Why Are We So Sick? Part II: Our Genetic Heritage

moran-warriorIn the first post in this series we took a look at the evolution of the human body and the birthplace of humanity: Africa. There’s good evidence to suggest that between 125,000 and 60,000 years ago, a band of hunter-gatherers successfully left Africa, travelled into Asia, and slowly started to colonize the rest of the world. Why did we leave? No one knows for sure. A widely held explanation is that changes in climate and/or food resources made it difficult to stay, but a need to explore the world could also have been a contributing factors. Because what is it that we know about humans? We are curious about the things we don’t yet know. Could it be greener on the other side of the fence…?

Colonizing the world

One of the things we’ve learned from the migration out of Africa is that the human ecological niche is very broad. As we started to colonize the rest of the world, we had to adjust to living conditions that were very different from those we encountered on the African savanna. While many animals need very specific environmental conditions to survive and reproduce, humans have evolved differently.

But why are we so adaptable? First of all, as we discussed in part 1, we have a very large brain. This mental  powerhouse allows us to think reasonably, form complex social networks, and produce tools, clothes, etc. that help us survive. In other words, we have the ability to evolve rapidly through culture.

Genetic adaptations also clearly play a role. Following the journey out of Africa, humans who settled down in areas of the world characterized by extreme cold slowly developed lighter skin, which is thought to be an adaptation to reduced sun exposure, and populations that moved into places where food was scarce evolved smaller bodies, which require fewer calories.

Contrary to many other animals, humans can eat a much greater variety of foods. This is something that can be clearly seen when we study the diet and health of indigenous people around the world. While some populations, such as the Inuit, have thrived on a high-fat diet that primarily consists of animal source food, other non-modernized societies, such as The Hadza and The Kitavans, eat a high-carbohydrate diet. That clearly doesn’t mean that all hunter-gatherer diets are equally healthy, but it does show that humans don’t need a very specific diet to survive.

map-spreading-homo-sapiens

Map of early human migrations.

It’s important to note that compared to today’s standards, the journey out of Africa and into the rest of the world was extremely slow, and it wasn’t until tens of thousands of years after the first migrators started leaving that we reached Australia, America, and other distant parts of the world. Also, we have to remember that although the environmental conditions in Europe, Asia, and other places of the world can be very different from those we were exposed to in Africa, the changes to our living conditions that took place tens of millenia ago fade in comparison to the changes that have occurred over the last several centuries.

Up until approximately 10.000 years ago, everyone still lived as hunter-gatherers, meaning that although the diet and lifestyle of different preagricultural populations around the world differed in several respects, they all subsided on the food groups that had nourished our species since the beginning, and they all kept the core lifestyle components that characterise a hunter-gatherer existence.

Hunter-gatherer fitness

For millions of years, our prehistoric ancestors – from simpler organisms, to tree-dwelling primates, to early Homo and Homo sapiens – lived in environments where physical activity was a demanded part of daily life and fast food and other modern commodities were clearly absent. During this time, evolutionary forces were steady at work, shaping the human genome.

In an ancestral, natural environment, physical fitness – meaning health and well-being or specifically the ability to perform aspects of sports or occupations – was closely linked with Darwinian fitness (the capacity of an organism to survive and transmit its genotype to reproductive offspring as compared to competing organisms). What this means is that when you live as a forager in the wild, survival until reproductive age (and the consequential care and survival of the offspring) is dependent upon being able to run, climb, and/or carry heavy things to get a hold of food, enduring periods of food shortage, and being able to ward of pathogenic organisms without the use of medicine. In other words, in Paleolithic times, those who were physically fit probably had a better chance of passing on their genes than those with low levels of physical fitness.

Compared to today’s standards in affluent nations, life expectancy in the Paleolithic era is thought to have been relatively low, as “harsh” living conditions ensured a constant battle between the human genome and selective pressures, which weeded out those who couldn’t make it in a Stone Age environment. This is clearly seen in contemporary hunter-gatherer populations, where high infant mortality, accidents, and infections keep the average life expectancy down (1).

However, as we know, hunter-gatherers tend to be in very good health in the sense that they are virtually free from modern diseases of civilization, such heart disease, cancer, acne vulgaris, IBD, myopia, obesity, and type-2 diabetes. Their relatively low average lifespan is merely a reflection of the fact that they don’t have access to modern medicine and technology – and thereby not the drugs and devices we use to combat infections, keep old and sick people alive, and fight infant mortality (1, 2).

Takeaway

A hunter-gatherer existence is deeply imprinted in the human genome. Studies consistently show that hunter-gatherers and traditional people minimally affected by modern habits are lean and “fit” and have very low incidence of degenerative disease. The two primary explanations for these findings are:

  • They follow a diet and lifestyle that the human body is well adapted for.
  • In an ancestral, natural environment, Darwinian fitness is linked to physical fitness. In other words, there is no surprise that obesity, asthma, myopia, and other conditions that significantly jeopardise a person’s ability to hunt, gather food, and escape dangerous animals are rare or nonexistent in forager populations.

Humans have clearly evolved since the Paleolithic era, but how adaptable are we really? And what happens when we diverge too far away from the hunter-gatherer niche? These are the questions I’ll look into next…

Main picture: Creative Commons Picture by Njambi Ndiba. Some rights reserved.

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Comments

  1. I am looking forward to Part III. 🙂

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  1. […] Why Are We So Sick? Part II: Our Genetic Heritage […]

  2. […] system, to metabolism, to brain function, and that Africa is the birth place of Homo sapiens. In part 2 we continued with our species’ journey out of Africa approximately 70.000 years ago and […]

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