathering, it propelled us rapidly toward our modern lifestyle. The effects of this transition are viewed most often from two perspectives. From a cultural standpoint, the agricultural revolution marks the beginning of a shift from a nomadic way of life to a settled one, the first steps toward industrialization. From an evolu- tionary standpoint, it marks the beginning of striking morphological and behavioral changes in the plants and animals we domesticated. Less well understood are the evolutionary changes that emerged in humans as a consequence of this transition. How have we adapted to our rela- tively recent way of life? On page 31 of this issue 1 , Tishkoff et al. shed new light on this enduring question, reporting that lactase expression in adults—which allows some, but not others, to exploit milk as a source of nutrition—has arisen at least twice, independently, in the course of human evolution. Lactose breakdown Most of us are familiar with the uncomfort- able problem of lactose digestion. Virtually all humans are born with the ability to digest this milk sugar, which allows us to drink mother’s milk until we are weaned, but most of us lose this ability by the time we are 12 or 13 years old. After that, even a modest nip of the white stuff causes intestinal symptoms best described in polite company as unpleasant, which arise in part from the gas-producing fermentation of lactose by bacteria in the gut 2 . A lucky minor- ity of us maintains the ability to digest lactose into adulthood and can go on enjoying milk throughout life. This ability is provided by an enzyme, lactase-phlorizin hydrolase (LPH), which performs the important task of breaking lactose (a disaccharide) into monosaccharides more readily absorbed by the gut. Variation in the ability of adults to digest milk has long attracted attention as an interest- ing developmental phenomenon, but success in understanding the genetic underpinnings of the trait has been slow in coming. The gene encod- ing lactase, LCT , was first mapped in the late 1980s and immediately became a top contender in the search for genetic variants that might account for lactase persistence
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This note was uploaded on 01/06/2010 for the course NS 2750 taught by Professor Haas&gu during the Fall '08 term at Cornell University (Engineering School).

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SarahTishkoff_Comments - NEWS AND VIEWS Following the herd...

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