Darwin could not explain why animal fossils seemed to appear from nowhere
In 1859, Darwin admitted a troubling gap in his theory. Trilobites and other complex animals burst into the rock record with no visible ancestors beneath them. He guessed that older fossils existed but had simply not survived or been found. More than a century of digging has partly proved him right, and partly deepened the mystery.
Starting roughly 538.8 million years ago, over a span of perhaps 13 to 25 million years, nearly every major animal group appears in the fossil record, including the earliest chordates, the lineage that would lead to us. Before then, life was mostly single cells or small multicellular organisms. Geologists noticed the abrupt change long before anyone could explain it: William Buckland remarked on the sudden richness of these strata in the 1840s, and Darwin, in On the Origin of Species, called it among the gravest difficulties for gradual evolution by natural selection. The American paleontologist Charles Walcott later proposed a missing interval, which he called the Lipalian, when Cambrian ancestors evolved but left no fossils.
Older life has since turned up. Complex cells have been found in Chinese and Montanan rocks some 1,400 million years old, and the strange, large Ediacaran organisms date from 580 to 543 million years ago. Modern fascination was ignited in the 1970s when Harry Whittington's team re-examined fossils from Canada's Burgess Shale and found creatures unlike anything alive, such as the five-eyed Opabinia and the spiny, slug-like Wiwaxia. Stephen Jay Gould's 1989 book Wonderful Life made the finds famous.
Much of what we know comes from rare sites that preserve soft tissue, called lagerstätten, unusually common in Cambrian rock. In 2019, researchers announced the Qingjiang biota from Hubei province in China: more than 20,000 specimens around 518 million years old, with muscles, gills, guts and eyes still visible, and about half the species previously unknown.
Interpreting all this is hard. Most animals are soft-bodied and rot before fossilising, so two-thirds of living animal phyla have never been found as fossils, and organisms typically exist long before they first show up in rock. Even the start date keeps moving, revised from 542 to 541 and then to 538.8 million years. Chemical signals hint at dramatic environmental upheaval around the boundary, but whether it triggered the burst of life or resulted from it remains open.
Source: Cambrian explosion
