Chemical reactions led to the first self-replicating molecules about four billion years ago. A half billion years later, the last common ancestor of all current life arose.  The evolution 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.  Among 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, and remains of biotic material found in 4. 1 billion-year-old rocks in Western Australia. The earliest direct evidence of life on Earth is contained in 3. 45 billion-year-old Australian rocks showing fossils of microorganisms.