Abiogenesis

See also: Origin of Life

Abiogenesis (pronounced ay-by-oh-jen-ə-siss[1]) or biopoiesis is the study of how biological life could arise from inorganic matter through natural processes. 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 (i.e. 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.

Spontaneous generation Main article: Spontaneous generation 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).[2] Classical notions of abiogenesis, now more precisely known as spontaneous generation, held that certain complex, living organisms are generated by decaying organic substances. According to Aristotle, it was a readily 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.[3] 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."[4]

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.[5] 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.[6]

The first experimental evidence against spontaneous generation came in 1668 when Francesco Redi proved that no maggots appeared in meat when flies were prevented from laying eggs. It was gradually shown that, at least in the case of all the higher and readily visible organisms, the previous sentiment regarding spontaneous generation was false. The alternative seemed to be biogenesis: that every living thing came from a pre-existing living thing (omne vivum ex ovo, Latin for "every living thing from an egg").

In 1768, Lazzaro Spallanzani demonstrated that microbes were present in the air, and could be killed by boiling. In 1861, Louis Pasteur performed a series of experiments that demonstrated that organisms such as bacteria and fungi do not spontaneously appear in sterile, nutrient-rich media, but only invade them from outside.

Complex biological molecules and protocells Sidney W. Fox also experimented with abiogenesis and the primordial soup theory. In one of his experiments, he allowed amino acids to dry out as if puddled in a warm, dry spot in prebiotic conditions. He found that, as they dried, the amino acids formed long, often cross-linked, thread-like, submicroscopic molecules now named "proteinoids".

In another experiment using a similar method to set suitable conditions for life to form, Fox collected volcanic material from a cinder cone in Hawaii. He discovered that the temperature was over 100 °C (212 °F) just 4 inches (100 mm) beneath the surface of the cinder cone, and suggested that this might have been the environment in which life was created—molecules could have formed and then been washed through the loose volcanic ash and into the sea. He placed lumps of lava over amino acids derived from methane, ammonia and water, sterilized all materials, and baked the lava over the amino acids for a few hours in a glass oven. A brown, sticky substance formed over the surface and when the lava was drenched in sterilized water a thick, brown liquid leached out. It turned out that the amino acids had combined to form proteinoids, and the proteinoids had combined to form small, cell-like spheres. Fox called these "microspheres", a name that subsequently was displaced by the more informative term protobionts. His protobionts were not cells, although they formed clumps and chains reminiscent of cyanobacteria. They contained no functional nucleic acids, but split asexually and formed within double membranes that had some attributes suggestive of cell membranes. Professor Colin S. Pittendrigh stated in December 1967 that "laboratories will be creating a living cell within ten years," a remark that reflected the typical contemporary levels of innocence of the complexity of cell structures.[16]

[edit]Early conditions

Main article: Timeline of evolution The Hadean Earth is thought to have had a secondary atmosphere, formed through degassing of the rocks that accumulated from planetesimal impactors. At first it was thought by scientists like Harold Urey, that the earth's atmosphere was made up of hydrates—methane, ammonia and water vapour, and that life began under such reducing conditions, conducive to the formation of organic molecules. However, it is now thought that the early atmosphere, based on today's volcanic evidence, would have contained 60% hydrogen, 20% oxygen (mostly in the form of water vapour), 10% carbon dioxide, 5 to 7% hydrogen sulphide, and smaller amounts of nitrogen, carbon monoxide, free hydrogen, methane and inert gases. As the earth lacks the gravity to hold any hydrogen, this component of the atmosphere was rapidly lost during the Hadean period. Solution of the carbon dioxide in water is thought to have made the seas slightly acid, with a pH of about 5.5.[17]

Morse and MacKenzie have suggested that oceans may have appeared first in the Hadean eon, as soon as two hundred million years (200 Ma) after the Earth was formed, in a hot 100 °C (212 °F) reducing environment, and that the pH of about 5.8 rose rapidly towards neutral.[18] This has been supported by Wilde[19] who has pushed the date of the zircon crystals found in the metamorphosed quartzite of Mount Narryer in Western Australia, previously thought to be 4.1–4.2 Ga, to 4.404 Ga. This means that oceans and continental crust existed within 150 Ma of Earth's formation. Rosing et al.,[20] suggest that between 4.4 and 4.3 Ga, the Earth was a water world, with little if any continental crust, with an extremely turbulent atmosphere and a hydrosphere subject to high uV, from a T Tauri sun and cosmic radiation and continued bolide impact.

As a result, the Hadean environment was one highly hazardous to modern life. Frequent collisions with large objects, up to 500 kilometres (310 mi) in diameter, would have been sufficient to vaporise the ocean within a few months of impact, with hot steam mixed with rock vapour leading to high altitude clouds completely covering the planet. After a few months the height of these clouds would have begun to decrease but the cloud base would still have been elevated for about the next thousand years. After that, it would have begun to rain at low altitude. For another two thousand years rains would slowly have drawn down the height of the clouds, returning the oceans to their original depth only 3,000 years after the impact event.[21]

Between 3.8 and 4.1 Ga, changes in the orbits of the gaseous giant planets may have caused a late heavy bombardment that pockmarked the Moon and the other inner planets (Mercury, Mars, and presumably Earth and Venus). This would likely have sterilized the planet, had life appeared before that time. Geologically the Hadean Earth would have been far more active than at any other time in its history. Studies of meteorites suggests that radioactive elements such as Aluminium-26 with a half-life of 7.17×105, and Potassium-40 with a half-life of 1.250×109 years, isotopes mainly produced in supernovae, were much more common, with the result that the earth was more than 96% more radioactive than it is today. Coupled with internal heating as a result of gravitational sorting between the core and the mantle generated a great deal of mantle convection, with the probable result that there would have been many more smaller very active tectonic plates, than in modern times.

