"Beginnings are apt to be shadowy, and so it is with the beginnings of the great mother of life, the sea." — Rachel Carson

	Cyanobacteria like this Bilyakh trichome are photosynthetic bacteria that live in water. These organisms are tiny and usually single-celled, but they often grow in colonies large enough to see with the naked eye. Here a 1.5 billion-year-old filament was found preserved in northern Siberia. © Dr. Andrew H. Knoll, Harvard University

The sea is the cradle of all life on Earth. Chemical evidence suggests that life was present in the oceans some 3.5 billion years ago, soon after the oceans themselves formed. The young planet was alien in appearance at this time, with hot springs dotting islands of barren, volcanic land and an atmosphere thick with carbon dioxide but poor in free oxygen. Still, marine fossils in the form of filaments of bacteria provide unambiguous evidence of life in the sea 3.2 billion years ago.

Single-celled, microscopic life-forms appeared in the oceans first, followed by multicellular organisms. But scientists today still hotly debate precisely how life began, and how these earliest life-forms evolved into the incredible diversity of creatures populating our planet.

Providing intriguing evidence of ancient life, stromatolites are not actually living organisms. Instead, they are structures that form when communities of microorganisms—primarily photosynthetic cyanobacteria—trap particles of sediment or cause calcium carbonate and other minerals to precipitate.