February 4, 2009 -- The oldest chemical traces of animal life on Earth have been discovered in ancient rock formations in Oman.

Scientists found evidence for primitive sponges dating back at least 635 million years, long before the sudden diversification of multicellular life in the Cambrian explosion around 530m years ago – which paved the way for all the major groups of animals 100 million years later.

Finding evidence of complex life pre-dating the Cambrian explosion has proved difficult until now. Charles Darwin was famously sceptical of the idea that creatures could have materialised fully formed out of nowhere, a view shared by some experts today.

Analysing sedimentary rocks in the South Oman Salt Basin, the scientists found remants of ancient steroid molecules that are only produced by one class of sponge alive today.

Gordon Love at the University of California at Riverside used sensitive tests to identify these "molecular fossils" in the rock that could only have been made by early sponges. The presence of a particular chemical called 24-isopropylcholestane dated the earliest life to the end of a great ice age known as the Marinoan glaciation.

The findings are described in tomorrow's issue of the journal Nature.