Seven hundred million years ago, Earth was in the grip of a winter beyond the imagining of George RR Martin. Even at the equator ice was sometimes kilometers thick. Shortly thereafter, the first animals appeared. A paper in Nature provides the strongest evidence yet that these events were connected.
Glaciers grind down mountains. As they retreat they release a pulse of nutrients into the oceans. After Earth's epic ice coverage back then, known as Snowball Earth, the pulses were phenomenal, leading to a matching flowering of algae. The paper reveals this abundant food supply prompted animals' rise.
The ice-nutrients-algae-animals theory has been around for a while, but Dr Amber Jarrett of the Australian National University (ANU) told IFLScience we have lacked complete geological sequences that could confirm it. However, Jarrett found locations in the Amadeus Basin, Central Australia, provide a record of conditions before, during and after the snowball. The timing of animals’ appearance is exactly as would be expected if the ice was responsible.
The team extracted sedimentary rocks from the basin. “We crushed these rocks to powder and extracted molecules of ancient organisms from them,” said first author, ANU’s Dr Jochen Brocks, in a statement. Although vast passage of time has destroyed the organisms DNA or any microfossils, the molecules can identify the relative frequencies of different categories of lifeforms.
Prior to 720 million years ago, bacteria were dominant. Then, between 659 and 645 million years ago there was a sharp rise in diversity, indicating the presence of planktonic algae, sponges and unicellular rhizarians, followed by the appearance of more complex animals. The timing neatly matches the gap between the Sturtian Glaciation – probably the greatest period of ice cover in Earth’s history – and the Marinoan Glaciation.
We know from molecular clocks that algae are far more ancient than this – originating somewhere between 900 and 1,900 million years ago – but the molecular evidence suggests they were not abundant prior to the snowball. The paper attributes this to bacteria's capacity to outcompete algae in phosphate-poor environments.
Exactly how these newly evolved animals survived the Marinoan Glaciation's bitter cold remains a mystery, but there is evidence it was not as complete as its predecessor, leaving some areas sufficiently ice-free that advanced life could survive, even if its abundance declined.
Previous attempts to date the rise of eukaryotes, including algae and animals, have been hampered by more recently formed oil intruding into older rocks and contaminating samples. Brocks and Jarrett are confident they have overcome this. Jarrett told IFLScience previous theories have associated the appearance of animals with increased atmospheric oxygen, something which the increased algal abundance would have contributed to.
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