This excerpt from Richard Dawkins' new book The Ancestor's Tale: a Pilgrimage to the Dawn of Evolution apparently engages with an intriguing line of inquiry: If evolution could be re-run, how might the story end differently? To quote Dawkins:
What systems have evolved many times independently, like eyes?
Or "several times", like echolocation? Have some things evolved only
once, or not at all? I suspect that we'd find certain potential evolutionary
pathways which life is "eager" to go down. Other pathways have more
The cool part of the excerpt, IMHO, is his consideration of evolutionary one-offs. Amazing stuff.