Speculating about what aliens look like has kept children, film producers and scientists amused for decades. If they exist, will extra terrestrials turn out to look similar to us, or might they take a form beyond our wildest imaginings? The answer to this question really depends on how we think evolution works at the deepest level.
Hollywood has given us its fair share of humanoid aliens over the years. Initially this was through necessity, as special effects required someone to clamber into a rubber suit. Ironically, now that CGI makes anything possible, aliens sometimes look even more human in order to help the cinema goer make an emotional connection with them – such as in James Cameron’s Avatar.
At present, the only life forms we can study are here on Earth. These had a single origin around 3.5 billion years ago, but this common ancestor gave rise to perhaps 20m living species of animals alone. These have bodies organised according to about 30 different body plans in major groups called phyla.
Nobu Tamura/wikimedia, CC BY-SA
But when animals first diversified some 542m or more years ago in the Cambrian “explosion”, there may have been an even greater diversity of fundamental body plans. Consider the five-eyed and trunked Opabina in the image above, or the stalked and almost flower-like Dinomischus alongside our own distant relative, the chordate Pikaia.
Rerunning the tape of life
In a famous thought experiment, biologist Stephen Jay Gould asked what might happen if we were to rewind the “tape of life” and rerun it. Gould argued for the importance of chance in evolution: change one small thing early on, and the consequences magnify through time. In the version of history we know, Pikaia (imaged below) or something very like it survived and ultimately gave rise to fishes, amphibians, reptiles, mammals and ultimately ourselves. But what if it had perished? Might some other group have given rise to intelligent beings, and might you now be reading this with five eyes rather than the customary two? If our own origins on Earth really turned on such fine hinges, why should aliens – evolving on different planets – even remotely resemble us?
Nobu Tamura/wikimedia, CC BY-SA
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.