Image: hydrothermal vent/Wiki
In the beginning, there was a dead sea. The sea spread across most of Earth and it was full of, mostly, vast amounts of dissolved iron. We can imagine it even with a dirty reddish tinge, like iron ore mining effluent except as far as the eye can see, but there was no oxygen in this ocean to react with, so it probably just looked a lifeless silver-gray. Without oxygen, fragile biology would seem to be the furthest thing from the planet's mind. But biology did happen, and that massive step from dead to alive is a mystery still somewhat submerged, where once upon a time disordered chemicals came together into ordered life-stuff and here we are, alive and breathing oxygen on a planet covered in oxygen machines. It worked out.
This process of life-happening is called abiogenesis and is considered to be an adjunct of regular evolution; abiogenesis is otherwise known as a step in molecular or chemical evolution. The basic idea is that of self-organization—the chaotic world forms patterns as it hunts, through vast barrages of pure dumb luck, for stability. It's not really a hunt of course; evolution in any form is far too random. It's like hunting blind.
The molecules floating around in that dead ocean will interact with other molecules, sometimes joining together into new, larger molecules as the result of simple chemical reactions between atoms. The result is a collection of atoms/molecules that, when stable, has lower energy than all of those atoms/molecules might have individually added together. Bigger and more stable molecules and structures form by a sort of natural selection: less stable and weaker molecules/structures will get destroyed easier, while the more stable ones persist and go on to form even bigger and more stable structures, which persist longer than weaker structures and have a greater chance to develop further. Self-organization.
That seems a long step from life, but research has demonstrated that this dead sea, the primordial stew, would have been capable of forming RNA molecules in the regions around superheated undersea vents. This RNA didn't suddenly start coding lifeforms, but simply catalyzed a specific set of reactions that led to more copies of that specific RNA. The catch is that RNA needs energy to do all of this, and that requires some sort of metabolism. But metabolic capabilities are something we associate mainly with relatively modern biological cells and not a prebiotic metallic stew. That problem may have been solved: research out yesterday in the journal Molecular Systems Biology makes the claim that metabolic reactions occurred in the stew not just before life-life but before RNA even developed, providing a possible energy source for that eventual replicator.
"Our results show that reaction sequences that resemble two essential reaction cascades of metabolism, glycolysis and the pentose-phosphate pathways, could have occurred spontaneously in the earth's ancient oceans," explains the University of Cambridge's Dr. Markus Ralser, the study's lead author, in a press release. Glycolysis, for example, is the process by which your cells turn various simple sugars into ATP, the cellular unit of energy. It turns out that nature doesn't need cells at all for this process to occur, and if it was happening in Earth's prelife stew, it could have provided RNA molecules the energy needed to continue replicating. And once replication is in motion, natural selection has a wide open highway on which to grow, eventually, biological life. In a way, the Cambridge discovery is the missing link in abiogenesis.
So, life finds a way—even before life existed. There's nothing particularly mystical about that, it's just chemicals doing what they do in the presence of other chemicals. In a good enough stew (not the "right" stew, just one with a few common cosmic basics, like dissolved metals and water), life finds a way because it's really the only way. Nonetheless, "we were surprised by how specific these reactions were," says Dr. Ralser.
"These results indicate that the basic architecture of the modern metabolic network could have originated from the chemical and physical constraints that existed on the prebiotic Earth," rather than being the product of a biotic Earth. That was sort of the chicken and egg quandary of yore: how did RNA come to exist and thrive in the absence of a metabolic process, a process which we would have expected to eventually result from RNA? The lower sequence in the diagram below shows the extra step:
Image: Ralser et al.
Prebiotic metabolism is an answer that asks another question. If these reactions were just happening in the primoridal soup, how then did they eventually get adopted by biological, intracellular enzymes, the eventual catalysts of metabolic reactions within lifeforms? The precise answer is yet to come, but we can readily enough look to evolution again, this time selecting for enzymes capable of exploiting chemical reactions that would be useful to whatever sort of very early life was hosting them. It's still interesting to consider the thing your body does as its very last step in response to ingested sugars from say, a regular old apple, as a feature of dead Earth. Maybe we shouldn't call prebiotic Earth "dead" at all.