AP BIOLOGY

Detailed explanation-1: -Human embryos never pass through a stage equivalent to an adult fish or bird. Rather, human embryos initially share characteristics in common with fish and avian embryos. Later, the mammalian and other embryos diverge, none of them passing through the stages of the others.

Detailed explanation-2: -The greatest similarity arises in the middle of embryonic development, during the “phylotypic stage"; species-specific differences predominate before and after this stage.

Detailed explanation-3: -During the phylotypic stage, embryos of birds, fish and even humans start to look the same-before they diverge again and become very different looking animals.

Detailed explanation-4: -Embryos for humans and other animals often look alike at certain developmental stages because they share ancient genes. These ancient genes are expressed during a middle “phylotypic period” of embryonic development for all species.

Detailed explanation-5: -So why do these embryos look so much alike? The basic design of all these animals is more similar than you might think. Since all vertebrates (animals with backbones) evolved from a common ancestor, the genetic information that guides their development is nearly the same.