Study: Earth’s earliest animals formed semi-complex ecosystems

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Study: Earth’s earliest animals formed semi-complex ecosystems

Simon Darroch
Trace fossils of what are thought to be Earth's first animals, which emerged more than 500 million years ago.

Soft-bodied. Thin-skinned. Immobile or, at best, slower than molasses in January. Lacking brains, guts, heart and virtually anything else resembling an organ.

Earth’s first animals seem primitive yet alien when compared with many of their modern-day counterparts — so much so that biologists only recently reached a consensus to classify them as animals. Yet a new study suggests that their ecosystems were more sophisticated than once thought.

For decades, many researchers suspected that the aquatic animals evolving between 570 million and 540 million years ago — the end of the Ediacaran Period — all competed for the same basic nutrients on a first-come, first-served basis. Many of those earliest animals probably absorbed nutrients and excreted waste directly through thin membranes, effectively operating like large amoebas.

But the University of Nebraska–Lincoln’s Peter Wagner and two colleagues have found statistical evidence that the emergence and divergence of this macroscopic life actually cultivated more complex ecosystems, with organisms regularly subsisting on different resources. In that scenario, organisms branching from different sections of the multicellular tree managed to carve out ecological niches for themselves and other organisms in much the same way as modern animals.

“You had a more complicated general ecosystem in the Ediacaran than in the (subsequent) Paleozoic Era, because the very basic ways in which organisms were doing things were more varied,” said Wagner, associate professor of earth and atmospheric sciences and of biological sciences.

“As you add more and more organisms, you certainly increase the chances for new niches becoming exploitable. In this case, we’re talking about niches at the most basic levels possible.”

The researchers identified and recorded new Ediacaran fossils from three of the world’s best-known colonies: the tip of a peninsula in eastern Canada, a mountain range in southern Australia and a basin in southern Namibia. After adding those data points to existing sets, Wagner statistically charted the proportional representation of each species in an ecosystem during certain intervals of the Ediacaran Period.

Simon Darroch
The researchers found, identified and recorded trace fossils of Ediacaran species on three continents.

Eight of the study’s 20 locale-timescale combinations did support the more conventional perspective on Ediacaran ecosystems: little biodiversity, as a few species appeared to reign at the expense of others. Yet the other 12 cases boasted a broader range of species, aligning more with ecological distributions found after the Cambrian Explosion — the rise of more specialized and advanced animal life, such as the trilobite, that also signaled the beginning of the end for Ediacaran species.

Why the shift? Rising calcium levels, driven partly by seafloor spreading and the composition of continental crust during the Ediacaran, may have contributed to new body types and feeding styles by raising the number of other nutrients and plankton, Wagner said. Rather than continuing to collect nutrients through osmosis, some Ediacaran species evolved into suspension feeders, consuming organic detritus that floated through both the deep and shallow waters they occupied.

That suspension-feeding might also have helped oxygenate water columns, Wagner said, favoring the evolution of more advanced organisms. Though none of the “strange-looking beasts” could likely swim, some apparently evolved the ability to creep along and burrow slightly into the seafloor, albeit at paces that would have made a snail seem speedy. In an environment of stationary counterparts, though, even that sluggish locomotion marked an initial move toward actively preying on other organisms.

Wagner compared the ecological diversification to the birth of the automobile industry in the United States, when a proliferation of fuel candidates — gasoline, oil, coal, wood, electricity — spawned early variety in engine designs.

Simon Darroch
The animals that emerged during the Ediacaran Period rapidly went extinct during the Cambrian Explosion, an unprecedented evolution of more advanced species that began appearing roughly 540 million years ago.

“I think it is particularly interesting that we got the results that we did, because it really is consistent with the idea that early animal life was … just doing all sort of different things that most animals don’t do anymore,” Wagner said. “What wound up winning out were the forms that basically were concentrating initially on suspension and really building from there.”

Despite the fact that all known Ediacaran species went extinct during the Cambrian Explosion, their diversification still appears to have prefaced the ecological specialization that now defines the animal kingdom, Wagner said.

“In some ways, it suggests that the basic bridge was there,” he said. “But you could also think of it as: We had a river to bridge, and there were three or four possible bridges in terms of what was going to become the way animals (would) make their livings. We burned two or three of them at that point and went all-in for one.

“We think it’s kind of cool, because it’s a different and initially non-intuitive way of thinking about how complexity can evolve. But when you sit back and start to think about it, it actually makes a bit of sense. It just really shows the quirkiness of evolution.”

The researchers reported their findings in the journal Nature Ecology and Evolution. Wagner authored the study with Simon Darroch of Vanderbilt University and Marc Laflamme from the University of Toronto Mississauga. The team received support from the Smithsonian Institution and National Geographic.

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