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Panspermia

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Panspermia is the hypothesis that life existed already in the Universe in the form of microscopic bacteria called Extremophiles, and that life on Earth may have originated through these Bacteria. The hypothesized mechanism for this process is that Earth was, is, and will continue to be hit with asteroids with the correct chemical composition to allow extraterrestrial bacterial endospores a measurable probability to survive the intense solar radiation and extreme cold of space and collide with our planet. It is then thought that this single species or group of Extremophiles then colonized our planet, and served as the common ancestor for all life on Earth to evolve from in the process of Evolution. The best guess as to where this asteroid could come from is Mars [1], because of the evidence that life on Mars did exist[2][3], and was most likely present before life on Earth; or that some other planetary body from the Kuiper Belt is the most likely source of this bacteria-harboring asteroid. Exogenesis is a more limited hypothesis that proposes life on Earth was transferred from elsewhere in the Universe but makes no prediction about how widespread it is. Because the term "panspermia" is more well-known, it tends to be used in reference to what would properly be called exogenesis.

Hypothesis

The first known mention of the idea was in the writings of the 5th century BCE Greek philosopher Anaxagoras, but panspermia hypothesis was dormant until the nineteenth century when it was revived in modern form by several scientists, including Hermann von Helmholtz in 1879 and, somewhat later, by Svante Arrhenius in 1903. Panspermia can be said to be either interstellar (between star systems) or interplanetary (between planets in the same solar system). There is as yet no compelling evidence to support or contradict it, although the majority view holds that panspermia — especially in its interstellar form — is unlikely given the challenges of survival and transport in space.

Sir Fred Hoyle (1915–2001) and Chandra Wickramasinghe (born 1939) were important proponents of the hypothesis who further contended that lifeforms continue to enter the Earth's atmosphere, and may be responsible for epidemic outbreaks, new diseases, and the genetic novelty necessary for macroevolution. This extension has also been adopted by proponents of Cosmic ancestry.

Panspermia per se does not remove the need for life to originate somewhere, but does extend the time frame and environments available. Similarly, it does not necessarily suggest that life originated only once and subsequently spread through the entire Universe, but instead that once started it may be able to spread to other environments suitable for replication. (In the strongest version of panspermia, life never originated, but always existed — this axiom would require amending the big bang theory.) The mechanisms proposed for interstellar panspermia are hypothetical and currently unproven. Interplanetary transfer of material is well documented, as evidenced by meteorites of Martian origin found on Earth. However, claims that these carry evidence of extraterrestrial lifeforms — let alone viable dormant lifeforms — have either been proven unfounded as a result of terrestrial contamination, misinterpretation, or hoaxing; or are currently hotly disputed. Interestingly, space probes may also be a viable transport mechanism for interplanetary panspermia in our solar system (or even beyond) especially given the claim (now no longer sustainable) that terrestrial bacteria were supposed to have survived in a dormant state on the Moon. Since then, however, NASA has implemented strict abiotic procedures to avoid planetary contamination.

Evidence

Evidence of Panspermia taking place within human history has been observed; however, it is extremely difficulty to prove that one of the millions of asteroids that have hit Earth in it's long history provided the common ancestor long hypothesized by Biologists. The most striking evidence of this phenomenon is the red colored rain that occurred in many places of Kerala in India during July to September 2001 due to the mixing of huge quantity of microscopic red cells in the rainwater. The red cells under study appeared to be the resting endospores of an extremophilic microorganism. [4] There is, however, circumstantial evidence for exogenesis:

Narrow time window for geogenesis

Pre-Cambrian stromatolites in the Siyeh Formation, Glacier National Park. It is in formations such as this that 3.5 billion year old fossilized algae microbes, the earliest known life on earth, were discovered.

The Precambrian fossil record indicates that life appeared soon after the Earth was formed. This would imply that life appears in several hundred million years when conditions are favourable.

