Abiogenesis

Abiogenesis

Abiogenesis (/?e?ba?.??d??n?s?s/ ay-by-oh-jen-?-siss[1]) or biopoiesis is the idea that life arose from inorganic matter.[2] In particular, the term usually refers to the processes by which life on Earth may have arisen. Abiogenesis likely occurred between 3.9 and 3.5 billion years ago, in the Eoarchean era (the time after the Hadean era in which the Earth was essentially molten). Hypotheses about the origins of life may be divided into several categories. Most approaches investigate how self-replicating molecules or their components came into existence. For example, the Miller–Urey experiment and similar experiments demonstrated that most amino acids, often called "the building blocks of life", were shown to be racemically synthesized in conditions thought to be similar to those of the early Earth. Several mechanisms have been investigated, including lightning and radiation. Other approaches ("metabolism first" hypotheses) focus on understanding how catalysis in chemical systems in the early Earth might have provided the precursor molecules necessary for self-replication. Belief in the ongoing spontaneous generation of certain forms of life from non-living matter goes back to ancient Greek philosophy and continued to have support in Western scholarship until the 19th century; this was paired with the belief in heterogenesis, i.e. that one form of life derived from a different form (e.g. bees from flowers).[3] Classical notions of spontaneous generation, which can be considered under the modern term, abiogenesis, held that certain complex, living organisms are generated by decaying organic substances. According to Aristotle, it was a

eadily observable truth that aphids arise from the dew which falls on plants, flies from putrid matter, mice from dirty hay, crocodiles from rotting logs at the bottom of bodies of water, and so on.[4] In the 17th century, such assumptions started to be questioned. In 1646, Sir Thomas Browne published his Pseudodoxia Epidemica (subtitled Enquiries into Very many Received Tenets, and Commonly Presumed Truths), which was an attack on false beliefs and "vulgar errors." His conclusions were not widely accepted at the time. His contemporary, Alexander Ross wrote: "To question this (i.e., spontaneous generation) is to question reason, sense and experience. If he doubts of this let him go to Egypt, and there he will find the fields swarming with mice, begot of the mud of Nylus, to the great calamity of the inhabitants."[5] In 1665, Robert Hooke published the first drawings of a microorganism. Hooke was followed in 1676 by Anton van Leeuwenhoek, who drew and described microorganisms that are now thought to have been protozoa and bacteria.[6] Many felt the existence of microorganisms was evidence in support of spontaneous generation, since microorganisms seemed too simplistic for sexual reproduction, and asexual reproduction through cell division had not yet been observed. van Leeuwenhoek took issue with the ideas common at the time that fleas and lice could spontaneously result from putrefaction, and that frogs could likewise arise from slime. Using a broad range of experiments ranging from sealed and open meat incubation and the close study of insect reproduction, by the 1680s he became convinced that spontaneous generation was incorrect.[7]