Earth is estimated to be about 4.5 billion years old, and for much of that history, it has been home to life in one weird form or another.
Indeed, some scientists think life appeared the moment our planet’s environment was stable enough to support it.
Most scientists agree that life went through a period when RNA was the head-honcho molecule, guiding life through its nascent stages. According to this “RNA World” hypothesis, RNA was the crux molecule for primitive life and only took a backseat when DNA and proteins—which perform their jobs much more efficiently than RNA—developed.
RNA is very similar to DNA, and today 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.