The first animals could possibly have lived 890 million years ago: On reefs in Canada, researchers have identified structures that may have come from sponges. If the results are confirmed, they are the earliest known fossil animals. At that time, the oxygen content of the earth was actually 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. The primitive animals without a brain are mostly grown together with their subsurface and swirl microorganisms out of the water with the help of tiny cilia to eat them. Genetic studies of modern sponges suggest that their evolutionary roots go back to the early Neoproterozoic between 1,000 and 541 million years ago. However, so far there has been no fossil evidence for this.
The oldest animal was probably a sponge
“Finding definitive physical evidence of multicellular animals from this period is made difficult by the uncertainty of what to look for,” says Elizabeth Turner of Laurentian University in Canada. For decades, the researcher collected and examined rock samples from a reef in northwest Canada that can only be reached by helicopter, which is dated to an age of 890 million years. In several samples she discovered branched networks of tubular structures surrounded by crystals of the mineral calcite.
“The shape, size, branching and the polygonal mesh of the tubes are similar to the sponge fiber networks that form modern horn sponges,” reports Turner. “They also have similarities with other tube-like microstructures that have been detected in various limestone rocks from more recent eras and interpreted as spongy.” For the researcher, this is a strong indication that the traces in the 890 million year old rock actually come from sponges. “If it is actually sponge body fossils, they are about 350 million years older than the next younger undisputed sponge body fossils, which come from the Cambrian,” said Turner.
Life even before oxygenation
This finding is astonishing in view of the fact that the oxygen content in the earth’s atmosphere and in water was so low until around 800 million years ago that animal life was considered almost impossible. It is well known that modern sponges can get by with comparatively little oxygen – but the oxygen would probably not have been sufficient for them 890 million years ago either.
Turner found in her samples, however, an indication of how the primeval sponges could have survived: “The organism only lived on, in and immediately next to reefs that were built up by calcifying cyanobacteria that carried out photosynthesis,” she reports. “The dissolved oxygen in the sea was probably low at that time – except in the vicinity of these microorganisms.” In contrast, the sponge itself apparently managed without light. “The worm-shaped, microstructured organism was unable to compete with the reef-forming cyanobacteria, but instead occupied niches in which the calcium microbes could not live due to poor lighting or high hydrodynamic energy,” says the researcher. In this way, he may be taking advantage of the neighborhood of the cyanobacteria without competing with them.
Survived the first ice ages
“If the structures are accepted as early spongy fossils, their age of around 890 million years would mean that the evolutionary development of the multicellular animals was decoupled from the oxygen enrichment in the Neoproterozoic,” explains Turner. In this case, the first animals would have existed before the early ice ages, between 720 and 635 million. “If the findings are correct, early animal life was not catastrophically affected by these glacial episodes,” said Turner.
Source: Elizabeth Turner (Laurentian University, Ontario, Canada), Nature, doi: 10.1038 / s41586-021-03773-z