The Cambrian explosion was a burst of evolutionary creativity during which most of the modern groups of animals, such as arthropods, made their debuts. Anomalocaris (left) was one of the animals that swam the seas at the time. Scientists have ascribed the period's rapid diversification to everything from continental drift to the end of a global deep freeze.
Geochemist L. Paul Knauth of Arizona State University, Tempe, and geologist Martin Kennedy of the University of California, Riverside, posited a botanical explanation after analyzing the abundance of two rare isotopes, oxygen-18 and carbon-13. During the Precambrian, the key events took place in shallow seas, where limestone was forming. Sea level ups and downs mean that runoff from land sometimes infiltrates nascent rocks and changes their isotope balance—fresh water tends to be richer in carbon-12 derived from organic matter and lower in oxygen-18.
To check for signs of this freshwater infusion, Knauth and Kennedy pored over all published records of carbon and oxygen isotope measurements, plotting the relative ratios of carbon-13 and oxygen-18 against each other. They found that values for rocks from late in the Precambrian clustered with those for more recent rocks, which researchers know have been invaded by runoff laden with organic matter. That correspondence suggests that the land was lush during the late Precambrian. What Knauth and Kennedy term the “Precambrian greening” probably was under way by about 850 million years ago, the paper suggests. The identity of the early landlubbers is still uncertain, the most likely candidates being protists and small plants such as mosses and liverworts.
That leaves one step: explaining how the colonization of land by photosynthetic organisms started the diversification of marine animals that occurred between about 540 million and 490 million years ago. As Knauth and Kennedy see it, terrestrial greening would hasten erosion and further enhance growth of photosynthesizers, leading to an increase in the amount of carbon buried in sediments on land and in the sea. In turn, this decline in readily available carbon would allow atmospheric levels of oxygen to surge, permitting the evolution of larger, more complex animals.
Credit: Arthur Weasley, Wikimedia Commons