Milk – poster child for rapid evolution in humans.
Marlene Zuk, a professor of ecology, evolution, and behavior at the University of Minnesota, describes milk – or more accurately the ability to digest it – as ‘the poster child for rapid evolution in humans.’ In Paleofantasy she explains how this ability came about, what it means at the genetic level, and what its consequences have been.
‘The first thing you have to do to study 4,000-year-old DNA is take off your clothes,’ writes Marlene Zuk in her introduction to Paleofantasy. ‘I am standing with Oddný Ósk Sverrisdóttir in the air lock room next to the ancient-DNA laboratory at Uppsala University in Sweden, preparing to see how she and her colleagues examine the bones of human beings and the animals they domesticated thousands of years ago. These scientists are looking for signs of changes in the genes that allow us to consume dairy products past the age of weaning, when all other mammals lose the ability to digest lactose, the sugar present in milk. After that time, dairy products can cause stomach upsets. But in some groups of humans, particularly those from northern Europe and parts of Africa, lactase - the enzyme that breaks down lactose - lingers throughout life, allowing them to take advantage of a previously unusable food source. Oddný and her PhD supervisor, Anders Götherström, study how and when this development occurred, and how it is related to the domestication of cows for their meat and milk. They examine minute changes in genes obtained from radiocarbon-dated bones from archaeological sites around Europe ...
To obtain the DNA, the bones are drilled and the powder from the interior is processed so that the genetic sequences inside are amplified - that is, replicated to yield a larger amount of material for easier analysis. Some bones are more likely to be fruitful than others; we heft the samples, since Oddný says that the most promising ones are heavy for their size, and glossy. Most of the samples are about 4,000 years old, but one of them is around 16,000 years old. It has already been rendered into powder, and I look at it closely, but it doesn’t seem any different from the others. One of the pieces is a flat section of skull, while others are sections of leg or arm bones, or a bit of pelvis. Oddný and I wonder briefly who all these people were, and what their lives were like. The details of their experiences, of course, are lost forever. But the signature of what they were able to eat and drink, and how their diet differed from that of their - our - ancestors, is forever recorded in their DNA.
Other than simple curiosity about our ancestors, why do we care whether an adult from 4,000 years ago could drink milk without getting a stomach ache? The answer is that these samples are revolutionizing our ideas about the speed at which our evolution has occurred, and this knowledge, in turn, has made us question the idea that we are stuck with ancient genes, and ancient bodies, in a modern environment. We can use this ancient DNA to show that we are not shackled by it.’
Adapted from Paleofantasy: What Evolution Really Tells Us About Sex, Diet, and How We Live by Marlene Zuk. Copyright © 2013 by Marlene Zuk. With the permission of the publisher, W.W. Norton & Company, Inc. Distributed in Australia by Wiley. In Paleofantasy, Marlene Zuk draws on fascinating evidence that examines everything from our ability to drink milk as adults to the texture of our ear wax to show that we’ve actually never stopped evolving. Our nostalgic visions of an ideal evolutionary past in which we ate, lived, and reproduced as we were “meant to” fail to recognize that we were never perfectly suited to our environment. Evolution is about change, and every organism is full of trade-offs.