History of the concept:
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
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.
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.
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.
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.
Main Nighet1 Nighet2abiogen1 abiogen2 abiogen3 abiogen4 abiogen5
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