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.
With few exceptions, animals either eat the primary producers or each other, and there are only so many ways of doing this. Pursuing food often necessitates moving with the mouth first, so the animal has a head and tail end. Teeth and probably jaws evolve to hold and tackle food items. Moving against a hard surface requires specialised structures (such as cilia, a muscular foot or legs) at the interface, so that there is a back and front side. Typically, this also imparts bilateral (left/right) symmetry: indeed, most animals belong to a “super-group” called the Bilateria.
Why not giant intelligent “insects”?
But what about the large brained and intelligent creatures that might be capable of crossing interstellar distances? Insects are 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.
New Zealand Department of Conservation, CC BY-SA
Relatively large brains, some degree of tool use and problem-solving abilities appear to be correlated on Earth, and have evolved multiple times: in apes, whales, dolphins, dogs, parrots, crows and octopuses. However, the apes have developed tool use to a vastly greater degree. This is at least partly the result of walking on two legs, which frees up the front limbs, and because of the dexterity of our fingers (which may also be a key to the origins of written language).
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 just two eyes and ears (just enough for stereo vision and hearing), and just two legs (reduced from the initially more stable four). 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. The fact that we have hands and feet with five digits is a consequence of the fixation on five in our early tetrapod ancestors – close relatives experimented with seven or eight.
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.
The main way we now search for intelligent life in space is by listening for radio or gamma transmissions. These efforts are increasingly being concentrated on star systems with Earth-like planets, as these are believed to be the most likely to harbour life. After all, it is easier to search for “life as we know it” than life as we don’t.