Scientists from Howard University (USA) conducted a detailed study of the development of human embryos. And they found a lot of interesting new facts - in particular, a previously unknown atavism associated with the muscles of the limbs. Like other embryonic atavisms, such as the primordial tail, it completely disappears by the 13th week of development.
The study showed that by the seventh week of pregnancy, up to 30 muscle groups are formed on each limb of the embryo - too many not only for humans, but also for mammals in general. But then, within 3-5 weeks, the number of these muscles is reduced to 20. Some merge with each other, the rest simply degrade as unnecessary.
This discovery made it possible to explain one of the mysteries - in extremely rare cases, adults have more muscles than they should. Research suggests that this is a consequence of the pathology of embryonic development. But where do these extra muscles come from in embryos? These muscles are not found in any living mammal - but some lizards do.
These reptiles tend to have ultra-flexible legs that use more muscle than humans or mammals. It is believed that about 250 million years ago, our species diverged, but since then, at the embryonic stage, humans briefly experience moments from the common with the ancient reptiles of the past.