Early in the nineteenth century, scientists noticed that embryos of all vertebrates look very similar. We now know that they all begin as a zygote that divides and becomes a hollow sphere of cells, undergo gastrulation, and seem to follow an identical path of development until their tissues begin to differentiate organs that vary greatly among fish, amphibians, reptiles, mammals, and birds.
We now know that the identical stages of develpment are controlled by DNA molecules they all have in common. Scientists believe they have inherited this DNA and pattern from common ancestors.