By examining the time interval between such devastating environmental events, the time interval when life might first have come into existence can be found for different early environments. The study by Maher and Stevenson shows that if the deep marine hydrothermal setting provides a suitable site for the origin of life, abiogenesis could have happened as early as 4.0 to 4.2 Ga, whereas if it occurred at the surface of the Earth abiogenesis could only have occurred between 3.7 and 4.0 Ga.[22]

Other research suggests a colder start to life. Work by Leslie Orgel and colleagues on the synthesis of purines has shown that freezing temperatures are advantageous, due to the concentrating effect for key precursors such as hydrogen cyanide.[23] Research by Stanley Miller and colleagues suggested that while adenine and guanine require freezing conditions for synthesis, cytosine and uracil may require boiling temperatures.[24] An article in Discover Magazine points to research by the Miller group indicating the formation of seven different amino acids and 11 types of nucleobases in ice when ammonia and cyanide were left in a freezer from 1972–1997.[25][26] This article also describes research by Christof Biebricher showing the formation of RNA molecules 400 bases long under freezing conditions using an RNA template, a single-strand chain of RNA that guides the formation of a new strand of RNA. As that new RNA strand grows, it adheres to the template.[27] The explanation given for the unusual speed of these reactions at such a low temperature is eutectic freezing. As an ice crystal forms, it stays pure: only molecules of water join the growing crystal, while impurities like salt or cyanide are excluded. These impurities become crowded in microscopic pockets of liquid within the ice, and this crowding causes the molecules to collide more often.

Evidence of the early appearance of life comes from the Isua supercrustal belt in Western Greenland and from similar formations in the nearby Akilia Islands. Carbon entering into rock formations has a ratio of Carbon-13 (13C) to Carbon-12 (12C) of about −5.5 (in units of δ13C), where because of a preferential biotic uptake of 12C, biomass has a δ13C of between −20 and −30. These isotopic fingerprints are preserved in the sediments, and Mojzis has used this technique to suggest that life existed on the planet already by 3.85 billion years ago.[28] Lazcano and Miller (1994) suggest that the rapidity of the evolution of life is dictated by the rate of recirculating water through mid-ocean submarine vents. Complete recirculation takes 10 million years, thus any organic compounds produced by then would be altered or destroyed by temperatures exceeding 300 °C (572 °F). They estimate that the development of a 100 kilobase genome of a DNA/protein primitive heterotroph into a 7000 gene filamentous cyanobacterium would have required only 7 Ma.[29] The Nobel Prize winning chemist, Christian de Duve, argues that the determination of chemistry means that "life has to emerge quickly... Chemical reactions happen quickly or not at all; if any reaction takes a millennium to complete then the chances are all the reagents will simply dissipate or breakdown in the meantime, unless they are replenished by other faster reactions".[30][31]

[edit] Current models

There is no "standard model" of the origin of life. Most currently accepted models draw at least some elements from the framework laid out by the Oparin-Haldane hypothesis. Under that umbrella, however, are a wide array of disparate discoveries and conjectures such as the following, listed in a rough order of postulated emergence:

Some theorists suggest that the atmosphere of the early Earth may have been chemically reducing in nature, composed primarily of methane (CH4), ammonia (NH3), water (H2O), hydrogen sulfide (H2S), carbon dioxide (CO2) or carbon monoxide (CO), and phosphate (PO43-), with molecular oxygen (O2) and ozone (O3) either rare or absent. In such a reducing atmosphere, electrical activity can catalyze the creation of certain basic small molecules (monomers) of life, such as amino acids. This was demonstrated in the Miller–Urey experiment by Stanley L. Miller and Harold C. Urey in 1953. Phospholipids (of an appropriate length) can form lipid bilayers, a basic component of the cell membrane. A fundamental question is about the nature of the first self-replicating molecule. Since replication is accomplished in modern cells through the cooperative action of proteins and nucleic acids, the major schools of thought about how the process originated can be broadly classified as "proteins first" and "nucleic acids first". The principal thrust of the "nucleic acids first" argument is as follows: The polymerization of nucleotides into random RNA molecules might have resulted in self-replicating ribozymes (RNA world hypothesis) Selection pressures for catalytic efficiency and diversity might have resulted in ribozymes which catalyse peptidyl transfer (hence formation of small proteins), since oligopeptides complex with RNA to form better catalysts. The first ribosome might have been created by such a process, resulting in more prevalent protein synthesis. Synthesized proteins might then outcompete ribozymes in catalytic ability, and therefore become the dominant biopolymer, relegating nucleic acids to their modern use, predominantly as a carrier of genomic information. No one has yet synthesized a "protocell" using basic components which would have the necessary properties of life (the so-called "bottom-up-approach"). Without such a proof-of-principle, explanations have tended to be focused on chemosynthesis of polymers. However, some researchers are working in this field, notably Steen Rasmussen at Los Alamos National Laboratory and Jack Szostak at Harvard University. Others have argued that a "top-down approach" is more feasible. One such approach, successfully attempted by Craig Venter and others at The Institute for Genomic Research, involves engineering existing prokaryotic cells with progressively fewer genes, attempting to discern at which point the most minimal requirements for life were reached.[32][33] The biologist John Desmond Bernal coined the term biopoiesis for this process,[34] and suggested that there were a number of clearly defined "stages" that could be recognised in explaining the origin of life.

Stage 1: The origin of biological monomers Stage 2: The origin of biological polymers Stage 3: The evolution from molecules to cell Bernal suggested that evolution commenced between Stage 1 and 2.[35