Archeans

The Archean, sometimes spelled Archaean, designates a major interval of Earth's history spanning roughly 4.0 to 2.5 billion years ago. It is the second eon in the geologic timescale, following the Hadean and preceding the Proterozoic, and it forms part of the Precambrian supereon. The name derives from Greek root meanings ancient and beginning, reflecting its position as an early phase in planetary development.

Geology and crustal evolution during the Archean focused on the formation and stabilization of Earth's first large-scale crustal blocks. Crustal differentiation, cooling, and differentiation produced early continents or cratons, while tectonic processes operated at potentially different rates than in the present day. The era also featured high rates of volcanic activity and meteorite bombardment early on, with gradual growth of stable continental cores over hundreds of millions of years.

The Archean environment supported a biosphere dominated by microscopic life. The fossil and geochemical record points to the presence of microbial ecosystems, including cyanobacteria capable of photosynthesis and forming stromatolites in some settings. Biological activity left isotopic and mineralogical signatures that indicate biogenic processes long before complex multicellular life emerged. The atmospheric composition was likely reducing, with oxygen accumulating only gradually toward the end of the Archean.

End of the Archean and transition into the Proterozoic were marked by changes in atmospheric oxygen levels and continued crustal growth. The Archean thus laid the foundation for later biological innovations and the emergence of more widespread oxygenation that preceded complex life.