By Jonathan Amos February 4, 2009
Science reporter, BBC News
Rocks in Oman
The rocks date to a time of dramatic glaciation on Earth
Traces of animal life have been found in rocks dating back 635 million years.
The evidence takes the form of chemical markers that are highly distinctive of sponges when they die and their bodies break down in rock-forming sediments.
The discovery in Oman pushes back the earliest accepted date for animal life on Earth by tens of millions of years.
Scientists tell Nature magazine that the creatures' existence will help them understand better what the planet looked like all that time ago.
"The fact that we can detect these signals shows that sponges were ecologically important on the seafloor at that time," said lead author Gordon Love, from the University of California, Riverside.
Photo taken from the bottom of the Grand Canyon, Arizona in January 2002. This is some of the oldest exposed rock in the world, with an average age of 1,700 to 2,000 million years. These rock layers were once the base of an ancient mountain range, comparable to the Rockies, which over time completely eroded away and now provides a tantalizing glimpse of the enormous time scale on which geologic events occur. The Colorado River is in the foreground.
© Mark Meyer Photography
Early Pre-Cambrian Rocks
* Vishnu Schist and Zoroaster Granite - This layer averages about 1,700 to 2,000 million years old and consists of mica schist. These were originally sediments of sandstone, limestone and shale that were metamorphosed and combined with metamorphosed lava flows to form the schist. This layer along with the Zoroaster Granite were once the roots of an ancient mountain range that could have been as high as todays Rocky Mountains. The mountains were eroded away over a long period of time and new sediments were they deposited over them by advancing and retreating seas. The color of this layer is dark grey or black.