Window to the past: New microfossils suggest earlier rise in complex life

Microfossils from Western Australia may capture a jump in the complexity of life that coincided with the rise of oxygen in Earth's atmosphere and oceans, according to an international team of scientists.

The findings, published in the journal Geobiology, provide a rare window into the Great Oxidation Event, a time roughly 2.4 billion years ago when the oxygen concentration increased on Earth, fundamentally changing the planet's surface. The 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, the scientists said.

"What we show is the first direct evidence linking the changing environment during the Great Oxidation Event with an increase in the complexity of life," said corresponding author Erica Barlow, an affiliate research professor in the Department of Geosciences at Penn State. "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, the scientists said. 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 microfossils were left behind by eukaryotic organisms, but the possibility would have significant implications, the scientists said. 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," 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."

Barlow discovered the rock containing the fossils while conducting her undergraduate research at the University of New South Wales (USNW) in Australia, and she conducted the current work as part of her doctoral work at UNSW and then while a postdoctoral researcher at Penn State.

"These specific fossils are remarkably well preserved, which allowed for the combined study of their morphology, composition, and complexity," said Christopher House, professor of geosciences at Penn State and a co-author of the study. "The results provide a great window into a changing biosphere billions of years ago."

The scientists analyzed the chemical makeup and carbon isotopic composition of the 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.

Barlow compared the samples to microfossils from before the Great Oxidation Event and could not find comparable organisms. The microfossils she found were larger and featured more complex cellular arrangements, she said.

"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," Barlow said.

Compared to modern organisms, Barlow said, 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," Barlow said. "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, the scientists said.

"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," 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."

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Geobiologists shine new light on Earth's first known mass extinction event 550 million years ago

A new study by Virginia Tech geobiologists traces the cause of the first known mass extinction of animals to decreased global oxygen availability, leading to the loss of a majority of animals present near the end of the Ediacaran Period some 550 million years ago.

The research spearheaded by Scott Evans, a postdoctoral researcher in the Department of Geosciences, part of the Virginia Tech College of Science, shows this earliest mass extinction of about 80 percent of animals across this interval. "This included the loss of many different types of animals, however those whose body plans and behaviors indicate that they relied on significant amounts of oxygen seem to have been hit particularly hard," Evans said. "This suggests that the extinction event was environmentally controlled, as are all other mass extinctions in the geologic record."

Evans' work was published Nov. 7 in the Proceedings of the National Academy of Sciences, a peer-reviewed journal of the National Academy of Sciences. The study was co-authored by Shuhai Xiao, also a professor in the Department of Geosciences, and several researchers led by Mary Droser from the University of California Riverside's Department of Earth and Planetary Sciences, where Evans earned his master's degree and Ph.D.

"Environmental changes, such as global warming and deoxygenation events, can lead to massive extinction of animals and profound disruption and reorganization of the ecosystem," said Xiao, who is an affiliated member of the Global Change Center, part of the Virginia Tech Fralin Life Sciences Institute. "This has been demonstrated repeatedly in the study of Earth history, including this work on the first extinction documented in the fossil record. This study thus informs us about the long-term impact of current environmental changes on the biosphere."

What exactly caused the drop in global oxygen? That's still up for debate. "The short answer to how this happened is we don't really know," Evans said. "It could be any number and combination of volcanic eruptions, tectonic plate motion, an asteroid impact, etc., but what we see is that the animals that go extinct seem to be responding to decreased global oxygen availability."

The study by Evans and Xiao is timelier than one would think. In an unconnected study, Virginia Tech scientists recently found that anoxia, the loss of oxygen availability, is affecting the world's fresh waters. The cause? The warming of waters brought on by climate change and excess pollutant runoff from land use. Warming waters diminish fresh water's capacity to hold oxygen, while the breakdown of nutrients in runoff by freshwater microbes gobbles up oxygen.

"Our study shows that, as with all other mass extinctions in Earth's past, this new, first mass extinction of animals was caused by major climate change -- another in a long list of cautionary tales demonstrating the dangers of our current climate crisis for animal life," said Evans, who is an Agouron Institute Geobiology fellow.

Some perspective: The Ediacaran Period spanned roughly 96 million years, bookended on either side by the end of Cryogenian Period -- 635 million years ago -- and the beginning of the Cambrian Period -- 539 million years ago. The extinction event comes just before a significant break in the geologic record, from the Proterozoic Eon to the Phanerozoic Eon.

There are five known mass extinctions that stand out in the history of animals, the "Big Five," according to Xiao, including the Ordovician-Silurian Extinction (440 million years ago), the late Devonian Extinction (370 million years ago), the Permian-Triassic Extinction (250 million years ago), the Triassic-Jurassic Extinction (200 million years ago), and the Cretaceous-Paleogene Extinction (65 million years ago).

"Mass extinctions are well recognized as significant steps in the evolutionary trajectory of life on this planet," Evans and team wrote in the study. Whatever the instigating cause of the mass extinction, the result was multiple major shifts in environmental conditions. "Particularly, we find support for decreased global oxygen availability as the mechanism responsible for this extinction. This suggests that abiotic controls have had significant impacts on diversity patterns throughout the more than 570 million-year history of animals on this planet," the authors wrote.

Fossil imprints in rock tell researchers how the creatures that perished in this extinction event would have looked. And they looked, in Evans' words, "weird."

"These organisms occur so early in the evolutionary history of animals that in many cases they appear to be experimenting with different ways to build large, sometimes mobile, multicellular bodies," Evans said. "There are lots of ways to recreate how they look, but the take-home is that before this extinction the fossils we find don't often fit nicely into the ways we classify animals today. Essentially, this extinction may have helped pave the way for the evolution of animals as we know them."

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