We don't know if the nearest aliens are single-celled organisms, or more complex animals, or full-fledged sentient societies. Frankly, we don't even know if they're there. But even with all of that uncertainty, there are a few things that we can predict with some amount of confidence.

A study that came out in 2017 made a very basic, yet paradigm-shifting point. Regardless of the specific ways that alien lifeforms might evolve, all evolution must follow the principles of natural selection (unless, of course, those aliens are being bred as intergalactic livestock). That means that, much like life on Earth, life on other planets will probably be somewhat hierarchical in nature — just as our single-cell origins can be read in our individual cells, alien lifeforms will probably take a similar path from lower- to higher-level complexity.

To demonstrate this principle, the researchers came up with the "octomite," which is a purely hypothetical organism made up of a hierarchy of other organisms. It's almost cute, in a purely hypothetical way.

Another prediction that seems like a (reasonably) sure bet: Alien lifeforms will probably be at least somewhat symmetrical. After all, virtually all forms of Earth life are — and so are inorganic phenomena such as crystals and galaxies. Symmetry is a useful baseline from which many evolutionary traits can arise.

Finally, there's another concept cribbed from evolutionary biology that could shine a light on possible alien forms — convergent evolution. Here's an example: Both bats and whales have evolved the ability to echolocate, and it's not because of any genetic relationship between each other. Instead, echolocation is a solution that works well for both, and so it developed more than once.

So you might say to yourself, "Self, you're very well put together. Your brain is big enough to do some awesome things. Your hands make it easy to manipulate and move stuff, and your vertical posture makes it so you can use that stuff. Any alien would do well to copy you!" Your pep talk, though a bit strange and specific, is quite right — we might expect aliens to look like us thanks to convergent evolution. But on the other hand, we might not.

The thing is, it's not really quite so simple. Convergent evolution explains why the eyeball has evolved so many different times (and why it's a pretty safe assumption that aliens would have some way of detecting light), but it doesn't mean that there's only one "best" solution to any given evolutionary hurdle, or that every single creature that encounters that hurdle will land on the same solution as another species.

Consider the Tyrannosaurus. It's a great design, isn't it? But it hasn't really seen a revival in the same way that other body types have. That's because, even though a thousand-pound head full of teeth and two long, powerful legs to run on is a great combination, it's not the only combination that adds up to a formidable predator. It's easy to assume that aliens might have body types similar to ours, and it shouldn't be surprising if it turns out to be the case. But it's important to remember that just because it works well for us, that doesn't mean it would work well in every environment — and more significantly, it doesn't mean that it's the only body type that would work.

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