Supercontinents helped life evolve on land by trapping soil

The changing flow of nutrient-rich soil across the planet as the continents shifted seems to have been a key driver of evolution and biodiversity. This suggests that the human-driven soil degradation we see today may be having a larger impact on ecosystems than previously considered.

Tristan Salles at the University of Sydney, Australia, and his colleagues made the discovery with a computer model that recreates the movement of Earth’s landmasses, using data on ancient precipitation, fossil and sediment records and tectonic plates. The model covers the past 540 million years and has a resolution down to as little as 10 square kilometres.

The modelling indicated that it wasn’t until supercontinents began to form, between 400 and 300 million years ago, that large volumes of soil – and therefore nutrients – stayed in terrestrial environments instead of being washed into the ocean.

“Rivers transport nutrients in the sediments and start to promote nice environments for biodiversity to really kick off and start growing,” says Salles.

Before the formation of supercontinents, he says, much of the world’s landmasses had coastal mountain ranges such as we see today in New Zealand and Chile. This meant, when it rained, sediment was swept into the marine environment, helping organisms there to develop but leaving terrestrial environments comparatively devoid of life.

With the formation of giant continents, sediments remained on land, creating thick soils that promoted the development of flowering plants with roots.

“Flowering plants really began to peak between 150 million and 100 million years ago because a lot of sediments stayed on the land instead of flowing into the sea,” says Salles.

The team was surprised to find a near-perfect correlation between the rate of sediment flow and the explosion of new organisms as the result of evolution. For both marine and terrestrial environments, the greater the availability of nutrient-rich sediment, the greater the biodiversity. The converse was also true: when volcanism, plate tectonics or a drop in rainfall led to a reduction in sedimentation, within five to 10 million years biodiversity followed suit.

“I am impressed by how striking the correlation between sedimentation and both terrestrial and marine biodiversity is,” says Alexander Skeels at the Australian National University in Canberra. “Usually in biodiversity studies the patterns are complex and muddy, but this seems like a really clear result.”

While the model is based on reconstructing the past environment, the work raises serious questions about future biodiversity, given how fast humans are driving land degradation along with soil and nutrient loss, says Salles.

Human-driven disruptions of nutrient cycling through erosion and changes in rates of sedimentation are likely to affect the land’s capacity for landscapes to host and evolve biodiverse communities, says Skeels.

“It’s an interesting result and might have biological significance”, says John Alroy at Macquarie University in Sydney, Australia, but it only shows that less sediment corresponds with fewer fossils, not necessarily less biodiversity. Major patterns of biodiversity in the fossil record are governed by preservation, he says. “Less sediments means less fossils, which means less biodiversity for researchers to be able to sample.”

This story is based on an article in New Scientist. The original research was published in Nature.

Bill Gray