  • Generally accepted scientific estimates of the age of the Earth place its formation (along with the rest of the Solar system) at about 4.55 Ga.
  • The oldest known sedimentary rocks are somewhat altered Hadean formations from the southern tip of Akilia island, West Greenland. These rocks have been dated as no younger than 3.85 Ga. The Greenland sediments include banded iron beds, thought to be the result of oxygen released by photosynthetic organisms combining with dissolved iron to form insoluble iron oxides. Carbon deposits in the rock show low levels of carbon-13. Kerogen deposits (derived from organic matter) are isotopically light (i.e. more negative δ13C values) which is indicative of photosynthesis (see Schidlowski, 1988). However, this interpretation is under doubt as the Akilia rocks have undergone high-temperature metamorphosis which is known to be fractionating itself (Gilmour & Wright, 1997). There is also a lack of corroborating sulphur isotope fractionation (Nisbet, 2000). Both the sedimentary origin and the carbon content of the rocks have been questioned (Lepland et al, 2005).
  • Fossilized stromatolites or bacterial aggregates, the oldest of which are dated at 3.5 billion years old, suggest that photosynthesis might be exogenic. The bacteria that form stromatolites, cyanobacteria, are photosynthetic. Most models of the origin of life have the earliest organisms obtaining energy from reduced chemicals, with the more complex mechanisms of photosynthesis evolving later.
  • During the Late Heavy Bombardment of the Earth's Moon about 3.9 Ga (as evidenced by Apollo lunar samples) impact intensities may have been up to 100x those immediately before or after (Cohen et al., 2000). From analysis of lunar melts and observations of similar cratering on Mars' highlands, Kring and Cohen (2002) suggest that the LHB was caused by asteroid impacts that affected the entire inner solar system. This is likely to have effectively sterilised Earth's entire planetary surface, including submarine hydrothermal systems that would be otherwise protected (Cohen et al., 2000).
  • The best estimate of the origin of the Universe, from the Wilkinson Microwave Anisotropy Probe, is 13700 million years ago (13.7 Ga). However, at least one subsequent cycle of star birth/death is required for nucleosynthesis of the C, N and O essential to life, and this process may have taken up to several Ga to produce sufficient quantities (Gilmour et al., 1997). This puts the earliest possible emergence of life in the Universe at ~12.7 Ga, although there is large uncertainty in the length of the necessary time period.

If life originated on Earth it did so in a window of at most 1 Ga (4.55 Ga to 3.5 Ga), most plausibly 400 Ma (3.9 Ga to 3.5 Ga), and possibly <100 Ma (3.9 Ga to 3.85 Ga) if the Greenland (3.85 Ga) isotope signal is correct. If life originated elsewhere, the window expands to ~9 Ga. That full length of time might not be available on a single planet, but the Earth has provided a life-friendly environment for at least 3.5 Ga.

Extremophiles

Evidence has accumulated that some bacteria and archaea are more resistant to extreme conditions than previously recognized, and may be able to survive for very long periods of time even in deep space. These extremophiles could possibly travel in a dormant state between environments suitable for ongoing life such as planetary surfaces.

  • Bacteria and more complex organisms have been found in more extreme environments than thought possible, such as black smokers or oceanic volcanic vents. Some extremophile bacteria have been found living at temperatures above 100 °C, others in strongly caustic environments, and others in extreme pressures 11 km under the ocean. [5]
  • Semi-dormant bacteria found in ice cores over a mile beneath the Antarctic lends credibility to the idea that the components of life might survive on the surface of icy comets.
  • There are bacteria that do not rely on photosynthesis for energy. In particular, endolithic bacteria using chemosynthesis has been found inside rocks and in subterranean lakes.
  • Deinococcus radiodurans is a radioresistant bacterium that can survive high radiation levels.
  • Dormant bacteria have been isolated from insects in amber 10s Ma old (Gilmour et al., 1997)
  • Recent experiments suggest that if bacteria were somehow sheltered from the radiation of space, perhaps inside a thick meteoroid, they could survive dormant for millions of years.
  • Duplicating the harsh conditions of cold interstellar space in their laboratory, NASA scientists have created primitive cells that mimic the membranous structures found in all living things. These chemical compounds may have played a part in the origin of life. [6]

However, it is worth stressing that the claim that Streptococcus mitis survived on the moon on the Surveyor 3 spacecraft is no longer sustainable .

