Origin Of Life PowerPoint PDF

The origin of life: Chemical reactions must have taken place on earth 4 billion years ago. Molecular building blocks of life can be synthesized from simpler molecules. Which existed in prebiotic seas. After the molecular building blocks, next is a origin of simple replicating molecule. Several studi...

The origin of life: Chemical reactions must have taken place on earth 4 billion years ago. Molecular building blocks of life can be synthesized from simpler molecules. Which existed in prebiotic seas. After the molecular building blocks, next is a origin of simple replicating molecule. Several studies suggest RNA preceded DNA. However, no one has yet discovered autocatalytic RNA that could catalyze its own replication. The early stage of life when it used RNA as the hereditary molecule is called ‘RNA world’. Life came to use DNA later in history. One reason for transition from RNA to DNA is that RNA based life was limited by relatively high mutation rate of RNA. More complex life forms could not evolve until mutation rate reduced. The evolution of DNA would have led to reduction of the mutation rate. The fossil record tells us little about origin of life because those events were on a molecular scale. Earth is 4.5 billion years old. For the first few hundred million years, earth was bombarded by huge asteroids that vaporized The oldest rocks are at a site at Isua, Greenland and are 3.8 billion years old. These rocks contain chemical traces that may or may not be chemical fossils of life forms. The rocks have undergone too much metamorphosis to have any chance of retaining fossil cells- if cells existed at that time. Fossil evidence of cells comes from various sites in the period 3-3.5 billion years ago. The earliest fossil cells were recently thought to come from 3.5 billion-year-old rocks form the Apex Chert in Western Australia. However, Brasier et al., (2002) argued that the alleged fossils in these rocks are artifacts and not fossils. Other evidence for fossil cells exists from 3-3.5 billion year period. Cells had therefore likely evolved by 3.5 billion years ago or soon afterwards. The origin of cells Fossils of cellular prokaryotic life has been found at several sites aged between 3.5 and 2 billion years ago. They often exist in the form of stromatolites. Stromatolites layer below them. As the process is repeated over time, a stromatolite builds up consisting of many mineralized layers. Thus, cellular layer was flourishing 2-3 billion years ago. The first cells were probably little more than replicating molecules either surrounded by or arranged within membranes. Modern prokaryotic cells are complex versions of this form of life. The deepest classificatory division of cellular life is three-way divide into archaeans, bacteria and eukaryotes. Archaea and bacteria are prokaryotes and existed on earth 2-3 billion years ago. The eukaryotes evolved after prokaryotes about 2.7 billion years ago. The earliest fossil cells proposed to be eukaryotic were found in abandoned mine in Michigan. The fossils are corkscrew shaped and closely resemble later algae. Molecular clock studies suggest eukaryotes originated 2.2-1.8 billion years ago. Thus the body fossil and molecular evidence agree, but chemical fossil evidence hints at earlier date. The origin of the eukaryotic cell could have spread over many hundreds of An important event with the origin of eukaryotes is the evolution of photosynthesis. Photosynthesis originated earlier-indeed Schopf’s 3.5 billion year old possible microbes may have been photosynthetic. The origin of multicellular life The molecular clock suggest that multicellular life originated 1.5 billion years ago. This is somewhat before the oldest multicellular fossils. Currently the earliest such fossils are algae from 1.2 billion years ago. The earliest definite fossils of multicellular animals come from Ediacaran deposits in Australia. These and similar deposits date to the period from 670 to 550 million years ago. The Ediacaran fossils are of soft-bodied aquatic animals such as jellyfish and worms.