Another line of evidence supporting evolution is embryology. If you look at fish, chicken, pig, and human embryos you will notice great similarities (right). It is now clear that embryos of more advanced animals resemble those of less advanced animals at different stages of development. Early mammalian embryos have many characteristics which are common to fish and amphibians. The young human embryo, for example, has gill slits, pairs of aortic arches, a fish-like heart with a single atrium and ventricle, as well as a tail with muscles for wagging.