Wider range of potential habitats for life

Another line of evidence comes from research that shows there are many more potential habitats for life than Earth-like planets.

  • The presence of past liquid water on Mars, suggested by river-like formations on the red planet, was confirmed by the Mars Exploration Rover missions. In December of 2006, Michael C. Malin of Malin Space Science Systems published a paper in the journal Science which argued that his camera (the Mars Observer Camera) had found evidence suggesting water was occasionally flowing on the surface of Mars within the last five years.
  • Water oceans might exist on Europa, Enceladus, Triton and perhaps other moons in the Solar system. Even moons that are now frozen ice balls might earlier have been melted internally by heat from radioactive rocky cores. Bodies like this may be extremely common throughout the Universe. Lake Vostok in Antarctica, which has been sealed for millions of years, and which may contain unusual life or be sterile, is a possible testing ground for ways to explore these moons.
  • Bacteria have been discovered living within warm rock deep in the Earth's crust.

Evidence of extraterrestrial life

No undisputed evidence has ever been published in a mainstream scientific journal to suggest that intelligent alien species have visited the Earth. The majority view in the scientific community seems to be an acceptance that the existence of intelligent life elsewhere in the Universe is at least highly probable, due to the sheer number of potential sites where life could take hold. However, the special theory of relativity holds that travel over the vast distances between stars would be limited to the speed of light, and so take such a long time that many scientists think it unlikely that such travel would be practical for life forms as we know them. Nevertheless, a small core of researchers continue to monitor the skies for signs of transmissions from other stars. The Search for Extra-Terrestrial Intelligence (SETI) project is the most popular example. Over the past century, thousands of people have reported UFO sightings in countries all over the world, but these reports have never been shown to be genuine. [7]

