February 5, 2009

Oldest fossil evidence of animals found

U.S. scientists say they have found fossil evidence of animals living on Earth more than 635 million years ago, the oldest such finds.

Researchers from the University of California-Riverside, the Massachusetts Institute of Technology and other institutions used compounds preserved in sedimentary rocks to confirm the existence of demosponges.

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 University of California-Riverside Assistant Professor Gordon Love. These sponges currently represent the oldest evidence for animals in the fossil record.

The preserved compounds Love and colleagues discovered in the sponges, called steranes, exist in a wide variety of biochemical configurations, said Stephen Macko, program director in the National Science Foundation, 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 has been observed in 635 million-year-old rocks but wasn't 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.

The research is reported in the journal Nature.