Question: Why is everyone different? I know it's to do with DNA and genes, but why is everyone's different?

Susan Cartwright answered on 19 Jun 2015:

Yes, it’s to do with DNA and genes. Most of our genes are pretty much fixed, because they code for proteins whose structure has to be right for us to survive, but a small percentage of them can occur in different forms (called polymorphisms), which cause differences between people. Some of these differences are obvious – black skin vs pink skin – while some aren’t – blood groups, differences in the immune system. Because genes constantly recombine and come up in different combinations, there are enough possibilities that almost everyone (except identical twins) is genetically different (just as there are only 49 different numbers on a lottery card, but 14 million possible combinations). Even genetically identical twins are not physically identical, because some genes just set the general pattern of growth but do not specify every last detail: identical twins have different fingerprints, for example.

Why is this a good thing? Well, some of the differences between us are directly and obviously adaptive. If you spend too long in bright sunlight, you increase the risk of skin cancer. This risk is decreased by dark skin, which blocks most of the UV light that causes the cancer. On the other hand, if you don;t get enough sunlight, you do not make enough vitamin D, and your bones may not grow properly (a disease called rickets). Therefore, it is not surprising that most people who live in the tropics, where there is a lot of bright sunlight, have dark skin, and most people who live at high latitudes, where there is much less sunlight, have light skin.

A general level of variation, especially in the immune system, is also valuable in protecting against disease. Generally, for any given disease, some people are more resistant to it than others. If we were all the same, then there would be a risk that some disease would develop that could get every single one of us – as it is, even really horrible diseases like Ebola don’t kill everyone they infect. (This is a big problem with modern food crops, which have been so heavily selected for yield and flavour that they have very little diversity, and could easily be wiped out by a bug. Bananas are currently under threat from a fungal disease called Panama disease, which has already pretty much exterminated the preferred banana variety from the early 20th century, Gros Michel (it does still survive in Malaysia and Thailand, and apparently tastes much nicer than the modern variety!), and has recently mutated so that it can now attack the new dominant commercial variety, the Cavendish. There are other strains of dessert banana, but the commercial plantations are almost all Cavendish, and it would be extremely difficult to restock with something else.)

Environment also affects us: for example, athletes go for high altitude training because exercising in thinner air with less oxygen makes you develop a higher concentration of red blood cells, and this gives you greater stamina when you return to sea level; poor nutrition in childhood can make you turn out fairly small even if you have genes for height (you’d still be taller than other poorly nourished people without the genes for height, but you could be quite small compared to people who grew up well fed). My grandparents on my father’s side were quite short, but my dad was 6’4″ (1.93 m), my brother is 6’5″ (1.95 m), and I’m 5’9″ (1.75 m). Clearly my dad’s family have genes for height (my mother is considerably shorter than me, so it’s not from her side), but my grandparents did not fulfil their potential, probably because working class Yorkshire folk in the late 19th and early 20th century didn’t get enough to eat.

In general, a certain amount of variation between people in a community is good for the survival of that community. It’s probably useful for a hunter-gatherer tribe to have some left-handed people, because they can sneak up on the prey round a boulder that would get in the way of a right-handed person – so left-handedness persists at a lowish level even though in many respects (e.g. standardisation of tools) it would be better if everyone had the same dominant hand – ever tried to use an ordinary pair of scissors left-handed? It’s useful for some people to be able to handle starvation better, even if they are small runty people who are not great hunters, because it means that in a bad year the community won’t die out. And so on.

Diversity is GOOD.