Anaximander was much impressed by the way trees grow and shed their bark. He used the same analogy to explain the origin of human beings. Other animals, he observed, can look after themselves soon after birth, but humans need a long nursing. If humans had always been as they are now, the race would not have survived. In an earlier age, he conjectured, humans had spent their childhood encased in a prickly bark, so that they looked like fish and lived in water. At puberty they shed their bark, and stepped out onto dry land, into an environment in which they could take care of themselves. Because of this, Anaximander, though not otherwise a vegetarian, recommended that we abstain from eating fish, as the ancestors of the human race.
A New History of Western Philosophy 1 & Ancient Philosophy - Anthony Kenny - Oxford University Press