Disputed

  • The Red Rain of Kerala. In 2003, Satyanarayana et al. proposed that the rain was coloured red by a dust cloud from the Gulf. [8] Their paper was then published in Aerosol Science and Technology. [9] Dr. Godfrey Louis has analyzed the dust and reported finding spores that he has hypothesised are of extraterrestrial origin. In April 2006, Louis published his findings in Astrophysics and Space Science. [10] [11] Louis claimed that the red particles "reproduce plentifully", and that they did so even in "water superheated to nearly 300 °C".
Microstructures in ALH84001 claimed to be of biogenic origin
  • A meteorite originating from Mars known as ALH84001 was shown in 1996 to contain microscopic structures resembling small terrestrial microfossils. When the discovery was announced, many immediately conjectured that the fossils were the first true evidence of extraterrestrial life — making headlines around the world, and even prompting U.S. President Bill Clinton to make a formal televised announcement to mark the event. As of 2003 however, most experts agree that these are not indicative of life, but may instead be formed abiotically from organic molecules. It has not yet conclusively been shown how they formed and recent advances in nanobe research has made the find interesting again.
  • On November 26, 2000, it was reported by Chandra Wickramasinghe that the Indian Space Research Organization have recovered microorganisms from an altitude of 16 kilometers in the Earth's atmosphere, and that the microorganisms were "unlike any known on Earth". NASA said the finding is likely to meet considerable skepticism in the scientific community, that "Aerobiologists might argue that 10 miles is not too high for Earth life to reside, a possibility that Wickramasinghe appears to accept." However, NASA said that it is plausible for microorganisms to be transported through space aboard comets and meteors, "A recent discovery indicates that microbes can remain dormant for millions of years -- enough time to travel from planet to planet."[4]
  • At the 46th annual meeting of the International Society for Optical Engineering (SPIE) in San Diego, California on April 29, 2001, scientists from India and the United Kingdom claimed to have found extraterrestial microbial life in the Earth's stratosphere. Professor Chandra Wickramasinghe of Cardiff University said "There is now unambiguous evidence for the presence of clumps of living cells in air samples from as high as 41 kilometers, well above the local tropopause, above which no air from lower down would normally be transported." Scientist Max Bernstein however said "it would strain one's credulity less to believe that terrestrial organisms had somehow been transported upwards than to assume that extraterrestrial organisms are falling inward."[5]
  • On May, 2001, two researchers from the University of Naples claimed to have found live extraterrestial bacteria inside a meteorite. Geologist Bruno D'Argenio and molecular biologist Giuseppe Geraci claim the bacteria were wedged inside the crystal structure of minerals, but were resurrected when a sample of the rock was placed in a culture medium. They believe that the bacteria were not terrestial because they survived when the sample was sterilized at very high temperature and washed with alcohol. They also claim that the bacteria's DNA is unlike any on Earth.[6] They presented a report on May 11, 2001, concluding that this is the first evidence of extraterrestial life, documented in its genetic and morphological properties. Some of the bacteria they discovered were found inside meteorites that have been estimated to be over 4.5 billion years old, and were determined to be related to modern day Bacillus subtilis and Bacillus pumilus bacteria on Earth but appears to be a different strain.[7]
  • Narlikar et al. (2003) took air samples at 41 km over Hyderabad, India — above the tropopause where mixing from the lower atmosphere is unexpected — from which rod and coccoid bacteria were isolated. Two bacterial and one fungal species were later independently isolated from these filters which were identified as Bacillus simplex, Staphylococcus pasteuri and Engyodontium album respectively (Wainwright, 2003)[8]. The experimental procedure suggested that these were not the result of laboratory contamination, although similar isolation experiments at separate laboratories were unsuccessful. That these are common terrestrial organisms is not necessarily contraindicative of panspermia, since a prediction of the hypothesis is that life throughout the Universe is derived from the same ancestral stock. Assuming they are not contaminants, did the micro-organisms come from the Earth or space? That there were no volcanic eruptions — the only known way for terrestrial particles to mix up beyond the tropopause — prior to sampling suggests against a terrestrial source. In either case, Wainright (2003) points out that some part of the panspermia hypothesis is validated: either terrestrial micro-organisms are indeed derived from space, or they are capable of contaminating our local space in a viable form. Measuring the isotope ratios of carbon and nitrogen in the micro-organisms from the stratosphere could reveal whether they come from Earth or space.
  • Of three biological experiments on the Mars lander Viking, two gave results that were initially indicative of life. However, the similar results from heated controls, how the release of indicative gas tapered off, and the lack of organic molecules in soil samples all suggest that the results were the result of an abiotic chemical reaction rather than biological metabolism. Later experiments showed that terrestrial clays could reproduce the results of the two positive Viking experiments. Despite this, some of the Viking experiments' designers remain convinced that they are diagnostic for life.

Debunked

  • In 1962, Claus et al. announced the discovery of 'organised elements' embedded in the Orgueil meteorite. These elements were subsequently shown to be either pollens (including that of ragwort) and fungal spores (Fitch & Anders, 1963) that had contaminated the sample, or crystals of the mineral olivine.

Hoaxes

  • A separate fragment of the Orgueil meteorite (kept in a sealed glass jar since its discovery) was found in 1965 to have a seed capsule embedded in it, whilst the original glassy layer on the outside remained undisturbed. Despite great initial excitement, it was found to be that of a European rush that had been glued into the fragment and camouflaged using coal dust. The outer 'fusion layer' was in fact glue. Whilst the perpetrator of this hoax is unknown, it is thought he sought to influence the 19th century debate on spontaneous generation — rather than panspermia — by demonstrating the transformation of inorganic to biological matter.

