7.12 Origins of Life on Earth

Geology sets the window for when life could have started. Radiometric dating of meteorites and the oldest rocks shows that Earth formed about 4.6 billion years ago (bya). For its first few hundred million years, Earth was extremely hostile: the surface was hot, and large asteroid impacts were frequent enough to melt or sterilize the surface. Conditions likely became suitable for life around 3.9 bya.

The earliest fossil evidence of life is about 3.5 billion years old. These are fossils of microbes, including stromatolites, layered rock mounds built by mats of bacteria. Since life was already present by 3.5 bya, and conditions allowed it only after about 3.9 bya, life most likely began somewhere in that window of a few hundred million years.

Most models describe life arising in steps: simple organic molecules formed from inorganic ones, then those monomers joined into polymers, then polymers became enclosed in membranes, and finally molecules that could copy themselves gave rise to heredity. Laboratory experiments support early steps. In 1953, Stanley Miller and Harold Urey ran electric sparks through a mix of gases thought to resemble the early atmosphere, plus water, and produced amino acids. Some meteorites have also been found to contain amino acids, suggesting organic molecules may have come from space as well.

Modern cells have a chicken-and-egg problem: DNA stores the instructions for proteins, but proteins (enzymes) are needed to copy DNA. Which came first? The RNA world hypothesis proposes that RNA came before both, because RNA can do both jobs. It stores information in its base sequence, and some RNA molecules, called ribozymes, act as catalysts.

The hypothesis rests on three assumptions:

• At some point, genetic continuity, meaning passing information to the next generation, was maintained by copying RNA.
• Base pairing was necessary for that copying, just as it is for DNA replication today.
• Genetically encoded proteins were not yet acting as catalysts; RNA itself did the catalyzing.

Several observations fit an RNA world. In today's ribosomes, it's the rRNA, not a protein, that catalyzes peptide bond formation, which looks like a leftover from a time when RNA ran the show. Scientists have made ribozymes in the lab that can copy short RNA strands. And RNA nucleotides are used throughout metabolism, as in ATP.

Later, DNA likely took over long-term information storage because its double strand is more stable and easier to repair, while proteins, with 20 different amino acids, became better, more varied catalysts. RNA kept its role as the go-between.