In nature, reproductive isolation is widespread, and even animals that look very similar have difficulty producing offspring. For example, there is reproductive isolation for the same cattle, buffalo and cattle, reproductive isolation for chimpanzees and orangutans, and reproductive isolation for horses and donkeys.
However, there is no reproductive isolation in the human world, and although we do not look similar and have great differences in skin color, we are able to produce offspring normally.
So the question is, why don't humans have reproductive isolation?
In life, we are accustomed to dividing human beings into yellow, black, and white people according to skin color. But in fact, in terms of biological classification, there is only one kind of human in the world: Homo sapiens, and there are no other subspecies, which means that there is no reproductive isolation of people all over the world.
According to the current mainstream theory, the ancient ancestors of human beings evolved in Africa, and it was not until about 200,000 years ago that early intellectuals began to appear in Africa, and they could not get out of this place due to the barriers between the ocean and the Great Rift Valley and the desert.
Around 100,000 years ago, the first Homo sapiens began to leave Africa, but they failed and left no descendants. About 65,000 years ago, the second group of Homo sapiens began to leave Africa, and this time they succeeded.
The Homo sapiens who came out of Africa gradually spread to all parts of the world in different ways, and thus continued to multiply and form who we are today.
Of course, when Homo sapiens first came out of Africa, there were some other human beings on the earth, such as Neanderthals, Denisovans, etc. There is evidence that these people and Homo sapiens once had genetic exchanges, and now in addition to Africans, we now carry Neanderthal genes to a greater or lesser extent.
In fact, Neanderthals do not belong to Homo sapiens, and it is reasonable to say that there should be reproductive isolation between the two, but reproductive isolation is not black and white, but there will be a gray area of broken threads, just like tigers and lions, although there is reproductive isolation, some lions and tigers can still reproduce, but the reproductive ability is reduced.
Mixed races with other races did not allow for reproductive isolation between Homo sapiens. On the contrary, because of natural proliferation, war, business, and affinity, human beings have been exchanging genes between them. Coupled with the fact that people around the world do not have a long time to separate their families, reproductive isolation does not exist for all people in the world today.
Since they are all homo sapiens, why are human skin colors so different?
At first, it was thought that humans had different skin tones to resist ultraviolet attacks. Ultraviolet rays are between ionizing radiation and non-ionizing radiation, in which ionizing radiation has a certain probability of destroying human DNA, leading to genetic mutations, causing diseases, and even leading to the occurrence of cancer.
In order to resist ultraviolet rays, people living in different regions have evolved different radiation, for example: people near the equator have black skin, which can reduce a lot of ultraviolet rays entering, and people in high latitudes have light skin, because the intensity of ultraviolet rays here is very low, even if it is light skin, there is no need to worry about the occurrence of skin diseases.
But there is a problem with this statement, that is, although ultraviolet rays may cause skin cancer in humans, it takes a very long time for ultraviolet rays to cause skin cancer in humans, and during this time, humans have completed the task of reproducing offspring and have passed on their genes to the next generation.
That is to say, although ultraviolet rays are harmful to humans, they do not affect the ability of humans to mate and reproduce, so ultraviolet rays are not the main reason why humans have evolved such diverse skin.
In fact, the difference in skin color in humans is more related to folic acid and vitamin D.
Vitamin D can prevent immune diseases, improve mood, promote bone formation, etc., and the way to get vitamin D is simple: bask in the sun every day.
But on the other hand, folic acid is very easy to be destroyed by ultraviolet rays, and the lack of folic acid will lead to a higher probability of fetal malformations, and may also lead to anemia of the pregnant woman herself, and the anemia of pregnant women will also endanger the fetus.
It is precisely because of the mutual restriction of folic acid and vitamin D, so that people have different skin tones, among which people living in Africa, the amount of solar radiation received is large, so the people here have darker skin color, can protect folic acid from being destroyed, and because of the large amount of solar radiation here, so even if the skin color is darker, the skin receives a slight amount of solar radiation is enough to help the synthesis of vitamin D.
On the other hand, people living in high latitudes have weak light intensity and folic acid is not easily lost, so in order to synthesize more vitamin D, lighter skin tones have to evolve here.
There are fewer genes that control skin color, which is not enough for humans to form reproductive isolation, so although the skin color of human beings around the world is different, all humans in the world are currently "Homo sapiens".
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