Rethinking the origins of complex life

Scientists from Australia, Germany, France and the US have shifted the timeline of the origin of complex life, overturning a discovery made a decade ago.

In two complementary studies they have shown that molecular fossils previously taken as the earliest signatures of animal life were in fact created by algae – and this has big implications for our understanding of evolution.

“It brings the oldest evidence for animals nearly 100 million years closer to the present day,” says Lennart van Maldegam from the Australian National University (ANU), co-author of one study.

“We were able to demonstrate that certain molecules from common algae can be altered by geological processes – leading to molecules which are indistinguishable from those produced by sponge-like organisms.”

Both papers are published in the journal Nature Ecology & Evolution. The other can be found here.

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Precambrian sedimentary rocks preserved in the Grand Canyon. Credit: L van Maldegam

For more than a century, scientists have been untangling the mystery of when our very earliest animal ancestors evolved. By delving into the rock record, we can see that eukaryotic cells first emerged around 1.6 billion years ago, but it isn’t until a billion years later that we begin to see them forming into more complex animal life with differentiated tissues like muscles and nervous systems.

The fossil record from around 541 million years ago is packed with examples of such animal life from the Cambrian explosion. Yet animals are thought to have existed before the Cambrian. By tracking how fast the genetic code is evolving, genetics research estimates that the last common ancestor of all animals evolved around 900-635 million years ago.

But traces of earlier animals are scarce.

“Most fossils we normally think of are bones or shells, yet biomineralisation – a trait which allows for production of these skeletal features – only evolved around 550 million years ago,” explains van Maldegam.

Organisms from the Ediacaran period – directly preceding the Cambrian, between 635–542 million years ago – had unusual bodies made entirely of soft parts. This means they only exist as trace fossils, like negative impressions in rock outlining their bodies. It also means that we have little information about them – some are even argued to have been plants, not animals.

Scientists can also use other methods to identify biological remains, such as looking at “molecular fossils”: molecules that must have been created by a living creature. Many of these are derived from lipids – fat molecules – and can be linked back to specific groups of organisms.

“Ten years ago, scientists discovered the molecular fossils of an animal steroid in rocks that were once at the bottom of an ancient sea in the Middle East,” says ANU co-author Jochen Brocks.

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Separation of algal lipids. Credit: I Bobrovskiy

Sponges are the only known living organisms that can produce this steroid, and since then similar molecular fossils have been found in twenty different locations around the world.

According to Brocks, “the big question was, how could these sponges have been so abundant, covering much of the seafloor across the world, but leave no body fossils?”

In the first paper, the team analysed the molecular fossils – lipid molecules – to determine where they came from. Specifically, they looked at samples from the Amazon Craton in Brazil, dating to 635 million years ago, and the Grand Canyon in the US, dating to 740 million years ago.

“Different organisms possess different sterols, similar to how different animals have different femur bones, allowing specialists to assign them to specific organisms,” explains van Maldegam.

They noticed that the molecules previously linked to sponges looked identical to other molecules that had been formed when algae sediments were converted into rock.

In the second paper, the team took sterols from modern algae and simulated these geological alteration conditions in the lab. They found they could produce the exact same molecules that had been thought to come from sponges – thus showing that chemical processes can mimic biology, transforming both red and green algae sterols into “animal” sterols.

This means that these types of molecular fossils can no longer be used as a diagnostic marker for early sponges, and so can’t help us reconstruct early animal evolution.

Van Maldegam notes that “with these molecular markers being overturned, the next oldest evidence for animals on Earth are Dickinsonia, a strange-looking soft-bodied animal which lived in the oceans 558 million years ago.”

The first specimen of this weird and wonderful organism was found in the Flinders Ranges in South Australia, as a rippled impression in sandstone.

But the genetics research still tells us that animals must have been around earlier than this – at least 600 million years ago and possibly beyond.

“It is absolutely feasible that someone will find new evidence placing animals further back in time,” van Maldegam says. “Yet, due to the lack of hard body fossils and a limited amount of Precambrian sediments preserved, it will be challenging.”

Related reading: The origins of life in hot springs

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