Question: so if we were once fish then why can't we breath under water

James Verdon answered on 28 Jun 2012:

Hi russia,

To breathe underwater, we need to have gills. However, when our ancestors moved on to land, the gills that they had were no longer useful, because they began using their lungs to breathe. Because the gills were not useful, natural selection favoured animals that had smaller gills. Generation by generation the animals with smaller gills were more successful, so gills got smaller and smaller, wasting away until the land animals had no gills at all, and they couldn’t breathe underwater (which didn’t bother them, because the water is a dangerous place for a land animal to be, because you’d be an easy target for predators).

However, we know that humans evolved from animals with gills. When a human embryo is developing in the womb, it actually begins by developing gill-like structures – arches and pouches – in the sides of the neck. They are called branchial clefts. However, rather than developing into gills, as they do for fish, while the embryo is growing they now develop into other important structures in our face and neck. Occasionally, something goes wrong during the development of the embryo, and branchial cysts develop that must be removed by surgery when the baby is born.