CANADA - NOVEMBER 01: This Acasta gneiss is a sample of the oldest known rock in the world. It was found in north-west Canada and is approximately 4 billion years old, just 500 million years younger than the estimated age of the Earth. It is possible that this rock was formed when the very earliest life was appearing on Earth, and perhaps on Mars. Conditions on Earth and Mars are thought to have been similar at this time. (Photo by SSPL/Getty Images)
