<P> Bacteria develop primitive forms of photosynthesis which at first did not produce oxygen . These organisms generated Adenosine triphosphate by exploiting a proton gradient, a mechanism still used in virtually all organisms . </P> <Tr> <Td> 3000 Ma </Td> <Td> Photosynthesizing cyanobacteria evolved; they used water as a reducing agent, thereby producing oxygen as a waste product . The oxygen initially oxidizes dissolved iron in the oceans, creating iron ore . The oxygen concentration in the atmosphere slowly rose, acting as a poison for many bacteria and eventually triggering the Great Oxygenation Event . The Moon, still very close to Earth, caused tides 1,000 feet (305 m) high . The Earth was continually wracked by hurricane - force winds . These extreme mixing influences are thought to have stimulated evolutionary processes . </Td> </Tr> <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>

When did living things begin to live on land