 Highly energetic chemical reactions are thought to have produced self – replicating molecules around four billion years ago . This was followed a half billion years later by the last common ancestor of all life . The development of photosynthesis allowed the Sun 's energy to be harvested directly by life forms ; the resultant molecular oxygen ( O2 ) accumulated in the atmosphere and due to interaction with ultraviolet solar radiation , formed a protective ozone layer ( O3 ) in the upper atmosphere . The incorporation of smaller cells within larger ones resulted in the development of complex cells called eukaryotes . True multicellular organisms formed as cells within colonies became increasingly specialized . Aided by the absorption of harmful ultraviolet radiation by the ozone layer , life colonized Earth 's surface . The earliest fossil evidence for life is microbial mat fossils found in 3 @.@ 48 billion @-@ year @-@ old sandstone in Western Australia , biogenic graphite found in 3 @.@ 7 billion @-@ year @-@ old metasedimentary rocks in Western Greenland , as well as , remains of biotic material found in 4 @.@ 1 billion @-@ year @-@ old rocks in Western Australia . 
