Our nearest relative in the animal kingdom is the chimpanzee. 97% of a chimp’s DNA is exactly the same as ours. It is the other three per cent that makes all the difference. We and the chimps are descended from a common ancestor, a tree-living ape, and we started diverging into two different species roughly 7 million years ago. Over that huge length of time, we have developed four obvious changes. We have a bigger and better brain; we stand upright on two feet; we have lost most of our body hair and we have the greatest ability on the planet to lose body heat though sweating. These four things are not random mutations; they are closely interlinked.

Hunter gatherers

Experts in evolution now believe that our distant ancestors, whom I will refer to as ‘we’, were forced to change from living in trees to living on the ground when a period of hot and dry weather killed the trees and turned the forests into grasslands. (Climate change is not a new thing.) This was a slow process but our very distant mothers and fathers had to adapt. We couldn’t sit on a tree branch eating the leaves and berries around us; we had to search for food. Standing upright meant we could see further, which allowed us to spot both food and danger.

We have never been able to digest grass but we were able to prosper by eating the animals that eat grass. Of course, we had to catch them first. At this stage of our evolution we had not developed any tools or weapons, so we had to chase animals until they collapsed from heat exhaustion. A typical grass eating animal, like the antelopes of today, would be chewing grass for hours and watching out for lions and tigers. They evolved to sprint away from a big cat but had no need to keep running for a long time. They either got away or they didn’t. Both they and the big cats were very poor at losing body heat and would sprint for a few seconds and then pant, like a domestic dog, to cool down.

We couldn’t sprint fast enough to catch any of these animals, so we took a different approach. We would work in a group and chase one animal in the heat of the day. This prevented us form being eaten by big cats because they don’t hunt at midday  Our prey would easily get away at first but we would catch up to it again before it had time to cool down. It would run away again and again but each time it would be getting hotter. Eventually it would collapse from heat exhaustion and we could walk up to it and enjoy a very large and nutritious meal. Because we were all hunting in this way we adapted to it. Over time we lost our body hair and improved our ability to sweat, both of which help to shed heat. The animals we hunted never adapted in this way because the herds they lived in survived well by sprinting away from danger. Chasing animals in the heat of the day for a million years is why we are upright, hairless apes with a fantastic ability to lose body heat and run long distances. (People who believe that we evolved as vegetarians have no explanation for our exceptional ability to sweat and run.) The ability of elite marathon runners to maintain an incredible pace for over two hours, and the ability of so many people to cover 26 miles, shows how strongly our species has adapted to this activity.

Brain power

This explains three of the differences between us and the chimpanzees but what about our superior brains? This way of hunting is responsible for that too. It all comes down to a combination of factors occurring at the same time.

  1. By eating the fat and meat of a large animal, we consumed enough energy and nutrients for a large brain to grow and flourish.
  2. Brain function and exercise are linked. A signal comes from the brain to move a muscle and, it has recently been discovered, a moving muscle produces a hormone called Irisin which travels back to the brain and triggers the production of molecules, called neurotrophic factors, which increase the connectivity of our brain cells. The more connectivity our brains have, the better they work.
  3. We hunted in groups. This requires communication and cooperation, which are both mentally stimulating activities.

In summary, we used our brains to solve the problems of catching food and working together. All the running and chasing provided the chemical stimulus to increase mental ability and the food we ate provided the nutrition and energy to allow our brains to grow.

But, of course, we are not carnivores, we are omnivores; we eat both animals and plants. A lot of the plants we would have eaten were not available all of the time. Fruits and berries could only be found for a few months each year when they were in season. These foods contain high levels of carbohydrate whereas animals provide only fat and protein. Understanding this part of our evolution is essential to understanding why we are all getting fat.

Seasonal food

The action of the hormone insulin is brilliant from an evolutionary point of view but disastrous in our modern world. When we eat berries in season, the carbohydrate and sugar they contain raises blood glucose levels. In response, insulin is produced to move the excess glucose from the blood to the cells of the body. When the cells and liver can store no more (maximum storage is about 2,500 calories) insulin converts glucose into fat and stores it in the fat cells. (Fat cells have no limit, as can be seen in some of the most obese people.) When insulin lowers the blood glucose it makes you feel hungry again so that you eat more berries. The sweetness of the sugar in these foods also makes you want to eat more. So, when carbohydrate is available you eat lots of it and store it as body fat. When there is no more carbohydrate to eat, because the season is finished, you have a layer of fat to keep you going during the times when food is hard to find. This clever piece of evolution kept our ancestors alive throughout many hard times. However, now that we longer need to wait nine months for carbohydrate rich plants to ripen and we can eat carbohydrate all day and every day, the action of insulin is making us fatter and fatter and fatter.

We only have a small carbohydrate store (glycogen) and insulin converts extra sugar into fat because fat is the body’s preferred fuel. (Most people think it is glucose but it is actually fat in the form of triglycerides and ketones.) Over the millions of years that we evolved, we were eating fat and protein nearly all the time. When we had the chance to eat carbohydrate we converted most of it into a store of fat. To lose weight and return our bodies to the way they are meant to be, we need to reduce our insulin levels by restricting carbohydrate. When we do this consistently, our bodies switch to burning fat and all the stored energy in our fat cells can be released and our excess weight will drop off.

Marathon runners should note that carbo-loading, which has been so highly recommended, does not fit with our evolution. Glycogen stores only contain about 2,500 calories and they run out towards the end of a marathon, whereas fat storage, even in a skinny runner, will contain at least ten times as much energy. Someone who is adapted to fat burning is never going to run out of energy during a marathon.

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Ref: Run for Your Life. Nature 487, 295–296 (19 July 2012)