The Rhyacian is the second geological period in the Palaeoproterozoic that began 2.3 billion years ago, and ended at 2.05 billion years ago. Before it was the Siderian It is followed by the Orosirian.
The first known eukaryotes began to evolve in the Rhyacian period. The multicellular Francevillian Group Fossils, at 2.1-Gyr-old are from the Rhyacian period, as well as the enigmatic fossil Diskagma.
- Kopp; Kirschvink, JL; Hilburn, IA; Nash, CZ; et al. (August 2005). "The Palaeoproterozoic Snowball: A climate disaster caused by the evolution of oxygenic photosynthesis". PNAS. 102 (32): 11131–6. Bibcode:2005PNAS..10211131K. doi:10.1073/pnas.0504878102. PMC 1183582. PMID 16061801.
- El Albani, Abderrazak; Bengtson, Stefan; Canfield, Donald E.; Bekker, Andrey; Macchiarelli, Reberto (July 2010). "Large colonial organisms with coordinated growth in oxygenated environments 2.1 Gyr ago". Nature. 466 (7302): 100–104. Bibcode:2010Natur.466..100A. doi:10.1038/nature09166. PMID 20596019. S2CID 4331375.