Everything that lives lives somewhere. All organisms exist in an environment of some sort. It may not necessarily be what we think of when we hear the word “environment”, but that’s merely our human bias creeping in. Animals live in a specific environment. So do plants. So do extremophile bacteria, though theirs and ours have essentially nothing in common. Aliens, too, will be found in a certain environment, but which one is very dependent on their evolution.
The nature of Nature
For a long time, scientists and philosophers wrestled with the question of how much an organism’s environment affects its life, the so-called “Nature vs. Nurture” debate. We know now that there is no debate, that both have an impact, but let’s focus on the Nature half for now.
We, as humans, live mostly in temperate and tropical climates with moderate to heavy rainfall. We’re adapted to a fairly narrow band of temperatures, but our technology—clothes, air conditioning, etc.—augments our ability to survive and thrive in more hostile environments. Indeed, technology has let us travel to nearly lifeless regions, such as deep, dry deserts like the Atacama, the frozen wastes of interior Antarctica, and that most deadly environment of all: space.
But puny little us can’t live in such places. Not by ourselves. Other organisms are the same way, and they don’t have the benefit of advanced life-support machinery. So most of them are stuck where they are. Look through history, and you’ll see numerous accounts of wild animals (and indigenous people!) being captured and returned to an explorer’s homeland, where they promptly die.
Now, evolution’s very premise, natural selection, says that the most successful organisms are those best adapted to their environment. Thus, for an alien species, you want to know where it lives, because that will play a role in determining how viable your alien is. An aquatic animal isn’t going to survive very long in rain-shadow desert. Jungle trees won’t grow at 60° latitude. And the list goes on.
Components of an environment
A few factors go into describing the kind of life that can exist in a specific environment, or biome. Most of these boil down to getting the things life needs to perform its ultimate goals: survival and reproduction. For instance, all kinds of life require some form of energy. Plants get it from sunlight and photosynthesis, while animals instead eat things. The environment serves as a kind of backdrop, but it’s also an integral part of an organism’s survival, which is why life’s goals are better suited by becoming more adapted.
On a more useful level, however, we can look at a biome as an area having the following characteristics in about the same quantities:
-
Temperature: Most species can only live effectively at a certain temperature. Too low, and things start to freeze; too high, and they boil. On Earth, of course, water is the primary limiting factor for temperature, though truly alien (i.e., not water-based) life will be constricted to somewhere near the range of its preferred chemical. (Not to say that freezing temperatures are an absolute barrier to life; penguins live just fine in subzero temps, for example.)
-
Sunlight: This is the “energy” component I mentioned earlier. Assuming we’re dealing with a surface-dweller, sunlight is likely going to be the main type of incoming energy. That’s especially true for plants or other autotrophs, organisms which produce their own food. As any horticulturist knows, most plants are also highly adapted to a certain amount of sunlight. They’ll bloom only when the day is long enough, for example, or they’ll die if the nights grow too long, even if the temperature stays just fine.
-
Proximity to water: I was going to label this as “precipitation”, but that turns out to be too specific. Water (or whatever your aliens use) is a vital substance. Every species requires it, and many absolutely must have a certain amount of it. If they, like plants, can’t move, then they must rely on water coming to them. That can fall from the sky as precipitation, or it can come across land in the form of tidal pools, or just about any other way you can think of.
-
Predators and prey: If you remember old science classes, you know about the food chain. Well, that’s something all life has to worry about, if you’ll pardon the anthropomorphizing. Predators adapt to the presence of certain kinds of prey, and vice versa. Take one away, and things go out of whack. Species can overrun the land or go extinct.
Humans get away with a lot in this. Once again, that’s because of our intelligence and technology, and it’s reasonable to assume that a sapient alien race would overcome their own obstacles in much the same way. But everything else has to limp along without the benefits of higher thinking, so other species must adapt to their environment, rather than, essentially, bringing their own with them.
Great upheaval
All environments are constantly in flux. Climate changes, from season to season or millennium to millennium. Rainfall patterns shift, oceanic currents move, and that’s before you get into anything that may be caused by humanity. Then there are “transient” changes in environment, from wildfires to hurricanes to asteroid impacts. These can outright destroy entire habitats, entire biomes, but so can the slower, more gradual shifts. Those just give more warning.
When the environment changes beyond the bounds of a species, one of two things can happen. That species can adapt, or it will die. History and prehistory are littered with examples of the latter, from dodos to dire wolves. Adaptation, on the other hand, can often give rise to entirely new species, distinct from the old. (For an example, take any extant organism, because that’s how evolution works.)
An alien race will have its own history of environmental upheaval, entirely different from anything on Earth. A different series of major impacts, larger tidal effects from a bigger moon, massive solar flares…and that’s just the astronomical effects. Aliens will be the result of their own Mother Nature.
That’s where they become different. Even if they’re your standard, boring carbon-based lifeforms, even if their “animal” kingdom looks suspiciously like an alternate-color version of ours, they can still be inhuman. On Earth, one branch of the mammalian tree gave rise to primates, some of which got bigger brains. On another world, it could have been the equivalent of reptiles instead. Or birds. Or plants, but I’m not exactly sure how that’d work. One thing’s for sure, though: they’ll live somewhere.