An example of atavism.

For example, a hairy child born with hair all over his body is an atavistic "degradation" phenomenon of human hair tissues and organs, and a person born with rotating ears can be classified as an atavistic "degradation" phenomenon of nervous system, and a person born with a tail can be classified as an atavistic "degradation" phenomenon of degraded organs, and the place where the atavistic phenomenon occurs is uncertain.

Some morphological characteristics of human ancestors have changed greatly in the process of human evolution, such as the increase of brain capacity, the change of body shape, the thinning of hair, the disappearance of tail and so on. The genes and gene regulation of human ancestors have also changed greatly in this process, and some genes have changed.

Some genes are shut down at some stage of human development. For example, when a human embryo develops, it has a tail at two months, and fine hairs all over the body at five or six months. In the process of fetal growth, the gene that controls the birth of the tail is turned off, so the fetal tail stops growing and becomes a cuboid bone; The dense body hair of the fetus disappeared before birth. This embryonic development process is considered to repeat the evolutionary process of human beings, indicating that some genes of human ancestors did not disappear, but were closed at the right time.

Extended data

As we all know, domestic chickens, ducks and geese have long lost their flying ability after long-term domestication by human beings. However, in domestic chickens, ducks and geese, sometimes there will be a kind of chickens, ducks and geese with particularly strong flying ability. The chicken, duck and goose regained their flying ability because of atavism. In addition, snakes with "feet" and dolphins with small fins beside their tail fins are also examples of animal atavism.

There are two explanations about atavism in modern genetics:

First, because two or more genes separated in the process of speciation were recombined through hybridization or other reasons, a certain trait was determined, and the traits of ancestors were re-expressed;

Secondly, the gene that determines this ancestral trait has been blocked by histone-based repressors for a long time in the evolution process, but for some reason, a specific non-histone is produced, which can combine with histone and make the repressors fall off. As a result, the blocked genes resumed their activities, and were transcribed and translated again, showing the characteristics of their ancestors.