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The origin of life are thought to have occurred sometime between 4.4 billion years ago, when the oceans and continents were just starting to form, and 2.7 billion years ago, when it is widely accepted that microorganisms existed in vast numbers due to their influence over isotope ratios in the relevant strata. Where exactly in this 1.7 billion year range the true origin of life can be found is less certain. A controversial paper published in 2002 by the UCLA paleontologist William Schopf argued that wavy geological formations called stromalites in fact contain 3.5 billion year old fossilized algae microbes. Some paleontologists disagree with Schopf’s conclusions and estimate the first life at around 3.0 billion years of age instead of 3.5 billion.
Evidence from the Isua supercrustal belt in Western Greenland suggests an even earlier date for the origin of life – 3.85 billion years ago. S. Mojzis makes this estimate based on isotope concentrations. Because life preferentially uptakes the isotope Carbon-12, areas where life has existed contain a higher-than-normal ratio of Carbon-12 to its heavier isotope, Carbon-13. This is widely known, but the interpretation of sediments is less straightforward, and paleontologists do not always agree on their colleague’s conclusions.
We do not know the exact geological conditions of this planet 3 billion years ago, but we do have a rough idea, and can recreate these conditions in a laboratory. Stanley Miller and Harold Urey recreated these conditions in their famous 1953 investigation, the Miller-Urey experiment. Using a highly reduced (non-oxygenated) mixture of gases such as methane, ammonia, and hydrogen, these scientists synthesized basic organic monomers, such as amino acids, in a completely inorganic environment. Now, free-floating amino acids are a far-cry from self-replicating, metabolism-imbued microorganisms, but they at least give a suggestion as to how things might have gotten started.
In the large warm oceans of early Earth, quintillions of these molecules would randomly collide and combine, eventually making a rudimentary proto-genome of some sort. However, this hypothesis is confused by the fact that the environment created in the Miller-Urey experiment had high concentrations of chemicals that would have prevented the formation of complex polymers from the monomer building blocks.
In the 1950s and 1960s, another researcher, Sidney Fox, made an early-Earth-like environment in a lab and studied the dynamics. He observed the spontaneous formation of peptides from amino acid precursors, and saw these chemicals sometimes arranged themselves into microspheres, or closed spherical membranes, which he suggested were protocells. If certain microspheres formed which were capable of encouraging the growth of additional microspheres around them, it would amount to a primitive form of self-replication, and eventually Darwinian evolution would take over, creating effective self-replicators like today’s cyanobacteria.
Another popular school of thought about the origin of life, the “RNA world hypothesis,” suggests that life forms when primitive RNA molecules became capable of catalyzing their own replication. Evidence for this is that RNA can both store information and catalyze chemical reactions. Its foundational importance in modern life also suggests that life today may have evolved from all-RNA precursors.
The origin of life continues to be a hot topic for research and speculation. Maybe one day there will be enough evidence, or someone smart enough, that we'll learn how it actually happened.
I remember seeing a documentary on public television years ago that suggested life began with a lightning bolt. All of the raw ingredients mentioned in this article were in place, but it took something as powerful as lightning to make the jump from inanimate to animate.
There was another theory that life began when meteors struck the earth and in a sense "infected" it with life. The first living things then started spreading like a virus until the entire surface was covered in a primordial ooze, like moss growing on rocks.