Earliest Animals Were Sea Sponges, Fossils Hint

Rebecca Carroll
for National Geographic News
February 4, 2009

Fossil steroids found underground in Oman show that early Earth was the scene of a sea sponge heyday more than 635 million years ago.

The ancient chemicals—similar to modern natural steroids such as estrogen and testosterone—are now the oldest known fossil evidence of animal life, says a new study led by Gordon Love of the University of California, Riverside.

Based on chemical signatures inside sedimentary rocks, Love and colleagues think the sponges likely grew in colonies that blanketed areas of the ocean floor.

Back then the supercontinent Rodinia, which had been Earth's dominant landmass for at least 350 million years, was in the process of breaking up, and the climate was extremely cold worldwide.

Sponges evolved in shallow ocean basins, because the deeper seas did not yet contain oxygen, a necessity for almost all life.

Although the environment was harsh at this time—about a hundred million years before the evolutionary growth spurt known as the Cambrian explosion—a lack of predators made life easier for the sponges.

"There was no competition from more complicated animals, so sponges were probably thriving," Love said. "Compared with other times in our history, there were enormously high amounts of them."

Spongy Roots

Love and colleagues were able to date the sea sponges because the animals' chemical traces were found in rocks beneath glacial deposits from an ice age that ended about 635 million years ago.

(Related: "'Alien' Atmosphere Helped Unfreeze Early Earth" [January 13, 2009].)

The scientists cut away the outer surfaces of the rock, cleaned the remaining core with solvents, and crushed what was left behind into a powder that could be chemically separated into its component parts.

"It just so happens that these sponges produce very distinctive chemical structures," said Love, whose team describes their results in this week's issue of the journal Nature.

Kevin Peterson of Dartmouth College and his colleagues had independently hypothesized that sponges lived about 650 million years ago based on biological clues in the genes of modern sponges.

"To see to a robust, geochemical record of a tremendous amount of sponge mass at this time is very exciting," said Peterson, who was not involved in the new study.

"At some point during this interval, sponges gave rise to more complex organisms, including eventually vertebrates," he said.

"The origin of complex life is rooted in sponge biology, and that's what makes it so exciting for us."




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