A geologist may have uncovered the oldest sign of animal life on Earth: sponge fossils that potentially date back 890 million years. That’s some 350 million years earlier than the oldest undisputed findings of animal fossils.
Recently, she went back to collect more samples and — aided by present-day advances in the study of far more modern sponges, which are simple aquatic animals with dense yet porous skeletons — she became confident enough in her theory to publish her findings in the journal Nature.
Scientists believe life on Earth emerged around 3.5 billion years ago. The earliest animals appeared much later, but exactly when is still the subject of debate because of a lack of fossilized evidence.
“The fossil record you see in a museum or geology teaching lab, fossils of animals, appear in the rock record in rocks that are younger than 540 million years,” Turner told The Washington Post. “Me suggesting that I have possibly found evidence of the bodies of sponges 890 million years ago seems pretty radical right at the outset.”
The oldest undisputed fossils date back to the Cambrian period when animals first developed hard skeletons, exoskeletons and shells, which were more likely to be preserved. Turner’s discovery could help prove the scientific theory that sponges existed before those more complicated creatures.
Examined under a microscope, the tiny sections of rock Turner uncovered contain a meshwork of three-dimensional structures — branching out in a distinctive way and rejoining — that closely resembles modern sponge skeletons.
Given that the sponge is among the most basic forms of animal life, “if we’re going to find early animals, it seems reasonable that they’re going to be spongelike,” Turner said in a Zoom interview.
If Turner’s discovery is proved correct, then the organisms would have lived on Earth before the time when most scientists previously believed there was enough oxygen to support animal life. They also would have lived before widespread ice ages on Earth.
Turner said it is possible the sponges predate these two major Earth system events, in part because modern rock records show sponges can be tolerant of low oxygen levels. Her samples were uncovered in giant fossil reefs, which she described as an “oxygen oasis.”
In the past five years or so, scientists have extensively researched how sponges get preserved — in what Turner describes as “a race between decay and preservation of their soft tissues.” They’ve experimented on modern sponges and worked on sponge preservation in ancient rocks, “all of which is very, very closely related to the structure that I describe,” Turner said.
Writing in the journal Nature, Turner leaned on those recent discoveries to describe how the fossils may have formed when sponges, measuring a few millimeters to a centimeter across, became mineralized. The soft tissue was first to fossilize, encasing the 3-D network of collagen-like fibers that formed the sponge’s skeleton. Over time, these skeletal fibers decayed, leaving hollow tubules that filled up with calcite crystals.
“I think she’s got a pretty strong case. I think this is very worthy of publishing — it puts the evidence out there for other people to consider,” David Bottjer, a paleobiologist at University of Southern California, who was not involved in the research, told the Associated Press.
“This is not a normal science story. This is slow science. It’s a story about early animal evolution,” Turner told The Post.
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