Lifeforms

The first thing you need in any story is a character, something to center the action around. Unless you have a very interesting plotline that revolves around stone, you're going to have some kind of life. The easiest thing to do in terms of lifeforms is simply to set your story here on Earth, but I'm guessing a majority of you won't do that, and so here I'll discuss what happens when you create new life. You might want to see the sections Flora and Fauna [not yet up] for more detailed information; this section will mostly pose questions to start your own thought processes, while those [will] give more detailed information.

Sentient Beings

I hesitate to put emphasis on anything in world-building, because one small mistake can screw up your whole system. But if you're creating sentient beings, especially if they're going to be your main characters, well...there are very few things you'll do that can screw up your whole system so badly.

Most sf and fantasy authors default to humanoids for sentient beings. Unless you're building up a huge, very specific ecosystem, that's what I'd recommend you do. The more different your characters are from humans, the more you'll have to describe how they're doing what they're doing, and that's usually a good way to throw your reader right out of the story. In a preface to the short story "Nightfall" by Isaac Asimov , he explains (roughly--I don't have a copy here) that you can think of someone "walking" as moving however strangely an alien would, and their "cars" may be hovering things we cannot imagine, but he would use familiar words because that's not what he wanted his readers to focus on. And that, in my opinion, is exactly right. You want to give your readers a handle, you don't want to distract them, and unless it's completely out of place in your ecosystem, most readers won't even notice that your sentient being is humanoid. It's like the word "said"--it may be boring, but it's invisible on the page, so you don't need to mess with it too much.

Of course, you can do something like...well, almost all science fiction with humanoids, really...and have every sentient race seeded throughout the galaxy by an originating race. But that's optional.

So, what if you want to do something else? You're perfectly free to--lots of people write sentient beings based on angels, demons, dragons, cats, dogs...virtually anything you can think of. Just remember that these lifeforms may have different values and different logic systems than humans. (So may your hominids, but again, that's something your readers won't really notice.) And if you have more than one sentient race, think about their interactions, and how different life would be if there were thinking beings other than humans.

Other Lifeforms

Your world will probably have nonsentient lifeforms as well: plants, viri, fungi, animals... The most important thing to think about is how your ecosystem ties together. Unless your world is some kind of created dimension where the laws of physics don't work the same, you have to have creatures which do not eat to survive, but get energy in some other way. On Earth, planets serve this purpose: they take a virtually unlimited source of energy input (sunlight) and change it into something they can use (through photosynthesis). Then, other organisms feed on the plants, using the chemical compounds the plants made from sunlight. So you'll need some sort of mechanism like that. It doesn't have to be explicit, but neither should you have the Amazing Barren Plain of Carnivorous Animals. Presumably most other creatures would be intelligent enough not to go there, so the Carnivorous Animals would eat each other, and the last ones left would starve. Yay. A very short-lived Amazing Barren Plain, then. And quite boring and unsustainable, without magic or divine intervention.

You have some latitude if your world is created, and less if has evolved...but either way, your ecosystem has to be self-sustaining. Think about what kinds of plants there are, what kinds of microorganisms, if there are herbivores, if there are carnivores...and make sure it makes sense. Also think about the effects on your main characters, i.e. the difference between a human who grew up around tigers and a human who grew up with guinea pigs, or between a human who grew up in a forest and a human who grew up in the desert or on the plains. The surroundings have an effect on your characters' mindset; again, just make sure it matches, and you shouldn't go wrong.

Glossary