650 million years ago, Earth was completely or almost completely frozen, according to the Snowball Earth Hypothesis. As the atmosphere changed and Earth warmed up, it heralded the beginning of the Ediacaran Period. The Ediacaran Period marks the first time multicellular life was widespread on the planet. It predates the more well-known Cambrian Period, when more complex life emerged, diversified, and flourished.
Life during the Ediacaran Period faced a mass extinction, and it was Earth’s first one.
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Fossils from the Ediacaran are pretty rare because shells and skeletons wouldn’t appear until the Cambrian Period. But some locations hold significant numbers and types of fossils of soft-bodied Ediacaran life. One such location is the period’s namesake region, the Ediacara Hills, in South Australia.
The Ediacaran Period saw life increase in complexity and become multicellular. Most of the Ediacaran biota was sessile, meaning it was anchored in place, like barnacles. Mobility would have to wait for evolution to create even more complexity. Life tended to be tubular or frond-shaped and anchored to the ocean’s bottom. Other lifeforms at the time resembled segmented worms and immobile mudbags. According to some research, Ediacaran lifeforms likely absorbed nutrients from the sea through their skin.
Earth was much different during the Ediacaran Period. The supercontinent Pannotia predated the more well-known Gondwana supercontinent, and Pannotia formed and broke apart during the period. The oceans also became more oxygenated and also oxygenated to greater depths. The oxygenation created more habitats for oxygen-loving lifeforms. Life thrived, became more complex, and spread around the globe during the Ediacaran.
But something happened that caused a mass extinction, and nearly all of the unique Ediacaran biota disappeared. The Cambrian Period followed the Ediacaran, and Cambrian life almost completely replaced Ediacaran life. Most existing body plans for animals alive now stem from the Cambrian rather than the Ediacaran. It’s almost like evolution had a clean slate to try again after the Ediacaran extinction.
The authors of a new paper say decreased oxygen availability drove the End-Ediacaran Extinction. Their paper is “Environmental drivers of the first major animal extinction across the Ediacaran White Sea-Nama transition.” The lead author is Scott Evans, a postdoctoral researcher in the Department of Geosciences at the Virginia Tech College of Science. Evans is the author and co-author of multiple papers examining the Ediacaran Period.
Some research suggests that life itself caused the End-Ediacaran extinction. That line of thinking says that as new lifeforms evolved, they behaved differently. Those behaviours brought a fundamental change to the ecosystem, in a phenomenon called “ecosystem engineering.” Proponents of this line of thinking say that mobile lifeforms appeared as the Cambrian explosion took effect. They fed on the sessile creatures of the Ediacaran and wiped them out. A 2015 paper said that “… this study provides the first quantitative palaeoecological evidence to suggest that evolutionary innovation, ecosystem engineering and biological interactions may have ultimately caused the first mass extinction of complex life.”
But Evans and his colleagues disagree. They say that plummeting oxygen levels caused the extinction of about 80% of Ediacaran life.
“This included the loss of many different types of animals. However, those whose body plans and behaviours 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.”
Scientists divide the Ediacaran biota into three distinct fossil assemblages: the Avalon (ca. 575–560 Ma), White Sea (ca. 560–550 Ma), and Nama (ca. 550–539 Ma). About 80% of the taxa in the White Sea assemblage are absent in the Nama assemblage. The ecological differences between the White Sea and the Nama are minimal, meaning that it’s unlikely that the lifeforms were outcompeted and replaced by other lifeforms, called biotic replacement.
The groups of lifeforms that disappeared during the extinction had diverse morphologies and behaviours. That indicates that some over-arching environmental characteristic changed, something so elemental that almost nothing could adapt. But the surviving creatures shared a common trait: high surface-to-volume ratios.
“The preferential survival of taxa with high surface area relative to volume may suggest that this was related to reduced global oceanic oxygen availability,” the paper states. “Thus, our data support a link between Ediacaran biotic turnover and environmental change, similar to other major mass extinctions in the geologic record.”
There are two extinctions attached to the Ediacaran, and they’re separated by about 10 million years. The first abrupt loss of diversity occurred during the White Sea and Nama assemblages. The second happened at the boundary between the Ediacaran and the Cambrian.
In the scientific back-and-forth over the Ediacaran extinction, some researchers point out the potential sampling bias in the fossil record. As part of their research, the team investigated any potential sampling bias that might exist. To do that, they examined the paleoenvironments and preservation modes for fossils from all three Ediacaran assemblages.
The researchers concluded that “… changes in diversity cannot be attributed to sampling biases…”
If falling oxygen levels doomed Ediacaran life, the next obvious question is why did the oxygen levels plummet?
This paper doesn’t provide an answer, but the authors share some thoughts. “The ultimate causes of mass extinctions include major drivers, such as bolide impacts, the eruption of large igneous provinces, and/or tectonic shifts, which produce major changes in redox state, temperature, and/or oceanic pH, leading to diversity decline. While our data do not provide direct information on the ultimate driver of this event, the broad range of paleoecologies impacted across the White Sea–Nama extinction is consistent with the predicted effects of a catastrophic environmental perturbation.”
Something about Earth changed, and it was terrible for life. Life survived, diversified, and flourished again in the Cambrian period that followed the Ediacaran, in what scientists sometimes call the Cambrian Explosion. All of the modern animal phyla were first represented in the Cambrian Explosion. But the peculiar lifeforms from the Ediacaran were gone, and life now looks nothing like Ediacaran life, with very few exceptions.
“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.”
Is there a lesson for humanity in all of this?
“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 Shuhai Xiao, a co-author also affiliated with Virginia Tech. “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.”
As Earth warms, so do the oceans. And warmer oceans hold less oxygen. Excessive nutrient run-off from agriculture also plays a role. As microbes break down the run-off, they consume more oxygen. Since we are responsible for the warming and the run-off, we are ecosystem engineers. But the engineering is haphazard.
“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.
Extinctions have played an over-arching role in the development of life on Earth. The fossils from the Ediacaran represent evolutionary designs that didn’t persist past the Ediacaran extinction.
“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.”
Life during the Ediacaran Period was largely immobile, so there was little predation. Sometimes the Ediacaran is called the “Ediacaran Garden” for this reason. Without the Ediacaran extinction, the Cambrian Explosion may never have happened. There’s no way to know for sure, but there may have been no dinosaurs, no mammals, and no homo sapiens. Maybe Earth would’ve stayed in a permanent Ediacaran state, with barely any predation or mobile animals, though that doesn’t seem likely.
Scientists talk about the “Big Five” extinctions in Earth’s history: 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,” the authors write in their study. Now we can add one more to the list, and it may have been the most critical in shaping life on Earth.
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