Rocks’ chemicals push back key date 100 million years

Organic chemicals locked into rocks buried in south Oman indicate that multicellular animal life on Earth originated 100 million years earlier than previously believed – well before the so-called Cambrian explosion 545 million years ago when such complex organisms were thought to have begun evolving and proliferating.

The chemicals, steroids distantly related to testosterone and estrogen, are a unique marker for sponges. Their presence in the Oman rock shows that these precursors of our bathtub sponges were the dominant species on the planet for as long as 100 million years, researchers reported Thursday in the journal Nature.

Sponges are one of the simplest multicellular organisms alive. They live on shallow sea floors and eat detritus floating down from above. Their cells’ walls contain steroids, 24-isopropylcholestanes, not present in other species.

“After four decades of research, they have never been seen in unicellular organisms other than in trace amounts,” said earth scientist Gordon Love of University of California, Riverside, lead author.

Love and his colleagues studied sedimentary rocks taken from the South Oman Salt Basin. The rocks have been dated to as many as 635 million years ago because they are covered with glacial deposits. (The oil, which is produced from multicellular plants and animals, dates from 530 to 545 million years ago.)

In chemicals from the rocks, the team found relatively high levels of the 24-isopropylcholestanes – high enough to convince them that sponges lived in the area throughout the entire period.

The discovery indicates that oxygen, which has long been seen as a precipitating factor in the Cambrian explosion, was present in sufficient quantities 100 million years earlier than previously believed.