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About 2.4 billion years ago, the Great oxidation event caused fundamental changes in the chemistry of Earth’s surface environments. However, the effect of these changes on the biosphere is unknown, due to the global lack of well-preserved fossils from this era. In new research, paleontologists examined exceptionally preserved large spherical microfossils permineralized in chert from the lower part of the 2.4 billion-year-old Turee Creek Group in Western Australia.
Archetypal microfossils of large spherical aggregates (SA) with wide, kerogen-free surrounding crusts from the 2.4 billion-year-old Turee Creek Group in Western Australia; The shape of microfossils is usually spherical, with radial symmetry: (a) planar polarized light (PPL) image of slightly ellipsoidal SAs oriented with long axes perpendicular to the bedding; note the uniform width of the crusts; Dense, stratified organic matter (OM) directly beneath the left SA appears to deflect downward, around the SA and its surrounding crust (arrow); (b,c) and (d,e) show specimens in both PPL and cross-polarized light (XPL), highlighting the very fine microquartz grain size within the crusts (arrow in e); Carbonate rhombuses are occasionally observed invading the shells of microfossils; these are visible in a (left SA, right side of cortex), in b, c (left side of cortex) and in e (right side of cortex). Image credit: Barlow et al., two: 10.1111/gbi.12576.
“The Great Oxidation Event is thought to have triggered a mass extinction and opened the door to the development of more complex life, but there was little direct evidence in the fossil record before the discovery of the new microfossils,” said Professor Erica Barlow, a researcher at the University of New South Wales and the Pennsylvania State University.
“What we show is the first direct evidence linking the changing environment during the event with an increase in the complexity of life.”
“This is something that has been hypothesized, but there is so little fossil record that we haven’t been able to prove it.”
“Compared to modern organisms, microfossils looked more like a type of algae than the simpler prokaryotic life (organisms like bacteria, for example) that existed before the Great Oxidation Event.”
Algae, along with all other plants and animals, are eukaryotes, more complex forms of life whose cells have a nucleus surrounded by membranes.
More work is required to determine whether the Turee Creek Group microfossils were left by eukaryotic organisms, but the possibility would have significant implications. It would push back the known record of eukaryotic microfossils by 750 million years.
“The microfossils bear a remarkable similarity to a modern family called Volvocáceas”said Professor Barlow.
“This suggests that the fossil is possibly an early eukaryotic fossil. “That’s an important claim and something that needs more work, but it raises an interesting question that the community can develop and test.”
“These specific fossils are remarkably well preserved, which allowed the combined study of their morphology, composition and complexity,” said Professor Christopher House of Pennsylvania State University.
“The results provide a great window into a changing biosphere billions of years ago.”
The authors analyzed the chemical composition and carbon isotopic composition of the Turee Creek Group microfossils and determined that the carbon was created by living organisms, confirming that the structures were indeed biological fossils.
They also discovered information about the habitat, reproduction and metabolism of microorganisms.
They compared the samples to microfossils from before the Great Oxidation Event and could not find comparable organisms.
Microfossils from the Turee Creek Group were larger and had more complex cellular arrangements.
“The record seems to reveal an explosion of life – there is an increase in the diversity and complexity of this fossilized life that we are finding,” Professor Barlow said.
“Compared to modern organisms, microfossils have explicit similarities to algal colonies, including in the shape, size and distribution of both the colony and the individual cells and membranes surrounding both the cell and the colony.”
“They have a remarkable similarity and therefore, for comparison, we could say that these fossils were relatively complex.”
“There is nothing like it in the fossil record, and yet they have quite striking similarities to modern algae.”
The findings have implications both for how long it took for complex life to form on early Earth (the oldest, indisputable evidence for life is 3.5 billion years old) and for what the search for life elsewhere in the Solar System may reveal.
“I think finding a fossil that is so large and complex, relatively early in the history of life on Earth, makes you wonder: if we find life elsewhere, it might not just be prokaryotic bacterial life,” said Professor Barlow. saying.
“Maybe there is a chance that something more complex is preserved; even if it is still microscopic, it could be something of a slightly higher order.”
He results were published in the magazine Geobiology.
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Erica V. Barlow et al. A distinctive microfossil supports the early Paleoproterozoic rise in complex cellular organization. Geobiology, published online October 6, 2023; doi:10.1111/gbi.12576
