Which evolutionary developments in living creatures are the results of rare mutations (those which would be unlikely to recur if evolution were 'rerun'), and which were virtually guaranteed from the outset? That's the subject of this extract on the Guardian website from zoologist Richard Dawkins' new book.
Eyes, for example, have evolved independently on 40-60 separate branches of the tree of life, i.e. in 40-60 organisms that share common but sightless ancestors. It hasn't always turned out the same -- there are nine 'kinds' of eye in the animal kingdom -- but sight is a path that evolution has gone down repeatedly.
Other developments are evolutionary one-offs, like the one genus of beetle that, uniquely in the whole animal kingdom, produces chemicals internally which, when mixed, explode! The beetle uses this as a defence mechanism.
From this, biologists can speculate that on an earthlike planet, vision is such a beneficial and realistic target that it is a likely evolutionary step. Assuming distant planets broadly similar to Earth also have their own evolved living creatures, they're very likely to have eyes. Explosive chemical reactions, however, may be a rare property of Earth life. It's conjecture, sure, but we've only seen the result of evolutionary processes (from single cell to complex organism) in a single environment -- our own planet -- and this small, isolated sample is all we have to look at if we want to think about the nature of life in the general case.
I like Dawkins' writing, and this excerpt is really interesting. Wonder if my local library has a copy of the book...