You can forget about little green men and predatory Xenomorphs: aliens are likely to look a lot like human beings, according to an expert in evolution from the University of Cambridge. In a new book, Simon Conway Morris says that any extra-terrestrial lifeforms will have evolved very much like we have - because an Earth-like planet would be necessary to support life in the first place.
The theory, called convergent evolution, is actually already well established: the idea is that different species evolve similar characteristics because they're living in a similar environment. One of the most commonly cited examples is the octopus camera eye, which works in much the same way as a human eye. Morris thinks that because alien species will be living on planets like our own, they'll evolve along similar lines.