THE FORTUNE OF LIFE- EARTH (HOW IT BEGAN AND ITS EVOLUTION)

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The earliest evidence for life on Earth is graphite found to be a biogenic substance in 3.7 billion-year-old metasedimentary rocks discovered in western Greenland and microbial mat fossils found in 3.48 billion-year-old sandstone discovered in Western Australia. More recently, in 2015, "remains of biotic life" were found in 4.1 billion-year-old rocks in Western Australia. According to one of the researchers, "If life arose relatively quickly on Earth ... then it could be common in the universe." Microbial mats of coexisting bacteria and archaea were the dominant form of life in the early Archean and many of the major steps in early evolution are thought to have taken place within them. The evolution of photosynthesis, around 3.5 Ga, eventually led to a buildup of its waste product, oxygen, in the atmosphere, leading to the great oxygenation event, beginning around 2.4 Ga. The earliest evidence of eukaryotes (complex cells with organelles) dates from 1.85 Ga, and while they may have been present earlier, their diversification accelerated when they started using oxygen in their metabolism. Later, around 1.7 Ga, multicellular organisms began to appear, with differentiated cells performing specialised functions. Sexual reproduction, which involves the fusion of male and female reproductive cells (gametes) to create a zygote in a process called fertilization is, in contrast to asexual reproduction, the primary method of reproduction for the vast majority of macroscopic organisms, including almost all eukaryotes (which includes animals and plants). However the origin and evolution of sexual reproduction remain a puzzle for biologists though it did evolve from a common ancestor that was a single celled eukaryotic species.[22] Bilateria, animals with a front and a back, appeared by 555 Ma (million years ago). It is generally agreed that all life today evolved by common descent from a single primitive lifeform. We do not know how this early form came about, but scientists think it was a natural process which took place perhaps 3,900 million years ago. This is in accord with a philosophy called naturalism: only natural causes are admitted. We do not know whether metabolism or genetics came first. The main hypothesis which supports genetics first is RNA world hypothesis, and the one which supports metabolism first is Protein world hypothesis. Another big problem is how cells develop. All existing forms of life are built out of cells