Around 2.4 billion years ago, the Great Oxidation Event caused fundamental changes to the chemistry of Earth’s surface environments. However, the effect of these changes on the biosphere is unknown, due to a worldwide lack of well-preserved fossils from this time. 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 large spherical aggregate (SA) microfossils with wide, kerogen-free surrounding rinds from the 2.4-billion-year-old Turee Creek Group in Western Australia; microfossil shape is most commonly spherical, with radial symmetry: (a) plane polarised light (PPL) image of slightly ellipsoidal SAs oriented with long axes perpendicular to bedding; note the even width of rinds; dense, bedded organic matter (OM) that directly underlies the left SA appears to be deflected downwards, around the SA and its surrounding rind (arrow); (b, c) and (d, e) show specimens in both PPL and cross polarised light (XPL), highlighting the very fine microquartz grainsize within the rinds (arrow in e); carbonate rhombs are occasionally observed intruding into microfossil rinds; these are visible in a (left SA, right side of rind), in b, c (left side of rind), and in e (right side of rind). Image credit: Barlow et al., doi: 10.1111/gbi.12576.
“The Great Oxidation Event is thought to have triggered a mass extinction and opened the door for the development of more complex life, but little direct evidence had existed 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’s been hypothesized, but there’s just such little fossil record that we haven’t been able to test it.”
“When compared to modern organisms, the microfossils more closely resembled a type of algae than simpler prokaryotic life — organisms like bacteria, for example — that existed prior to the Great Oxidation Event.”
Algae, along with all other plants and animals, are eukaryotes, more complex life whose cells have a membrane-bound nucleus.
More work is required to determine if the Turee Creek Group microfossils were left behind by eukaryotic organisms, but the possibility would have significant implications. It would push back the known eukaryotic microfossil record by 750 million years.
“The microfossils have a remarkable similarity to a modern family called Volvocaceae,” Professor Barlow said.
“This hints at the fossil being possibly an early eukaryotic fossil. That’s a big claim, and something that needs more work, but it raises an exciting question that the community can build on and test.”
“These specific fossils are remarkably well preserved, which allowed for the combined study of their morphology, composition, and complexity,” said Pennsylvania State University’s Professor Christopher House.
“The results provide a great window into a changing biosphere billions of years ago.”
The authors analyzed the chemical makeup and carbon isotopic composition of the Turee Creek Group microfossils and determined the carbon was created by living organisms, confirming that the structures were indeed biologic fossils.
They also uncovered insights into the habitat, reproduction and metabolism of the microorganisms.
They compared the samples to microfossils from before the Great Oxidation Event and could not find comparable organisms.
The Turee Creek Group microfossils were larger and featured more complex cellular arrangements.
“The record seems to reveal a burst of life — there’s an increase in diversity and complexity of this fossilized life that we are finding,” Professor Barlow said.
“Compared to modern organisms, the microfossils have explicit similarities with algal colonies, including in the shape, size and distribution of both the colony and individual cells and membranes around both cell and colony.”
“They have a remarkable similarity and so, by that way of comparison, we could say these fossils were relatively complex.”
“There is nothing like them in the fossil record, and yet, they have quite striking similarities to modern algae.”
The findings have implications for both how long it took complex life to form on early Earth — the earliest, uncontroversial evidence of life is 3.5 billion years old — and what the search for life elsewhere in the Solar System may reveal.
“I think finding a fossil that is this relatively large and complex, relatively early on in the history of life on Earth, kind of makes you question — if we do find life elsewhere, it might not just be bacterial prokaryotic life,” Professor Barlow said.
“Maybe there’s a chance there could be something more complex preserved — even if it’s still microscopic, it could be something of a slightly higher order.”
The results were published in the journal Geobiology.
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Erica V. Barlow et al. Distinctive microfossil supports early Paleoproterozoic rise in complex cellular organisation. Geobiology, published online October 6, 2023; doi: 10.1111/gbi.12576
