A continental crash that raised one of the biggest mountain chains in the Earth’s history may be responsible for the explosive diversification of animals more than 500 million years ago.

Sediments washed from the mountains – dubbed the Transgondwanan Supermountain – added vital nutrients to the ocean, opening new evolutionary opportunities, says Rick Squire, now at Monash University in Clayton, Victoria, Australia.

The rapid proliferation of animals that occurred at that time is one of evolution’s biggest enigmas. Life had remained simple and largely single-celled for nearly three billion years, until the multi-celled Ediacara fauna evolved, 575 million years ago.

Most major groups of animals evolved during a second radiation, called the Cambrian explosion, from 530 to 510 million years ago. The mystery of what suddenly kick-started animal evolution has been a topic of hot debate among experts.


Block war

Rising oxygen levels were clearly vital, but may not have been the only factor, Squire argues. Other suggestions include changes in the Earth’s orbit or exceptionally rapid motion of the continental plates on the Earth’s surface. Yet those candidates did not occur at the right time, Squire says.

He suggests the trigger was the collision of a series of three large continental blocks roughly corresponding to Arabia, India, and Antarctica – with the eastern edge of Africa from 650 to 515 million years ago.

The drawn-out continental impact raised a vast 1000-kilometres-wide mountain range that stretched for more than 8000 kilometres along the equator on the ancient land mass known as Gondwana. Heavy rains typically fall along the equator, which would have produced a high level of erosion – it was before the evolution of land plants.

Squire’s research group has traced the resulting offshore sediment deposits around the world, and say they eventually amounted to more than 100 million cubic kilometres – enough to cover the entire US up to 10 kilometres deep. (The Petra cliffs of Jordan are a good example of such sediment deposits, and is famed for structures carved into the stone).

Protective shells

Minerals in the sediments then dissolved, releasing chemicals essential for life, including phosphorous, iron, calcium, and bicarbonate ions. By the time India collided with Africa 580 million years ago, “the mountains got to the crucial size, [releasing] enough nutrients into the ocean” that Ediacara animals could evolve, Squire told New Scientist.

Big deposits of phosphate – a critical nutrient – accumulated, he says. Then Antarctica collided with Africa near the start of the Cambrian explosion, raising more mountains and releasing more sediment. From 543 to 515 million years ago the concentration of calcium tripled. Animals from trilobites to bivalves used the calcium to build protective carbonate shells, which first appeared during that period, Squire says.

There may be no single explanation, says Mary Droser at the University of California at Riverside, US, who was not involved in the research. “Oxygen clearly goes across the board,” but the Ediacara was extraordinarily complex time physically, chemically and biologically. It is “not going to be simple” to explain, she says.

Journal reference: Earth & Planetary Science Letters (vol 250, p 116)

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