<Tr> <Td> 2800 Ma </Td> <Td> Oldest evidence for microbial life on land in the form of organic matter - rich paleosols, ephemeral ponds and alluvial sequences, some of them bearing microfossils . </Td> </Tr> <P> 2500 Ma--542 Ma . Contains the Palaeoproterozoic, Mesoproterozoic and Neoproterozoic eras . </P> <Table> <Tr> <Th> Date </Th> <Th> Event </Th> </Tr> <Tr> <Td> 2500 Ma </Td> <Td> Great Oxygenation Event led by cyanobacteria's oxygenic photosynthesis . Commencement of plate tectonics with old marine crust dense enough to subduct . </Td> </Tr> <Tr> <Td> 2000 Ma </Td> <Td> Diversification and expansion of acritarchs . </Td> </Tr> <Tr> <Td> By 1850 Ma </Td> <Td> Eukaryotic cells appear . Eukaryotes contain membrane - bound organelles with diverse functions, probably derived from prokaryotes engulfing each other via phagocytosis . (See Symbiogenesis and Endosymbiont). Bacterial viruses (bacteriophage) emerge before, or soon after, the divergence of the prokaryotic and eukaryotic lineages . The appearance of red beds show that an oxidising atmosphere had been produced . Incentives now favoured the spread of eukaryotic life . </Td> </Tr> <Tr> <Td> 1400 Ma </Td> <Td> Great increase in stromatolite diversity . </Td> </Tr> <Tr> <Td> By 1200 Ma </Td> <Td> Meiosis and sexual reproduction are present in single - celled eukaryotes, and possibly in the common ancestor of all eukaryotes . Sex may even have arisen earlier in the RNA world . Sexual reproduction first appears in the fossil records; it may have increased the rate of evolution . </Td> </Tr> <Tr> <Td> 800 Ma </Td> <Td> First multicellular organism may have arisen . </Td> </Tr> <Tr> <Td> 750 Ma </Td> <Td> First protozoa (ex: Melanocyrillium) </Td> </Tr> <Tr> <Td> 850--630 Ma </Td> <Td> A global glaciation may have occurred . Opinion is divided on whether it increased or decreased biodiversity or the rate of evolution . </Td> </Tr> <Tr> <Td> 600 Ma </Td> <Td> The accumulation of atmospheric oxygen allows the formation of an ozone layer . Prior to this, land - based life would probably have required other chemicals to attenuate ultraviolet radiation enough to permit colonisation of the land . </Td> </Tr> <Tr> <Td> 580--542 Ma </Td> <Td> The Ediacara biota represent the first large, complex multicellular organisms--although their affinities remain a subject of debate . </Td> </Tr> <Tr> <Td> 580--500 Ma </Td> <Td> Most modern phyla of animals begin to appear in the fossil record during the Cambrian explosion . </Td> </Tr> <Tr> <Td> 560 Ma </Td> <Td> Earliest fungi </Td> </Tr> <Tr> <Td> 550 Ma </Td> <Td> First fossil evidence for Ctenophora (comb jellies), Porifera (sponges), Anthozoa (corals and sea anemones) </Td> </Tr> </Table> <Tr> <Th> Date </Th> <Th> Event </Th> </Tr>

When did the first protozoa appear on earth