听力文摘：为什么人类身上没有斑点或条纹?（在线收听）

Have you ever wondered why we humans don't have spots or stripes?

你是否曾经好奇过为什么我们人类身上没有斑点或条纹？

Let me think,Humans and animals get their skin color from melanin pigments, which can be either black or yellow.

让我想想，人类和动物的皮肤颜色来自黑色素，它不是黑色就是黄色。

Together, these two pigments account for just about every color you might see on the skin of mammals: white, red, yellow, brown, and black and every shade in between.

二者混合可呈现你能见到的各种哺乳动物的皮肤颜色：白色、黄色、棕色、黑色和他们之间的过渡色。产生黑色素的皮

The skin cells that produce melanin are known as melanocytes.

肤细胞是黑素细胞，

In the embryos of mammals, the melanocytes originate from the same group of cells that produces the spinal cord and the brain.

哺乳动物的胚胎中，黑素细胞源自制造出脊髓和大脑的同组细胞。

From there, these cells migrate through the developing skin towards the belly of the animal.

这些细胞从那里出来通过皮肤的生长迁往动物的肚子。

If the melanocytes that produce dark pigments and light pigments migrate in streaks, you end up with stripes.

如果产生深浅色素的黑素细胞以条纹状迁移，那么你身上就有条纹。

The same goes for spots, but in this case, only random patches of certain pigments survive.

斑点也是这么来的。但在这种情况下，只有随机的某些色素斑点能够存活。

In either case, the migration pattern is genetically determined.

不论哪种情况，其迁移方式由基因决定。

For certain species, like zebras, stripes provided an evolutionary advantage because they worked as camouflage.

对于特定物种 ，比如斑马， 条纹因可作为伪装对进化有利。

Those animals with stripes were more likely to survive, and to pass along the genes that produce stripes to their offspring.

身上有条纹的动物更有可能生存下来，并把能产生条纹的基因传给后代。

For humans, this apparently wasn't the case.

对于人类来说，情况显然不同。

However, it's likely that our overall skin color probably is a result of adaptation as well.

但是我们全身皮肤的颜色也可能是适应环境的结果。