The Cambrian explosion was caused by a lack of oxygen, not an abundance

DURING the Cambrian period, which began 541m years ago, animal life took a remarkable leap forward. The first creatures believed by most (though not all) palaeontologists to be multicellular animals appear in the previous geological period, the Ediacaran. But though they are abundant and reasonably diverse, Ediacaran creatures do not look ancestral to modern animals. That is in contradistinction to Cambrian fossils, among which are found representatives of all the main animal groups (annelids, arthropods, brachiopods, chordates, cnidarians, echinoderms, molluscs and so on) that are around today. And these groups appear in what is, in geological terms, an eyeblink.

Several explanations have been put forward to explain the Cambrian explosion of animal life. One of the most popular is that it was fuelled by a dramatic rise in oxygen levels, permitting large and active creatures to thrive. However, a study just published in Geology by Wei Guangyi of Nanjing University suggests this tale is not true. Though oxygen levels certainly did rise in the late Ediacaran and early Cambrian, he and his colleagues suggest that what propelled animal evolution was actually two occasions when they crashed.

Dr Wei’s evidence comes from rocks in the Chinese provinces of Hubei and Yunnan. Specifically, he and his colleagues looked at uranium in limestone that had formed in shallow Ediacaran and Cambrian seas. Uranium has two isotopes, ²³⁵U and ²³⁸U, that differ by three in the number of neutrons in their nuclei. This small mass difference is enough to affect the speed of chemical reactions involving them, particularly if those reactions are biologically mediated by, say, bacteria, and thus involve interactions with enzymes. The upshot is that uranium compounds precipitated in well-oxygenated water have more ²³⁸U in them than those from anoxic water.

Armed with this information, Dr Wei and his colleagues looked at the ratio of uranium isotopes in their rock samples and found two moments when ²³⁸U levels plunged with respect to those of ²³⁵U. The first was between 542m years and 541m years ago—that is, immediately before the Cambrian. The second was between 524m years and 523m years ago, after the Cambrian had been going for some time. Crucially, these dates match what look, from the fossil record, like two pulses of evolution in the history of the animals. The first saw the emergence of brachiopods and molluscs, the second that of annelids, cnidarians, echinoderms and chordates (a group that includes the vertebrates).

In Dr Wei’s view, then, what happened in the Cambrian was similar to subsequent incidents of biological diversification, such as that of the mammals after the extinction of the dinosaurs. First, some sort of environmental catastrophe wiped out many of the previous incumbents. Then, new groups emerged to fill the empty ecological niches. In the case of the dinosaurs, the catastrophe was an asteroid impact. In the case of the Cambrian it was periods of anoxia of as-yet-unknown cause. It remains true that rising oxygen levels on Earth at the time were necessary to permit animals to prosper. But the trigger for their diversity may well have been the reverse.