Why do we all look so different?

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29 November
16:29
December
2016

It’s a fascinating question, and my initial challenge to it is: do we? The variations that we see in human populations are enough to warrant human behaviour reflecting them, and those differences in things like skin colour and facial features are the source of many conflicts and battles over human history – but I’m not sure it’s correct.

"If you look at the amount of genetic difference around the world, you see something very interesting – there is more genetic variation within Africa than in the rest of the world put together."

When it comes to the very obvious things, like skin colour, this is a phenomenon in which biology is effectively deceiving us. Because the number of genetic changes that result in all the skin hues we see around the world are a) very small – only a few genes, and b) isn’t reflected in the overall variation that we see within genomes. We do know that pale skin probably originated about 10,000 years ago, probably in an individual somewhere in Europe, and that all pale-skinned people inherit their pale skin-ness from that individual.

The human species, when it began in East Africa, was dark-skinned. But if you look at the amount of genetic difference around the world, you see something very interesting – there is more genetic variation within Africa than in the rest of the world put together. And that makes perfect sense, because only a small population left Africa, and effectively populated the entire world, which meant that there was not very much genetic variation within that group. The people who remained in Africa are much more genetically diverse as a result. If you take two black-skinned people from Uganda and Ethiopia, and compare their entire genomes with each other, they will be much more different to each other than either one of them is to anyone of European descent. A Chinese man is genetically more similar to a Ugandan than a Ugandan is to a man from Sudan.

"A big misunderstanding about evolution, especially human evolution, is that all of our physical characteristics are the result of positive natural selection"

So it’s gene/culture co-evolution. We have strong evidence that pale skin is an adaptation to northern climes with less sunlight. Vitamin D deficiency occurs when you’re not exposed to much sunlight, so reducing the amount of melanin, which blocks sunlight, increases the level of Vitamin D production. So that definitely looks like an advantageous trait which evolved in the people of northern Europe at that time, and has spread around the world through migration ever since.

There are other factors that convey obvious physical differences – but we don’t know whether they were selected. A big misunderstanding about evolution, especially human evolution, is that all of our physical characteristics are the result of positive natural selection. We just don’t know that. So many of our physical characteristics are just there because they’re there – a phenomenon in evolution called genetic drift. So we can broadly say that East Asians have a different skin tone, and certain typical characteristics – thicker black hair, different density of sweat glands, almost all of them have a different shape of the back of their teeth than almost everyone else on Earth, and the fold of skin above the eyes which gives them a distinctive shape. A lot of those characteristics that we associate with East Asian looks come from a single mutation that occurred approximately 31,000 years ago somewhere in Middle Asia.

Does that mean that they were selected, and that this conferred some great advantage to the people who had it, compared with people who didn’t? We don’t know. Probably not. It’s more likely that this is just something that happened, and spread through a population over the following tens of thousands of years. What that means is there’s a genetic difference which means these people look a bit like this. Does it mean anything biologically? No. Not at all.

Adam Rutherford is the author of Creation: The Origin Of Life/The Future Of Life and A Brief History of Everyone Who Ever Lived.

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