The answer, according to evolutionary biologist Simon Conway Morris, lies in the phenomenon of evolutionary convergence: the process by which distantly related animals come to closely resemble each other. For example, the similar streamlined shape of dolphins, tuna fish and the extinct ichthyosaurs all evolved independently in response to the same selective pressures for moving efficiently through water at speed.

But what aspects of alien biology might we expect? Carbon-based biochemistry is likely given that carbon forms stable backbone chains, and makes stable but readily breakable bonds with other elements. Other elements, notably silicon and sulphur, make less stable bonds at Earth-like temperatures. Water or some other solvent also seems necessary. For evolution to occur there needs to be some mechanism for storing and replicating information with moderate fidelity, such as DNA, RNA or some analogue. Although the first cells appeared on Earth quite early, multicellular animals took nearly 3 billion more years to evolve. So it may well be that life on other planets could get stuck at the single-celled stage.

On an Earth-like planet it is also likely that radiation from the alien sun or suns would be used in biochemical pathways as a source of energy. For moderately large multicellular primary producers, harnessing light efficiently probably necessitates a light gathering system of leaves and branches. Similar shapes and habits have evolved convergently on Earth, so we might expect “plants” with broadly familiar forms on Earth-like planets.