Religious accounts of life's origins tend to favor a little outside influence. A divine hand reaches down from the sky, perhaps, and stirs this biology into existence.
As fantastic as this sounds, scientist consider a similar god-free possibility in the form of panspermia.
Panspermia theory holds that life didn't originate on this planet at all, but traveled here on an asteroid some 4 billion years ago. This vital fragment might even have originated on Mars, blasted off the surface by a primordial bombardment of cosmic bodies.
See, bacteria are some tough mama jamas, capable of thriving in extreme conditions -- including a jaunt through the harsh void of outer space.
So think of our planet as a huge Petri dish, filled with excellent fodder for life. An incoming asteroid is like a q-tip coated in bacteria. Get the idea? The seed of life travels from one environment to a new one, and there thrives into a new ecosystem.
We call this theory exogenesis, the notion that life on Earth originated elsewhere in the universe.
Robert Lamb is a senior writer and podcaster at HowStuffWorks, where he co-hosts Stuff to Blow Your Mind with Julie Douglas. He has a love for monsters, an aversion to slugs and a hankering for electronic music.