I realised that for some reason, I still don’t know this. Why do we have different skin colors, hair textures, eyes or such? Is it just a random thing that happened or are there evolutionary reasons to it?
Some are random and have no disadvantage, so they stick around. Others have an advantage that may or may not still be relevant.
High melanin levels help with bright equatorial sun. Low melanin levels help with vitamin D synthesis in areas where there’s less sun.
Curly African hair is better at protecting the scalp from the sun and heat. There’s less hair follicles overall, allowing for better airflow and the tight curls keep the hair away from the scalp allowing it to cool better. This also meant less sweating, which made it easier to remain hydrated and clean.
Straighter hair tends to be more dense, and to do a better job keeping you warm.A lot of the other traits are random, or in genes connected to the general melanin genes, since evolution is unlikely to specifically target just the melanin levels of skin, and not the overall melanin level.
Some traits are also a result of sexual selection. Peacocks have large, vibrant plumage because it helps them attract a mate. Some human characteristics are the same. We essentially selectively bred ourselves based on the whim of aesthetics.
Finally, much of what we consider racial differences between people are social constructs.
That’s not to say that the differences aren’t real, but that the racial division is a relatively arbitrary line.
For example, I’m nearly a foot taller than my wife. My ancestors wandered up from Africa, landed in Scandinavia and then drifted to Scotland and southern England before coming to the Americas and getting mixed up in the Canadian fur trade in the 1600s. My wife’s ancestors stopped in Germany before coming to the Americas in the late 1800s.
Our children are not considered mixed race because our skin is the same color, even though the actual lineage is pretty distinct.We decided that skin color is a race marker, but not things like “height”, “toe and finger length”, or things like that.
Except for where we did, like when European colonizers relatively arbitrarily decided that different traits were racial markers amongst the colonized, like nose shape and chin thickness.All that to say, much of what we consider obvious racial differences that stand out are only such because we decided to pay attention to them. Other perfectly visible variations are just normal individual variations.
Without the evolutionary pressure to maintain high melanin levels in the skin, and possibly also from interbreeding with Neanderthaal, European people’s got paler.
Someone mutated to have eyes that were paler and more sensitive to light, which was useful during the longer nights further north, and that mutation spread.
So it’s a bit of both.
Put differently (so as not to imply high melanin is fundamentally superior):
Someone mutated to have lower melanin levels in their skin, which was useful to absorb more sunlight for Vitamin D production further north, and that mutation spread.
Could animals that are related but separated by vast distances be used as an example as well? Like the black swan, or even trees and crops?
Without the evolutionary pressure to maintain high melanin levels in the skin, and possibly also from interbreeding with Neanderthaal, European people’s got paler.
But what’s the evolutionary pressure keeping melanin levels among ethnicities that stayed black? And why does it affect people in Central and South Africa but not in North Africa and the Middle East, when both regions are about equally hot?
It’s important to think about the time scale that evolution works on. These changes happened very slowly 50,000+ years ago.
The regions near the equator where people still tend to be lighter skinned have been in contact with and interbreeding with lighter skinned people for thousands of years, plus many migrations and invasions.over the past 10,000 years.
I seem to remember that the majority traits south of Sahara (black/very dark skin, and curly hair) can be traced back to something called the “great Bantu expansion”, which was essentially the result of a group of people with these traits developing agriculture and wiping out most other peoples south of Sahara, much like the Europeans did to the Americas.
Some cultures south of Sahara did survive, which can be seen both genetically, and in some languages that are completely from other languages in the area (I believe the family of languages with “clicking” sounds is an example).
I’m on my phone now, but I’ll have a double check and come back.
Ok, I’ve done some double checking: The Bantu expansion is approximately what I thought it was. I believe the language group I was thinking about that survived the Bantu expansion was the Khoisan.
My (very coarse) knowledge of this comes from a mixture of reading Jared Diamond (Guns, Germs and Steel) and from following it up with some Wikipedia. In short: The genetic makeup in a lot of the world is relatively dominated by the groups that were the first to adopt agriculture in their respective regions. Before the Bantu expansion, phenotypes south of Sahara were more varied, just like the phenotypes in the Americas were more varied before the corresponding “European expansion”, or the equivalent expansion that happened in South-East Asia (I don’t remember which society stood behind that one).
