Humans belong to a sub-clasification of animals called deuterostomes (or second-mouth). During embryological development there is a period where part of the tissue “folds” back into itself creating what’s called a blastopore. The blastopore, in insects (and other members of the “other” group called protostomes, or first-mouth), forms the mouth. Not so for us and other deuterostomes. Oh, no. Our blastopore develops into the anus. We come out butt first.
For discussion on such phylogenetic fun and sillyness, I stumbled across this tidbit from Greg Morrow:
So that’s it for our animal family reunion: empty bags of water, worms with heads shaped like penises, and spiny things that hurt if you step on them. And all of us talk out of our ass.
However, one finds that the HOX genes work similarly in mice and flies (see illustration in the Wikipedia article). So the bilaterality of vertebrates and arthropods goes deeper than the distinction in embryonic development.
But then, why shouldn't embryos evolve too? Earlier today, I was reading about the very early development of the human embryo, and it is quite different from that of the chick embryos in the open eggs that we had at school many years ago, and very different from the tadpoles in frog spawn.