What triggered the Cambrian explosion?

For roughly three billion years, life on Earth was microbial mats and, late in the day, soft frond-like things of uncertain affinity. Then, beginning around 539 million years ago and concentrated in a window of perhaps twenty-five million years, the fossil record erupts: eyes, shells, legs, guts, predators, prey, and nearly every basic animal body plan still in use. Darwin himself flagged the suddenness in On the Origin of Species as a legitimate objection to his theory. The objection has been softened by better fossils. It has not been closed.

Why it matters

This is the origin story of animal form, including ours; every skeleton, eye, and brain traces to some Cambrian pioneer. It is also a rare natural experiment in what evolution does when a door opens. Whether the explosion was set off by the environment, by genes, or by ecology decides how we think about the tempo of evolution generally, and about how likely complex animal life is anywhere else. A universe where the Cambrian needed a rare coincidence is lonelier than one where it was waiting to happen.

What has been tried

The candidate triggers have each been seriously developed. Oxygen: animal bodies are expensive, and rising oxygen in the ocean around that time may have lifted a ceiling, though the record of exactly when and how much is disputed. Genes: the developmental toolkit that builds animal bodies, including the Hox genes, evolved well before the explosion, which suggests the fuse was long and the genes were necessary but not the spark. Ecology: the first good predators, and the first good eyes, may have ignited an arms race in which shells, speed, and burrowing all pay at once; one version credits the evolution of vision itself as the light switch. Ocean chemistry, rising calcium that made shells cheap, and the churn of the Snowball Earth glaciations shortly before have their advocates too. Precursor fossils, the Ediacaran biota and small shelly fauna, show the start was less instantaneous than it once appeared.

Where the edge is

The live question is not whether these factors mattered but their order and dependency: which were preconditions, which was the trigger, and whether "trigger" is even the right model for a cascade in which ecology feeds back on environment.

What would count as an answer

A sequenced causal chain, dated tightly enough by fossils and geochemistry to show what changed first and what could not have happened without it.