According to Diamond, we can trace a lot of (most?) surviving human phenotypes and languages back to relatively few societies, which after adopting agriculture, more or less wiped out / displaced neighbouring cultures due to increased resistance to a lot of infectious diseases and massively increased food production / need for land. This mostly happened less than 10 000 years ago, i.e. far too recently for natural selection to have a major impact on things like skin colour, hair type, height, facial features, etc. afterwards.
So: While major trends in phenotypes are of course a result of natural selection / evolutionary pressure in specific regions (resistance to skin cancer / sunburn vs. vitamin D production, or cooling down more efficiently with a wider nose vs. retaining heat with a slimmer one, or having an eye-shape that lets in more light vs. provides more shade), a lot of what we see today is simply a result of what phenotype the first group a given region that adopted agriculture had. This means that looking at the dominant phenotype in a region today will not necessarily give a good impression of what phenotype that is “optimally designed” to survive in the conditions of that region.
Most likely a conbination of the amount of sun exposure and other poaitive genetic mutations that happened alongside melanin levels.
An example of the complexity of genetic changes being intertwined is resistance to malaria and sickle cell anemia. The benefits of resistance to sickle cell anemia outweigh the negstives of the increased chance of sickle cell anemia so the mutation has persisted.
It is likely that North African populations had something that was beneficial as a tradeoff for their comparably lighter skin. Wearing clothing that covers a lot more of their body and having shelters from the sun could also help to mitigate some of the sun damage.
So it is complicated and most differences are due to a combination of genetic traits, they don’t get passed down one trait at a time.
“Evolutionary reason” does not necessarily mean that a feature is an advantage. Mutations are random and then positive or negative selective pressure act on them untill the mutation is either extinct or is adopted by the whole population. For features without selective pressure the same thing still happens it just takes longer and is basically random. So different populations of a species will always develop different features even when given the same environment.
So for most of the features you listed: yeah it just happened.
Skin color is directly related to latitude. Darker skin means more melanin, which absorbs more light and protects against sunburns and thus skin cancer.
Eye color factors are less confidently known, but darker eyes generally have a better time in bright daylight.
Regarding those two, it’s also worth mentioning that the Inuit people don’t follow those patterns, as while they have less sunlight, they also have to deal with reflected light off the snow.
Hair texture is like eye color in that we’re only mostly confident, but tight curls also probably protect from the sun.
It’s also been posited that epicanthic folds might help against freezing winds, but there’s no real evidence for that.
the simple reason is geographic isolation of populations. over time groups of people in different places gain minute genetic differences relating to their region/breeding population.
its geography + genetic drift
Came to the comments and was pleasantly surprised how well genetic drift was represented. People seem to have a much more sophisticated understanding of evolution than 30 years ago.
Humans were domesticated long ago and broken up into breeds, like dogs or cats, and now that we no longer build pyramids the folks who domesticated the humans left. Now it’s just us, us, and racism.
Time to get off the internet, Dr. Jackson. I’m sure those goa’uld will show up in their spaceships any day now.
Some features, such as melanin levels, are due to evolutionary pressures. High levels of melanin protect you from the sun, lower levels of melanin boost your vitamin D production.
Other features are just random. Over time, human populations were affected by random changes that weren’t exactly useful but not detremental either. They’re called sprandels. According to a quick Google search, blue eyes don’t seem to have any advantages, but not really any major disadvantages either, so that would be an example of that.
For height and weight, there are links to the climate that the people live in. Typically, hotter climates favor skinny people while colder climates favor more husky people. As for height, my understand is that a dry heat favors taller people as they can take advantage of sweating more than a wet heat.
I think the impact of wealth and access to nutrition and an environment that does not stunt development plays a major role here as well. A good example of this is height in Europe post-industrial revolution and improvements in medicine:
Most notable being a 20cm increase in the Netherlands in just over 100 years as well as changes in places that industrialised quickly like China:
Not to say that geography plays no role but it’s closer to 10cm gap than a 30cm one.
