ORIGIN OF LIFE AND EVOLUTION ON EARTH

ORIGIN OF LIFE AND EVOLUTION ON EARTH

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 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, remains of biotic material found in 4.1 billion-year-old rocks in Western Australia.During the Neoproterozoic, 750 to 580 mya, much of Earth might have been covered in ice. This hypothesis has been termed "Snowball Earth", and it is of particular interest because it preceded the Cambrian explosion, when multicellular life forms significantly increased in complexity. Following the Cambrian explosion, 535 mya, there have been five major mass extinctions. The most recent such event was 66 mya, when an asteroid impact triggered the extinction of the non-aviandinosaurs and other large reptiles, but spared some small animals such as mammals, which then resembled shrews. Over the past 66 Ma, mammalian life has diversified, and several million years ago an African ape-like animal such as Orrorin tugenensis gained the ability to stand upright. This facilitated tool use and encouraged communication that provided the nutrition and stimulation needed for a larger brain, which allowed the evolution of humans. The development of agriculture, and then civilization, led to humans having an influence on Earth and the nature and quantity of other life forms that continues today.