What evolution can tell us about sex, diets and how we live

First aired on Quirks & Quarks (27/04/13)

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Modern humans have been around for about 200,000 years, most of it in the Paleolithic period, or Stone Age. But about 10,000 years ago, our ancestors began planting crops and breeding animals, which led to a dramatic change in diet and lifestyle. There are those who believe those changes in diet and lifestyle that came along with agriculture -- and subsequently with industry and technology -- happened too fast for evolution to catch up. They believe we are out of sync with our modern environment, and should live and eat more like our Paleolithic ancestors. Dr. Marlene Zuk, a professor of Ecology, Evolution and Behaviour at the University of Minnesota, disagrees. In her new book, Paleofantasy: What Evolution Really Tells Us About Sex, Diet and How We Live, she argues that neither we, nor any other species, have ever been perfectly suited to their environment. She also contends that, just like our "caveman" ancestors, we deal with change by continually evolving.

In a recent interview with Quirks & Quarks, Zuk explained that the term "paleofantasy" was coined by anthropologist Leslie Aiello. "She was frustrated by our inability to get really concrete information about what human brains were like, and what our behaviour was like, tens or hundreds of thousands of years ago, and she said, 'Well, do we really have scientific evidence or are we just coming up with paleofantasies?'" Zuk thought it was a good description of how we tend to think about the past. "We think of it as a time when we were more suited to our environment. "

That view comes, in part, because such major changes have taken place in the last 10,000 years of human existence. Zuk pointed to agriculture in particular. "Our ancestors went from living in small groups, perhaps nomadic, to settling down, growing crops, domesticating both animals and plants, and living in much larger units." To many people, this was a mistake -- and nowadays, some people try to emulate our prehistoric ancestors in terms of diet, choosing not to eat foods that have come with modernity, like dairy or grains.

But Zuk rejects the notion that humans lived "in perfect harmony" with their environment in Paleolithic times, because evolution is a continuous, ever-changing process. "The idea that organisms, whether they're humans or animals or plants, get to a point in terms of evolution and then are perfectly adapted to their environment and any change you make kind of disturbs that balance is kind of a misapprehension of the way evolution works," Zuk told host Bob McDonald.

Zuk says she wrote Paleofantasy because she was impressed with new information about recent changes in human genes. It may seem, at least in the western, industrialized world, that because of medical advances that cure us of many diseases that would have been fatal just a few decades ago, and the use of contraception, natural selection isn't influencing us any longer. But Zuk said that's not the case. "All evolution means is that there's differential reproduction that's resulting in different genes in the gene pool in future generations," she said. "That's happening in western society, and lots of parts of the world where western medicine and contraception aren't widely available. So any time you've got people leaving different numbers of copies of their genes in future generations, you've got the opportunity for evolution."

In her book, Zuk discusses milk and lactose intolerance as a sign of recent evolutionary change. Human beings are the only mammals that consume milk past weaning (most mammals lose the digestive enzyme that breaks down lactose after they're weaned). But in some human populations, the ability to break down lactose has persisted. Scientists believe that this evolutionary change came about through a process called "gene culture co-evolution."

When our ancestors started herding cattle or other milk-producing animals, about 10,000 years ago, it was for their meat and hides. "But within that population, a few individuals could digest dairy," Zuk said. This gave them an advantage, "because they had an additional food source and...a source of uncontaminated fluid in the form of milk." These individuals had more children who inherited the ability to digest dairy. "That then in turn made it more advantageous to have a population that herded cattle, which then in turn selected for more people who had lactase persistence." What develops is a "feedback between the cultural practice of herding cattle and the biological characteristic of lactase persistence."

Nowadays, "about 35 per cent of people all over the world have lactase persistence, and can break down the sugar in milk," Zuk said. Her response to people who say we shouldn't eat dairy because our ancestors didn't? "Sure, our ancestors didn't. But we, or at least some of us, can."

Because of technological and industrial changes, our lifestyle is changing rapidly. When asked if we are we moving too quickly for our own evolutionary good, Zuk responded that framing the question in these terms is a value judgment that "doesn't quite suit how the process works." Evolution isn't like "models of cars where one form replaces another...and the newer form is always better than the old one," she said. "We're not better evolved than our ape-like ancestors...we've just evolved to suit our environment in a different way. Evolution doesn't progress toward a particular goal, so there's no right or wrong."



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