Humans Have Evolved to Reproduce, Not to be Healthy

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Daniel E. Lieberman, chair of the department of human evolutionary biology at Harvard University, tells us how the human body evolved over millions of years, and explains how we’ve ended up living longer but with more chronic disease. The Story of the Human Body: Evolution, Health, and Disease shows how cultural evolution differs from biological evolution, and looks at the modern conditions to which our bodies are not well adapted, resulting in more obesity and diseases such as type 2 diabetes.

Some Interview Highlights

Are our bodies adapted to be a hunter-gatherer on the Serengeti? The answer is complicated.

We're still evolving, Daniel Lieberman pointed out, and that cultural evolution is as important in shaping humans as biological evolution. When our ancestors invented special clothing (like heavy wool), it enabled us live in different environments, which has led to natural selection to operating in important ways. "Cultural and biological evolution are not independent. In fact they're deeply woven together," said Lieberman.

We are hardwired to crave foods with salt, sugar, and fat. For example: babies take a long time before they're independent enough to feed themselves, and mothers need a lot of energy to be pregnant, breastfeed, and to take care of children. Humans needed a lot of fat and calories to fuel active lives and to get through times when there wasn't enough food.

Lieberman said there are good and bad things about the Paleo Diet. There's no question that the healthiest diets are low carb, low sugar because we're poorly adapted to handle simple carbs and sugars. But he thinks the Paleo Diet's restricting dairy and legumes is going too far. "It makes the assumption that if we've evolved to eat it, it must be healthy," Lieberman said. "But we evolved to reproduce, not to be healthy."

Processed foods change the way our digestive system functions. When food is ground, cooked, etc., our digestive systems can break it down faster. One of the problems with our digestive system is that it has never evolved to adapt to the amount of sugar we eat today. 

Cavities are a modern affliction. The modern human diet, which includes more cereals and sugars and starches, has led to tooth decay because the bacteria that live in our mouths love simple starches and sugar. The bacteria feed on them, produce acid, and that acid causes cavities.

How did allergies evolve? Lieberman explained that we used to be bombarded with germs and bugs and worms and dirt. Now we've removed a lot of the demands our immune system with cleanliness, and the leftover immune responses are attack the wrong things and are attacking our own bodies. 

We're living longer but we're sicker. We spend too much time treating symptoms of diseases rather than causes. Looking at evolution can help us identify the causes of these diseases and know how to prevent them.