How Did Life Arise on Earth?

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.

The earliest evidence for life on Earth comes from fossilized mats of cyanobacteria called stromatolites in Australia that are about 3.4 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, perhaps as early as 3.8 billion years ago.

Today, there are several competing theories for how life arose on Earth. Some question whether life began on Earth at all, asserting 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 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.

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.

Suppose, Dawkins says, the universe contains a billion billion planets (a conservative estimate, he says), then the chances that life will arise on one of them is not really so remarkable.

Furthermore, if, as some physicists say, 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.

Robert Shapiro, a chemist at New York University thinks 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.