The findings, published about a week before the 200th birthday of Charles Darwin, are also proof of the simple organisms the English naturalist said must have existed before evolving into more complex creatures, they researchers said.

"Basically we have found a thread of that evidence that he predicted should be there," said Roger Summons, a geobiologist at the Massachusetts Institute of Technology who worked on the study.

"There is a great wealth of evidence these sponges were the first multi-cellular organisms to exist."

Using a chemical analysis of molecules in rocks dated to 635 million years ago, the researchers discovered a modified form of cholesterol only produced by the sponges.

This suggests the creatures existed before a monstrous ice age that occurred around 630 million years ago, an interesting finding because many scientists believe the frozen periods spurred the development of complex forms of life, Summons said.

These simple forms of animal life came about 200 million years before land plants appeared on Earth, Summons added. The first single cell bacteria and other similar forms of life appeared around two-and-a-half billion years ago.

The oldest visible fossils of animals found in rocks are 580 million years old but the findings from this paper published in the journal Nature show that looking at molecular evidence is also key to better understanding evolution, he said.

"People who look at fossils in rock usually just consider the visible picture," Summons said in a telephone interview. "This is showing this isn' + char(39)+ N't the only evidence you should look for."

(Reporting by Michael Kahn; Editing by Maggie Fox and Jon Boyle)