Life originated more than 3.5 billion years ago. However, the evidence does suggest that life can be traced back even further. Scientists speculate that the first lifeforms were nothing more than a “proto-cell” with a self-replicating molecule.
The question of “how life started” remains unanswered to this day. There are many theories, some of which may seem too far fetched, yet almost plausible. One of the oldest fossil evidence for life goes back to 3.45 billion years. Fossilized cyanobacteria formed hard, rock-like structures called stromatolites. Moreover, organisms that lived before the stromatolites left evidence of their existence in the form of “chemical signatures.” Measurements of carbon isotopes in rock formations that date back billions of years have confirmed these findings.
Plausible Theories
The origin of life is a question that is unanswered to this day. Hence, there is no “standard model”; instead, there are just plausible theories which are built on cell biology and molecular biology.
Miller–Urey Experiment
This experiment partially demonstrated how organic molecules could form from inorganic substances on the early earth. Moreover, the experiment accounted for the early earth’s atmosphere, which was quite different from today. Every biogeochemical process was also vastly different or non-existent. For instance, if one were to observe the nitrogen cycle diagram during that time period, it would look nothing like today. Though the experiment was able to recreate most of the organic molecules required for life, it was not able to replicate all the compounds associated with life.
Panspermia
This theory proposed that life developed elsewhere in the universe and then arrived on earth. Microbial spores from space were supposedly brought to earth by asteroids or meteors; which jump-started life on earth. However, this theory states how life spread, not how life began. The validity of this theory is fiercely debated since no conclusive proof exists. Furthermore, this theory ignores the fact that space is incredibly vast, so travelling between planets would take thousands and thousands of years. It would be highly improbable for a lifeform to remain dormant or alive for such long durations.
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