Earth is estimated to be about 4.5 billion years old, and for much of that history it has had life in one form or another.
Life appeared as soon as the environment of our planet was stable and good enough to support life.
There are hints of life in 4.1-billion-year-old zircons rocks fn Western Australia They contain high amounts of a form of carbon typically used in biological processes.
The earliest evidence for life on Earth comes from fossilized cyanobacteria called stromatolites in Greenland that are about 3.7 billion years old. Ancient as their origins are, these bacteria (which are still around today) are already biologically complex—they have cell walls protecting their protein-producing DNA, so scientists think life must have begun much earlier.
But despite knowing approximately when life first appeared on Earth, scientists are still far from answering how it appeared.
Many theories of the origin of life have been proposed, but since it’s hard to prove or disprove them, no fully accepted theory exists.
The answer to this question would not only fill one of the largest gaps in scientists’ understanding of nature, but also would have important implications for the likelihood of finding life elsewhere in the universe.
There are several theories for how life came and started on Earth. Some question whether life began on Earth at all, suggesting instead that it came from a distant world or the heart of a fallen comet or asteroid. Some even say life might have arisen here more than once.
Most scientists agree that life went through a period when a so called RNA molecule went through the first form of life. According to this “RNA World” hypothesis, RNA was the crux molecule for primitive life and got overtaken by better functioning DNA and proteins.
RNA is very similar to DNA, and still carries out numerous important functions in each of our cells, including acting as a transitional-molecule between DNA and protein synthesis, and functioning as an on-and-off switch for some genes.
But the RNA World hypothesis doesn’t explain how RNA itself first arose. Like DNA, RNA is a complex molecule made of repeating units of thousands of smaller molecules called nucleotides that link together in very specific, patterned ways. While there are scientists who think RNA could have arisen spontaneously on early Earth, others say the odds of such a thing happening are astronomical.
The appearance of such a molecule, given the way chemistry functions, is incredibly improbable. It would be a once-in-a-universe long shot. To adopt this view you have to believe we were incredibly lucky.
Many believe that, since the universe contian billions of billions of planet, the chances that life has arisen on one of them is more likely that that it originated on earth.
If our universe is just one of many, and each universe contained a billion billion planets, then it’s nearly a certainty that life will arise on at least one of them.
It is now believed that life started with molecules that were smaller and less complex than RNA, which performed simple chemical reactions that eventually led to a self-sustaining system involving the formation of more complex molecules. And these molecules traveled the universe on asteroids. We only needed one of those to hit the earth that started life evolving in to everything you see on our planet today.