Origin Of Life

History of the concept: abiogenesis
Research into the origin of life is the modern incarnation of the ancient concept of abiogenesis. Abiogenesis, in its most general sense, is the generation of life from non-living matter. The term is primarily used in the context of biology and the origin of life. Abiogenesis was long considered to be a very common occurrence until the Law of Biogenesis (omne vivum ex ovo or "all life from other life") became firmly established in modern biology.

The modern definition of abiogenesis is concerned with the formation of the simplest forms of life from primordial chemicals. This is significantly different from the concept of Aristotelian abiogenesis, which postulated the formation of complex organisms. This article reviews different hypotheses for modern abiogenetic processes that are currently under debate.

From organic molecules to protocells
As the question how organic molecules form a protocell is largely unanswered, there are many different hypotheses regarding the path that might have been taken from simple organic molecules to protocells, cells, and metabolism. Some of these postulate early appearance of nucleic acids ("genes-first"), whereas the evolution of biochemical reactions and pathways is regarded as moving force of early evolution ("metabolism-first"). Recently, trends are emerging to create hybrid models that combine aspects of both.

"Genes first" models: the RNA world

The RNA world hypothesis, for example, suggests that short RNA molecules could have spontaneously formed that would then catalyze their own continuing replication. Early cell membranes could have formed spontaneously from proteinoids, protein-like molecules that are produced when amino acid solutions are heated. Other possibilities include systems of chemical reactions taking place within clay substrates or on the surface of pyrite rocks. At this time however, these various hypotheses have incomplete evidence supporting them. Many of them can be simulated and tested in the lab, but a lack of undisturbed sedimentary rock from that early in Earth's history leaves few opportunities to determine what may have actually happened in reality.

"Metabolism first" models: iron-sulfur world and others
Several models reject the idea of the self-replication of a "naked-gene" and postulate the emergence of a primitive metabolism which could provide an environment for the later emergence of RNA replication. One of the earliest incarnations of this idea was put forward in 1924 with Alexander Oparin's notion of primitive self-replicating vesicles which predated the discovery of the structure of DNA. More recent variants in the 1980s and 1990s include Günter Wächtershäuser's iron-sulfur world theory and models introduced by Christian de Duve based on the chemistry of thioesters. More abstract and theoretical arguments for the plausibility of the emergence of metabolism without the presence of genes include a mathematical model introduced by Freeman Dyson in the early 1980s, and Stuart Kauffman's notion of collectively autocatalytic sets discussed later in that decade.

Hybrid models
A growing realization of the inadequacy of either pure "genes-first" or "metabolism-first" models is leading the trend towards models that incorporate aspects of each.

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