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About 535 million years ago (the cause is still debated)Reputable source · 2 sourcesDebated

Why so fast? The unresolved cause of the explosion

On the timeline · around About 535 million years ago (the cause is still debated) · The Explosion BeginsThe Explosion BeginsWhy so fast? The unresolved cause of the explosion538 Ma537 Ma536 Ma535 Ma534 Ma533 Ma532 Ma

What happened

No single explanation for the speed of the Cambrian explosion has won out. The Royal Ontario Museum's own account of the debate lays out the leading, competing hypotheses. A rise in atmospheric and ocean oxygen could have allowed larger, more active animals to exist, though oxygen levels show little clear change right at the Ediacaran-Cambrian boundary. Small shifts in Hox genes, the master switches that lay out an animal's body plan during development, could have unlocked a much larger range of possible body shapes from only minor genetic changes. Predation itself may have driven the burst: once some animals began hunting others, the hunted evolved shells and burrowing to escape, and the hunters evolved better senses and weapons in response, an evolutionary feedback loop with no obvious stopping point. Zoologist Andrew Parker has argued more specifically that the sudden evolution of camera-like vision was the trigger, since sight let predators hunt at a distance for the first time, but other researchers, including Martin Brasier, counter that sharp eyes appear too late in the sequence to have started it.

Why it matters

None of these explanations is disqualifying on its own, and most working paleontologists now favor some combination of them rather than a single cause. The debate matters because it is really a question about whether evolution's biggest bursts need an external trigger or can arise from biology alone, once a threshold of complexity is crossed.

How we know

Each hypothesis rests on a different kind of evidence: oxygen from chemical proxies in ancient rock, Hox genes from comparing DNA across living animal groups, predation from bite marks and defensive shells preserved in the fossils themselves, and vision from the compound eyes fossilized directly in Cambrian predators. No single dataset settles the question, which is why it remains open.

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Part of a timelineThe Cambrian Explosion9 events · How nearly every animal body plan alive today appeared within a geological blink, from the first mineral skeletons to the apex predators, calcite eyes, and first fish the fossil record preserves.View all →
Why so fast? The unresolved cause of the explosion · The Cambrian Explosion · SourcedStory