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 new techniques make 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.
Yet it seems unlikely that the truly alien would resemble us in any way whatsoever. Professor Nnedi Okorafor thinks they could be anything: "I don't think aliens have to be humanoid, carbon-based, or even alive by our definition of alive. Human beings aren't made to survive in space. If aliens arrive on Earth here, it means they have survived in space and have the means to adapt to our atmosphere."
"I think it highly possible that they'll be nothing like us. I don't see why aliens couldn't be microscopic, only be seen at wavelengths beyond human detection, be built in a way so outside of human understanding that to look upon them would cause one to faint. I don't think aliens have been, are, or will be what we are expecting."
Some scientists, however, argue that life on other planets may well follow a theory known as convergent evolution. It suggests species will independently evolve similar traits and features to solve the problems presented to them by their environment. A famous example is the eye of the octopus being identical to the human eye. As a result of this, life on other Earth-like planets would follow similar evolutionary paths to those on our own planet, including the emergence of higher intelligence.
But what about insects, by far the most species rich group on Earth: why shouldn’t aliens look more like them? Unfortunately, having your skeleton on the outside makes growth difficult, and entails periodic shedding and regrowth. On Earth-like planets, all but relatively small terrestrial animals with external skeletons would collapse under their own weight during moulting, and some critical size may be necessary for suitably complex brains.
Ultimately, the jury is out on the extent to which intelligent aliens - if they exist - would resemble us. It may or may not be significant that humans have two eyes and ears, and just two legs (reduced from the initially more stable four), to free up the front limbs. Or that many other organs also come in pairs as a consequence of our evolutionarily deep-seated - and perhaps inevitable - bilateral symmetry.
Still other elements of our body plan are probably nothing more than chance. Indeed, most species have been subject to an accidental 'locking down' during development - making body plans become stereotyped and inflexible with evolutionary time. Untangling the functional from the accidental is one of the big outstanding challenges in evolutionary biology - and may help us better understand how alien lifeforms could differ from us.
Science fiction writer Aaron Rosenberg doesn't think aliens look like us at all: "If we had occurred on a completely different world, with different temperatures and topography and flora and fauna, we would have evolved differently. And if that other world had a completely different chemical composition, so would we. All life on Earth is carbon-based, but that wouldn't be the case elsewhere. Life forms could be silicon-based or iron-based or anything else at all. They could have any number of arms and legs—or none at all."
SETI researcher Seth Shostak argues that if an extraterrestrial species becomes advanced enough to send signals Earthlings can pick up, it will likely shed its traditional biological trappings and become a form of machine intelligence in rather short order. "The human species invented the radio around 1900 and the computer in 1945, and it's already manufacturing relatively cheap devices with greater computing power than the human brain," he explains.
"Unlike Earth organisms, super-advanced extraterrestrial machines would not require water or other chemicals to survive, so they would not be tied to their ancestors' home worlds tightly at all," says Shostak. "And journeying tremendous distances would not be a big deal to these machines, provided they could access enough energy and raw materials to keep repairing themselves over the millennia."
Aaron Rosenberg goes even further than that: "Perhaps life on other planets evolved without physical form or with no fixed form—perhaps there are aliens who are nothing more than sentient clouds, or who have mutable bodies that can alter to suit the needs of the moment. Maybe they can sail through space unaided, and use stellar radiation for a food source and a sensory array, detecting changes in the radiation the same way bats detect sound waves."
That might seem a bit too much imagination because after all, the laws of nature impose limits to what is possible. For example, an organism that functions coherently as a system would ultimately be limited in size by its ability to send signals within itself at the speed of light - the fastest allowable speed in the Universe.
But what if the universal laws of physics no longer apply? Other information systems may exist that correspond to other physical realities, in universes based on logic completely different from ours and not based on space and time as we know it.
Advanced civilizations might have changed the algorithms so that instead of being trapped in the linear dimensions we find ourselves in, their consciousness moves through the multiverse and beyond. Why would aliens build massive ships and spend thousands of years to colonize planetary systems (most of which are probably useless anyway), when they could simply tinker with the data and go whereever they want?
One thing is certain: diversity is a key feature of life on Earth. Try describing a snake to someone who has never heard of it: "There is this animal on Earth that stalks its prey with infrared detectors, swallows whole live animals up to five times bigger than its head, has no arms or legs or any other appendage, yet can slide along level ground at a speed of three meters per second!"
Image: Octopus marginatus seeking shelter between two shells.