People in China have black eyes. Why are American eyes blue? Why do people have black and white skin colors?

As we all know, orientals have black eyes and westerners have blue eyes. So, why do people of different races have different eye colors?

The eyeball mentioned here refers to the front and middle part of the eyeball, which is composed of cornea, iris and pupil. Because the cornea is colorless and transparent, the color of the eyeball is the color of the iris.

Then, why are the eyes of orientals black and those of westerners light blue? This should be discussed from the structure of eyeball iris. Scientists have found that the iris of our human eye consists of five layers of tissues. They are endothelial cell layer, anterior limiting membrane, matrix layer, posterior limiting membrane and posterior epithelial layer. Among these five layers of tissues, there are many pigment cells in the stroma, anterior limiting membrane and posterior epithelial layer, and the amount of pigment contained in these cells determines the color of iris. The more pigment contained in pigment cells, the darker the iris and the darker the eyeball. The less pigment, the lighter the iris color and the lighter the eyeball color. The pigment content in pigment cells is consistent with skin color, which is related to ethnic inheritance.

Orientals are colored people, and there are many pigments in the iris, so the eyes look black; Westerners are white, with less iris pigment and less interstitial blood vessels, so their eyes look light blue.

The color of eyes is the most important physical feature of human body, but scientists say that the color of eyes can be changed.

Scientists point out that it is melanin in the matrix in front of iris that determines the color of human eyes. The more melanin, the darker the human eye, and vice versa. However, the melanin content in the matrix usually remains unchanged for life, unless something abnormal happens to cause it to change permanently. The key to the color change of human eyes is heredity and trauma.

Why do people have different skin colors?

When you watch TV, you will see black people, white people and people of other colors. Why are their skin colors different from ours?

If you have studied geography or read other children's books, you can know that there are many kinds of skin colors, such as black, yellow, red, white and brown. People of the same race are also very different.

For example, yellow people also have whiter or darker skin colors. Although people's skin color is different due to different seasons, different environments and pathological influences, the inherent skin color of the human body is determined by parents' genes.

There are three factors that can change the color of skin: the thickness of skin, the blood supply and the pigment in skin. Especially the skin pigment, the most important is melanin, which can range from yellow to black. The skin is divided into several layers, the outer layer is called epidermis, the inner layer is called dermis, and melanin is between dermis and epidermis.

If a person's skin is rich and dense in melanin, the skin is darker; Less melanin, the skin will appear yellow; If there is little melanin, the skin will look very white. Human skin is like a mirror, reflecting people's vitality and health. People with different health conditions have different skin colors, while the skin colors of different races have extremely obvious differences. Yellow people have yellow skin color; Black people have dark skin; White people have light skin color.

So, why are there people with different skin colors in the world?

The color of skin is mainly determined by the content of melanin in skin. Melanin is a kind of black or brown particle, whose main function is to block the damage of ultraviolet rays in sunlight to human subcutaneous cells. When the sun is strong, the content of melanin increases, so we will be tanned in summer.

Human skin color is the result of adapting to nature in the process of evolution. Africans living in the equatorial region have black skin, because their skin is often exposed to strong sunlight, and melanin is produced in large quantities. It is rare in cold areas, so the skin is white. Yellow people generally live in temperate regions, with moderate sunshine intensity and melanin in between, so their skin color is also in between.

After research, scientists found that there was no difference between human ancestors at first, and their skin colors were basically the same. Only later people moved to different areas, and in order to adapt to the external environment, skin color differences gradually appeared.