The origin of life is estimated to have occurred between 4.4 and 2.7 billion years ago. The true origin is uncertain, with some suggesting it may be found in stromalites or evidence from the Isua supercrustal belt. The Miller-Urey experiment and Sidney Fox’s research provide insight into how basic organic monomers and protocells may have formed. The RNA world hypothesis suggests primitive RNA molecules catalyzed their own replication. The origin of life remains a topic of research and speculation.
The origin of life is thought to have occurred between 4.4 billion years ago, when the oceans and continents were just beginning to form, and 2.7 billion years ago, when it is widely accepted that microorganisms existed in large numbers due to their influence on isotope ratios in the relevant layers. Where exactly in this 1.7 billion year interval the true origin of life can be found is less certain. A controversial paper published in 2002 by UCLA paleontologist William Schopf argued that undulating geological formations called stromalites actually contain 3.5 billion-year-old fossil algae microbes. Some paleontologists disagree with Schopf’s conclusions and estimate early life at about 3.0 billion years 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 absorbs 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 interpretation of the sediments is less straightforward and paleontologists don’t always agree on their colleagues’ conclusions.
We don’t know the exact geological conditions of this planet 3 billion years ago, but we 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) gas mixture such as methane, ammonia and hydrogen, these scientists synthesized basic organic monomers, such as amino acids, in a completely inorganic environment. Now, floating amino acids are a far cry from metabolism-soaked, self-replicating microorganisms, but at least they give a hint as to how things might have started.
In the great warm oceans of the early Earth, quintillions of these molecules collide and combine randomly, eventually creating a rudimentary protogenome of some kind. However, this assumption is confounded 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, created an Earth-like environment in a laboratory and studied its dynamics. He observed the spontaneous formation of peptides from amino acid precursors and saw that these chemicals sometimes organized into microspheres or closed spherical membranes, which he suggested were protocells. If a few microspheres were formed that encouraged the growth of more microspheres around them, it would be 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 on the origin of life, the “RNA world hypothesis,” suggests that life formed when primitive RNA molecules became capable of catalyzing their own replication. The evidence for this is that RNA can both store information and catalyze chemical reactions. Its critical importance in modern life also suggests that today’s life may have evolved from precursors of all RNAs.
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 find out how it really happened.
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