Origins of Life on Earth

Earth formed about 4.6 billion years ago as a molten ball, then cooled enough for a solid crust and liquid water within a few hundred million years. The oldest widely accepted fossil evidence of life, stromatolites and chemical signatures from ancient rock, dates to roughly 3.5 billion years ago, meaning life arose relatively quickly once conditions allowed liquid water and basic chemistry to interact. That gap between a cooling planet and the first clear fossils is where origin-of-life chemistry has to happen, and nobody watched it occur, so scientists build models from chemistry, geology, and lab experiments rather than direct observation.

The biggest puzzle is the chicken-and-egg problem of modern cells: DNA stores information but cannot copy itself without proteins, and proteins do the catalyzing but need DNA's instructions to be built. Neither works alone. RNA offers a way out, because it can both hold a sequence of information and fold into shapes that catalyze reactions, including copying itself. A self-replicating RNA molecule that made occasional errors would produce variants; some variants would replicate faster or more accurately than others, and those would become more common over time. That is natural selection acting on chemistry before proteins, DNA, or even a cell existed.

This matters for the rest of the unit because it extends natural selection's logic, differential success among variants, all the way back past the first true organisms. It also connects to how scientists revise timelines: when older fossils or chemical biosignatures are found, the window for life's origin shrinks, pushing the estimated start of life earlier without changing the underlying mechanism proposed.