Origin of life

Research into the origin of life is a limited field of research despite its profound impact on biology and human understanding of our world. Progress in this field is slow and sporadic, but it still draws the attention of many. A few facts give insight into the conditions in which life may have emerged, but the mechanisms by which non-life became life are elusive.

Table of contents

1 History of the concept: abiogenesis
2 Current models of the origin of life

2.1 Origin of organic molecules: Miller experiments
2.2 From organic molecules to protocells

2.2.1 "Genes first" models: the RNA world
2.2.2 "Metabolism first" models: iron-sulfur world and others
2.2.3 Hybrid models

3 Other models

3.3 "Deep-hot biosphere" model of Gold
3.4 "Primitive" extraterrestrial life

4 Relevant fields
5 See also:
6 External links

History of the concept: abiogenesis

Main article: 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.

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

Current models of the origin of life

There is no truly "standard" model of the origin of life, however most currently accepted models build in one way or another upon the following discoveries, which are listed in a rough order of postulated emergence:

Plausible pre-biotic conditions result in the creation of the basic small molecules of life. This was demonstrated in the Urey-Miller experiment by Stanley L. Miller and Harold C. Urey in 1953.
Phospholipids spontaneously form lipid bilayers, the basic structure of a cell membrane.
Procedures for producing random RNA molecules can produce "ribozymes", which are able to produce more of themselves under very specific conditions.

The origin (see Origin of organic molecules) of basic biomolecules such as components of amino acids, while not settled, is less controversial than the significance and order of steps 2 and 3 around which much of the current debate revolves around (see From organic molecules to protocells).

Origin of organic molecules: Miller experiments

"Miller experiments" (including the original Miller-Urey experiment of 1953) have shown that under simulated conditions resembling those thought to have existed shortly after Earth first accreted, many of the basic organic molecules that form the building blocks of modern life are able to spontaneously form. Simple organic molecules are of course a far cry from a fully functional self-replicating life form, but in an environment with no pre-existing life these molecules could accumulate and provide a rich environment for chemical evolution. The spontaneous formation of complex polymers from abiotically generated monomers under these conditions is straightforward.

Other sources of complex molecules have been postulated including sources of extra-terrestrial, stellar or interstellar origin. In 2004, a team detected traces of PAH (polycyclic aromatic hydrocarbons) in a nebula, the most complex molecule, to that date, found in space.

From organic molecules to protocells

There are many different hypotheses regarding the path that might have been taken from simple organic molecules to protocells cells and metabolism. Many of the possibilities have tended to fall into either "genes-first" or "metabolism-first", a recent trend is the emergence of hybrid models that combine aspects of both.

"Genes first" models: the RNA world

Main article: RNA world hypothesis

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. None of these various hypotheses have strong evidence behind them at this time, however. 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 practice.

"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 G�nter W�chtersh�user'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.

Other models

"Deep-hot biosphere" model of Gold

A theory put forward by Thomas Gold in the 1990s has life first developing not on the surface of the earth, but several kilometers below the surface. We now know that microbial life is plentiful up to five kilometers below the earth's surface in the form of archaea, which are generally considered to have originated around the same time or earler than bacteria, most of which live on the surface including the oceans. Discovery of microbial life below the surface of another body in our solar system would lend significant credence to this theory.

"Primitive" extraterrestrial life

An alternative to Earthly abiogenesis is the hypothesis that primitive life may have originally formed extraterrestrially (note this is related to, but is not the same as the notion of panspermia). Organic compounds are relatively common in space, especially in the outer solar system where volatiles are not evaporated by solar heating. Comets are encrusted by outer layers of dark material, thought to be a tar-like substance composed of complex organic material formed from simple carbon compounds and ultraviolet light. The rain of cometary material on the early Earth could have brought significant quantities of complex organic molecules, and it is possible that primitive life itself may have formed in space and been brought to the surface along with it. A related hypothesis holds that life may have formed first on early Mars, and been transported to Earth when crustal material was blasted off of Mars by asteroid and comet impacts to later fall to Earth's surface. Both of these hypotheses are even more difficult to find evidence for, and may have to wait for samples to be taken from comets and Mars for study.

Relevant fields

Astrobiology is a field that may shed light on the nature of life in general, instead of just life as we know it (on Earth), and may give clues as to how life originates.
Complex systems