Fossil steroids record the advent of earliest known animals

Using compounds preserved in sedimentary rocks more than 635 million years old, researchers have found some of the earliest evidence for the existence of animals.

Demosponges thrived in the shallow coastal waters of what is now Oman, according to scientist Gordon Love of the University of California at Riverside and colleagues from MIT and other institutions.

They report the results of their research in this week's issue of the journal Nature.

"Demosponges appeared during the Neoproterozoic era, 1,000 to 542 million years ago, an era of climatic extremes and biological evolutionary developments culminating in the emergence of animals and new ecosystems," said Love.

"These sponges currently represent the oldest evidence for animals in the fossil record."

The preserved compounds Love and colleagues discovered in these sponges, called steranes, exist in a wide variety of biochemical configurations, according to Stephen Macko, program director in the National Science Foundation (NSF)'s Division of Earth Sciences, which funded the research.

"The compounds are also known as 'biomarkers,' indicating that they can be traced directly to living organisms," said Macko.

The biomarker Love and colleagues identified, 24-isopropylcholestane, is found in living demosponges, and now has been observed in 635 million-year-old rocks, but was not seen in older samples of the same rock formation.

"The fact that these biomarkers were found in samples associated with sedimentary rocks that formed in shallow waters," said Macko, "lends support to the hypothesis that demosponges arose in warm shallow coastal seas."

Feeding on dissolved and particulate organic debris in the water, these animals eventually migrated to the deep sea. They now reside there, as well as in shallower coastal waters.

February 4, 2009