Approximately 2.7 billion years ago, braided rivers transported mud and sand from the land of a small continent to the sea. The sand on the riverbanks and beaches contained pyrite crystals preserved in an atmosphere low in oxygen. The continent possessed no life form. However, the ocean teemed with single-celled organisms that floated close to the surface. In the shallows, microorganisms lived in matlike communities on the sea floor and grew layer by layer closer to the sunlight, forming domelike, finely laminated structures. Some of these stromatolites attained a height of several meters. Later, this continent, called the Pilbara, would collide with other plates of continental crust to form the Australian mainland.