National Geographic News
A fossilized sponge.
A scanning electron microscope view of Otavia antiqua.

Image courtesy Anthony Prave, University of St. Andrews

Ker Than

for National Geographic News

Published February 7, 2012

Microscopic, sponge-like African fossils could be the earliest known animals—and possibly our earliest evolutionary ancestors, scientists say.

(Related: "Earliest Animals Were Sea Sponges, Fossils Hint.")

The creature, Otavia antiqua, was found in 760-million-year-old rock in Namibia and was as tiny as it may be important.

"The fossils are small, about the size of a grain of sand, and we have found many hundreds of them," said study leader Anthony Prave, a geologist at the University of St. Andrews in the U.K.

"In fact, when we look at thin sections of the rocks, certain samples would likely yield thousands of specimens. Thus, it is possible that the organisms were very abundant."

From these tiny "sponges" sprang very big things, the authors suggest. As possibly the first muticellular animals, Otavia could well be the forerunner of dinosaurs, humans-basically everything we think of as "animal."

(Related: "405-Year-Old Clam Called Longest-Lived Animal.")

Oldest Animal Built to Last?

Prior to the new discovery, the previous earliest known "metazoan"—animals with cells differentiated into tissues and organs—was another primitive sponge, dated to about 650 million years ago.

Based on where the new fossils were found, Prave and his colleagues think Otavia lived in calm waters, including lagoons and other shallow environments.

The team thinks Otavia fed on algae and bacteria, which the animal drew through pores on its tubelike body into a central space. There the food was digested and absorbed directly into Otavia's cells.

(Also see "Was The Humble Sponge Earth's First Animal?")

The simple setup seems to have worked.

The fossil record indicates Otavia survived at least two long-term, severe cold snaps known as "snowball Earth" events, when the planet was almost completely covered in ice.

Despite such wild environmental swings, "the oldest and youngest Otavia fossils all have the same quasi-ovid form, with large openings leading from the exterior," Prave said in an email.

In short, the animals didn't evolve much, he said—suggesting that, at least for its roughly 200 million years of existence, Otavia was built to last.

The new oldest-animal study is detailed in the current issue of the South African Journal of Science.

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