The first animals may have lived 890 million years ago: On Canada’s coral reefs, researchers have identified structures that may have come from sponges. If the results are confirmed, they are the oldest known fossil animals. At that time, the Earth’s oxygen content was still too low for animal life. The sponges could have compensated for this by settling near oxygen-producing microorganisms.
Sponges live in all seas of the world today. Primitive animals that do not have a brain mostly grow with microorganisms under their surface and circulate out of the water with the help of small cilia to eat them. Genetic studies of modern sponges indicate that their evolutionary roots go back to the beginnings of early life between 1,000 and 541 million years ago. However, there is no fossil evidence of this yet.
Perhaps the oldest animal was a sponge
“Finding definitive physical evidence for multicellular animals from this period is made difficult by uncertainty about what to look for,” says Elizabeth Turner of Laurentian University in Canada. For decades, the researcher has collected and examined rock samples from a reef in northwest Canada that can only be reached by helicopter, dating back 890 million years. Branched networks of tubular structures surrounded by crystals of the mineral calcite were detected in several samples.
“The shape, size, branching, and polygonal network of tubes resemble the networks of spongy fibers that make up modern tubal sponges,” Turner reports. “They also have similarities with other tube-like microstructures that have been discovered in many limestone rocks from modern times and have been interpreted as spongy.” of sponge. “If it’s actually a sponge body fossil, it’s about 350 million years older than the next indisputably younger sponge body fossil, which came from the Cambrian period,” Turner said.
Life even before oxygen
This discovery is astonishing in view of the fact that the oxygen content in the Earth’s atmosphere and in the water was very low until about 800 million years ago, when animal life was considered almost impossible. It’s well known that modern sponges can get relatively little oxygen – but perhaps 890 million years ago there wasn’t enough oxygen for them either.
However, Turner found in her samples an indication of how primitive sponges survived: “The organism lived only on corals formed by the calcification of cyanobacteria that carried out photosynthesis.” “It is likely that dissolved oxygen in the sea was low at that time – except in the vicinity of these microorganisms.” “The worm-shaped microstructure was not able to compete with the reef-forming cyanobacteria, but instead occupied niches where calcium microbes could not survive due to poor lighting or high hydrodynamic energy,” says the researcher. In this way, he may have exploited the cyanobacteria’s neighborhood without competing with it.
Survived the first ice ages
“If the structures are accepted as early sponge fossils, their age of about 890 million years would mean that the evolutionary evolution of multicellular animals was decoupled from the oxygen enrichment of modern life,” Turner explains. In this case, the first animals existed before the early ice ages, between 720 and 635 Ma. “If the results are correct, early animal life was not catastrophically affected by these glacial episodes,” Turner said.
Coyle: Elizabeth Turner (Laurentian University, Ontario, Kannada), Nature, doi: 10.1038/s41586-021-03773-z
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