\section{The Cambrian Explosion}

\lettrine[lraise=0.1]{A}{bout} 540 million years ago, after roughly 3 billion years of almost exclusively microbial life, the fossil record shows an extraordinary proliferation of complex animal body structures appearing in geologically very rapid time --- perhaps 20 million years, a geological blink of an eye.

It's known as the Cambrian Explosion.

Virtually every major animal body plan alive today --- left-and-right symmetry, eyes, appendages, nervous systems --- appears in this window. Before it: microbial mats and simple soft-bodied organisms. After it: the full complexity of animal life.

The trigger is still debated --- rising oxygen levels, ecological feedback loops, genetic innovations, possibly all three. But the threshold is unmistakable: something altered the equilibrium sufficiently that an entirely new order of organism complexity became possible.