Continental drift, climate change, and evolution

If the time-traveling hero of the science fiction series Doctor Who took you on a trip to Earth’s Jurassic period, what would you see and notice when you and the Doctor stepped out of his iconic spaceship, the TARDIS?

You’d probably feel the heat and humidity. Compared with our modern era, Earth was more volcanically active during and before the Jurassic, which lasted from around 200 Ma (million years ago) to 145 Ma. As a result of that volcanism, the Jurassic atmosphere contained several times more carbon dioxide than it does now. Earth’s mean temperature was 3 °C higher.

Among the fauna you might see are herbivorous dinosaurs, such as iguanodons and diplodocuses, which throve during the Jurassic. If you arrived at the tail end of the period, you might see an archaeopteryx flying in the sky. You’d also see giant pine trees, ferns, and other plants, but one thing would be missing: flowers.

Besides their blooms, flowering plants are characterized by seeds that have a protective covering; they also produce fruit. Scientifically, flowering plants are known as angiosperms, from the ancient Greek αγγείον (bottle, vessel) and σπέρμα (seed). By contrast, the pine trees and ferns of the Jurassic, as well as and their modern successors, lack seed coverings. They are known as gymnosperms; γυμνόσ means naked.

The first angiosperm fossils date from around 130 Ma in the Cretaceous period, which followed the Jurassic and lasted until 66 Ma. In that span, the number of angiosperm species exploded. By the end of the Cretaceous, angiosperms accounted for about 80% of all species of land plant. The rapid rise puzzled Charles Darwin. Twenty years after he had published On the Origin of Species, he wrote a letter to his closest friend, the botanist Joseph Hooker, in which he referred to the diversification of the angiosperms as an “abominable mystery.”

A paper published this week in the Proceedings of the National Academy of Sciences proposes a partial answer to Darwin’s mystery. Anne-Claire Chaboureau, who’s currently a postdoctoral researcher at the French oil and gas company Total, and her collaborators simulated the Earth’s climate from the Triassic period, which preceded the Jurassic and ran from about 250 Ma to 200 Ma, through the Cretaceous.

Chaboureau and her collaborators identify continental drift and the climate change that it caused as a major factor in the angiosperms’ evolutionary success. At the beginning of the Triassic, Earth’s landmass consisted of one giant supercontinent, Pangaea. Regions far from its coast were largely arid. As the period wore on, Pangaea began to split into two separate landmasses: Laurasia to the north and Gondwana to the south. Further splitting and dispersal continued during the Jurassic, which accounts for the period’s more humid climate compared with the Triassic. Still, the continents remained clustered together around the equator.

But by the end of the Cretaceous, Earth’s continental configuration had begun to resemble its current form. The spreading of new ocean crust and the high atmospheric temperature raised the sea levels, which further divided the landmasses.

Charboureau and her collaborators simulated Earth’s climate for a sequence of five continental configurations from the Middle Triassic (225 Ma) to the Late Cretaceous (70 Ma). Because the atmospheric concentration of CO₂ over that span is uncertain, the researchers simulated each continental configuration with three different CO₂ levels.

As the continents dispersed, distances from continental interiors to coasts shrank and more and more of a continent’s land came within reach of abundant rainfall. The poleward shift of landmasses also boosted rainfall. Then, as now, the meridional circulation of air over the tropics creates a moist equatorial belt sandwiched between arid areas. Continental drift reduced the fraction of land in those arid areas and raised the fraction at temperate midlatitudes.

From their simulations, Charboreau and her collaborators could quantify that shift in climate. At the start of the Middle Triassic, 33–50% of land had a distinctly arid climate, whereas 22–27% had a distinctly temperate climate (the range is due to the different CO₂ levels). By the end of the Cretaceous, the proportions had reversed: 20–45% was distinctly temperate; 5–10% was distinctly arid.

That change in climate, together with the fragmentation of the landmasses, created new environments that angiosperms were poised to exploit. Angiosperms’ fruit-enclosed seeds are dispersed by animals. Bees and other pollinators evolved alongside angiosperms to help them reproduce efficiently. Thanks to those advantages, angiosperms quickly and successfully adapted to the new environmental niches.

Darwin surely knew that angiosperms had the potential to out-compete gymnosperms. He knew that Earth’s climate changed in the past and that its crust subsides and folds. But he was unaware of continental drift, a theory proposed in 1912 by Alfred Wegener and eventually accepted by almost all geophysicists by the 1960s. Conceivably, all that Darwin lacked to solve his own abominable mystery was the realization that a geophysical process—continental drift—could accelerate his great discovery, evolution.