A Common Beginning
As adults, pigs, chickens, and zebrafish are very different animals. But surprisingly, all three look very similar in their early stages of development, and chickens and pigs retain many similarities nearly until they are born. As Darwin realized, such developmental resemblances hint at common ancestry, with more closely related animals following more similar paths of development. Today we understand better than Darwin could the reasons for these early resemblances.
Animals, including humans, share many nearly identical genes--"blueprints" for building the body. Different animals develop along similar pathways because they all have inherited the same genes for building limbs, or eyes, or heads. In fact, swapping the gene that triggers eye development in one organism with the corresponding gene in another has little effect--it's the same gene! Differences arise among various species because of changes in when and where such genes become active during development. Ultimately those variations in timing spell the difference between fish, chickens, pigs--and us.