What Is The Oldest Fossil Ever Found
In the case of stromatolites, the "cast" is composed of thin layers of sediment and calcium carbonate -- the same substance that forms limestone -- that build up around complex colonies of cyanobacteria (aka blue-green algae) and other single-celled organisms. Stromatolites form very slowly, preserving a record of thousands of years of life in the process. Fortunately, stromatolites from every geological period still exist. By carefully dissecting and exploring these structures, scientists have access to some of the only remaining clues about what the first life on Earth was like. After the Earth first formed nearly 4.5 billion years ago, it was entirely uninhabitable. After the Earth's surface cooled and solidified into continental plates, the first microorganisms appeared. Among the most significant of these were cyanobacteria, which thrived in basins of shallow saltwater where they were protected from the sun's intense rays but were still close enough to the surfac...