Evolution is an entirely uncontroversial idea, at this point, but how could it get started? Life begets life, but where did the first life come from?
If we want to explain the origin of life without invoking a deity (which, contrary to popular belief, creates a lot more questions than it could possibly answer), it must have come from non-life at some point: abiogenesis.
Let’s start by saying what abiogenesis is not: it’s not spontaneous generation.
Spontaneous generation is the idea that complex life forms can appear fully formed, and that’s what Pasteur disproved (kind of; you can’t prove a negative, obviously). The important part about abiogenesis is that it’s gradual, like evolution itself. Early life would have been less complex than even modern viruses.
Now, the exact details of abiogenesis are still far from certain, so what I’ll do is first describe the best guess we have so far, and then describe a model that takes it from there, and is probably similar to the kind of mechanism that led to abiogenesis.
The primordial soup hypothesis states that prebiotic Earth had oceans that formed a type of warm soup with tons of simple organic molecules (such as CO2) dissolved into them. In this soup, these simple molecules combined into more and more complex molecules, eventually forming the amino acids which form the basis of life.
In 1936, Aleksandr Ivanovich Oparin demonstrated that organic molecules could be created in an oxygenless atmosphere, under the influence of sunlight.
He suggested these molecules would combine in growing complexity until they dissolved into a coacervate droplet.
These droplets could then fuse with other droplets and break apart into more droplets, and whatnot. A simple form of reproduction with partial inheritance.
Favorable attributes such as increased durability in the structure would survive more often than nonfavorable attributes, so they’d become dominant. So there’s a basic selection as well.
Influenced by this idea, and by the work of J. B. S. Haldane, Stanley L. Miller and Harold C. Urey formulated and carried out the now-famous Miller-Urey experiments, in which they attempted to recreated the chemical conditions the primitive Eart would have had. They started out with methane, ammonia, water vapor, and hydrogen, mostly. An external energy source was provided in the form of electric sparks, simulating lightning.
Within two weeks, thirteen of the twenty amino acids used in life had formed spontaneously. Very impressive results.
It’s worth pointing out that larger organic molecules probably wouldn’t have been stable, and would have fallen apart relatively quickly under most circumstances.
Still, perhaps not.
This was first suggested by Graham Cairns-Smith. The idea is simple and elegant.
Clay crystals form naturally from silicates in solution, and like all crystals, they preserve their structure as they grow. When bits snap off from the main crystal, they continue to grow, preserving the structure of the main crystal. This is a simple form of reproduction with inheritance. Nobody would say this is life, though.
There is a simple selection as well: crystals of forms that grow more easily will eventually dominate.
From here, it’s not hard to imagine some structures might happen to influence their environment in ways that are condusive to more efficient replication. A stickier clay crystal is more likely to silt a stream, perhaps, thereby creating an environment that’ll make further sedimentation, and thus growth, easier.
It’s conceivable that some crystals could find ways to trap certain kinds of molecules (perhaps, indeed, the amino acids of the Miller-Urey experiment, or even simpler organic molecules) on their surface to replicate even more efficiently, perhaps arranging them in certain combinations. This could create rather complex (comparatively speaking) organic molecules, catalysed by the surface properties of the silicates.
Eventually, these organic molecules learn to replicate on their own: they’ve become proto-DNA.
They no longer depend on the clay for their reproduction, and because they need so much less space and time to reproduce than the clay crystals do, they become the dominant type of “life” (the cut-off between life and non-life being, of course, mostly arbitrary). Clay crystals might continue to reproduce, but they’re a dead end.
Now, while this is a good model of the kind of process that might have been involved in the prehistory of DNA, it’s very unlikely that this was the exact mechanism.
Still, it’s useful to help us think about abiogenesis.
If you want to know more, TalkOrigins has a section on abiogenesis as well.
It should be noted that I’m not an expert, so if got anything wrong, feel free to point it out. We’re all here to learn~