While we wait for our first conclusive evidence of life beyond Earth, we can contemplate its possible nature. In particular, we can ask the following question. To what extent should we expect evolution elsewhere to take a similar course to the one it has taken on Earth? That could be described as the key question about the biology, as opposed to the geography, of extraterrestrial life. But the way I’ve just put it isn’t ideal – it’s too centred on our home planet as a reference point. Let’s try to rephrase it in a Copernican manner, so that Earth doesn’t occupy a special place. Here’s one such rephrased version. To what extent does evolution follow similar courses on different inhabited planets? Earth is implicit here, but just as one of many inhabited planets, and almost certainly not the first one.