For most of Earth’s history, life was small and simple. Then, starting around 539 million years ago, the fossil record fills with animals: creatures with shells, legs, eyes, guts and jaws, including early relatives of many of the main groups of animals alive today. In geological terms it happened in a flash, over perhaps 13 to 25 million years. This is the Cambrian explosion, and what set it off is still one of the great open questions in the history of life.
Darwin’s problem
The sudden appearance of complex animals worried Darwin. In On the Origin of Species, in 1859, he called the abrupt appearance of trilobites in the oldest fossil-bearing rocks then known a difficulty “of the gravest nature” for his theory, and put it down to gaps in the fossil record. He was partly right. Older life has since been found, including the strange soft-bodied Ediacaran organisms of about 580 to 543 million years ago. But the burst of animal diversity in the Cambrian remains real.
Windows into the Cambrian
Most fossils preserve only hard parts. A few rare sites preserve soft bodies too, and they have transformed what we know. The most famous is the Burgess Shale in Yoho National Park, British Columbia, about 508 to 505 million years old. It was discovered by the palaeontologist Walcott in 1909, and by 1924 he had collected more than 65 000 specimens. From the 1960s a new generation of researchers re-examined them and reinterpreted many of the animals. The fossils formed at the foot of an underwater cliff about 160 m high, below the depth reached by storm waves, where animals were buried rapidly by mud carried down in currents.
An even older window opened in China in 1984, when the palaeontologist Hou found soft-bodied fossils at Chengjiang in Yunnan, no more than about 518 million years old. They were laid down in a shallow, muddy sea near a river delta and include about 185 species described by 2006, among them some of the earliest fish-like animals. In 2019 researchers reported the Qingjiang site, with more than 20 000 specimens, where about half of the species were new.
Anomalocaris
A top predator of these seas was Anomalocaris, a swimming animal about 38 to 60 cm long, depending on the estimate. It had a pair of spiny arms in front of its head, flaps along its sides for swimming, a fan-shaped tail, and large compound eyes on stalks; one eye from Australia has more than 24 000 lenses. Its parts, and those of its close relatives, were found separately long before anyone realised they belonged together: an arm was first taken for a shrimp, a round mouth for a jellyfish and a body for a sponge.
It was long imagined crushing trilobites in its arms. A 2023 study modelled those arms and found them flexible and their spines too fragile for that. They were built for speed, so Anomalocaris more likely snatched fast, soft-bodied animals in open water.
When and why
The start of the Cambrian is officially dated to about 538.8 million years ago. A 2023 study of volcanic crystals in rocks in Nevada argued it was younger than 533 million years, which would squeeze the early burst into an even shorter time. Not everyone agrees: a 2024 study of the same formation found no need to move the date.
Why it happened is unresolved. A rise in oxygen in the oceans was long the favourite trigger, but a 2025 study found that oxygen in the deep ocean was still swinging up and down between about 485 and 380 million years ago, long after the explosion. Another 2025 study analysed more than 170 trace fossils: by about 545 million years ago tracks were becoming smoother, with fewer abrupt turns, a sign that small animals were gaining control of their movement, so some of the groundwork was laid before the Cambrian began.