Objections to panspermia and exogenesis

  • Life as we know it requires heavy elements carbon, nitrogen and oxygen (C, N and O, respectively) to exist at sufficient densities and temperatures for the chemical reactions between them to occur. These conditions are not widespread in the Universe, so this limits the distribution of life as an ongoing process. First, the elements C, N and O are only created after at least one cycle of star birth/death: this is a limit to the earliest time life could have arisen. Second, densities of elements sufficient for the formation of more complex molecules necessary to life (such as amino acids) only occur in molecular dust clouds (109–1012 particles/m3), and (following their collapse) in solar systems. Third, temperatures must be lower than those in stars (elements are stripped of electrons: a plasma state) but higher than in interstellar space (reaction rates are too low). This restricts ongoing life to planetary environments where heavy elements are present at high densities, so long as temperatures are sufficient for plausible reaction rates. Note this does not restrict dormant forms of life to these environments, so this argument only contradicts the widest interpretation of panspermia — that life is ongoing and is spread across many different environments throughout the Universe — and presupposes that any life needs those elements, which the proponents of alternative biochemistries do not consider certain.
  • Space is a damaging environment for life, as it would be exposed to radiation, cosmic rays and stellar winds. However, some bacteria may be able to survive these conditions. Also, environments may exist within meteorites or comets that are somewhat shielded from these hazards.
  • Bacteria would not survive the immense heat and forces of an impact on earth — no conclusions (whether positive or negative) have yet been reached on this point. However most of the heat generated when a meteor enters the Earth's atmosphere is carried away by ablation and the interiors of freshly landed meteorites are rarely heated much and are often cold. For example, a sample of hundreds of nematode worms on the space shuttle Columbia survived its crash landing from 63 km inside a 4 kg locker, and samples of already dead moss were not damaged. Though this is not a very good example, being protected by the man-made locker and possibly pieces of the shuttle, it lends some support to the idea that life could survive a trip through the atmosphere. [13] The existence of Martian meteorites and Lunar meteorites on Earth suggests that transfer of material from other planets to Earth happens regularly.
  • Occam's Razor implies that when developing a hypothesis, we should avoid making evidentially unsupported presumptions about things if at all possible. See heuristic arguments. From this perspective, geogenesis appears to be the default assumption when compared with panspermia or exogenesis. The former assumes a single step: that life originated on Earth, where it is now commonly observed, excluding the presumption that life formed elsewhere, in places where it has never been observed before. Geogenesis eliminates the unsupported presumption of life existing beyond the Earth, but requires a lot to happen in a relatively short time frame in order for life to arise. Exogenesis assumes that it must require a longer period of time than could be offered on Earth. Given that an understanding of life's emergence remains speculative, however, the perception of which presumption (life beyond Earth or life emerging rapidly) is preferable can be less than clear, however.
  • Supporters of exogenesis also argue that on a larger scale, for life to emerge in one place in the Universe and subsequently spread to other planets would be simpler than similar life emerging separately on different planets. Thus, finding any evidence of extraterrestrial life similar to ours would lend credibility to exogenesis. However, this again assumes that the emergence of life in the entire Universe is rare enough as to limit it to one or few events or origination sites. Exogenesis still requires life to have originated from somewhere, most probably some form of geogenesis. Given the immense expanse of the entire Universe, there is a higher probability that there exists (or has existed) another Earth-like planet that has yielded life (geogenesis) than not. This explanation is more preferred under Occam's Razor than exogenesis since it theorizes that the creation of life is a matter of probability and can occur when the correct conditions are met rather than in exogenesis that assumes it is a singular event or that Earth did not meet those conditions on its own. In other words, exogenesis theorizes only one or few origins of life in the Universe, whereas geogenesis theorizes that it is a matter of probability depending on the conditions of the celestial body. Consider that even the most rare events on Earth can happen multiple times and independent of one another. However, since to date no extraterrestrial life has been confirmed, both theories still suffer from lack of information and too many unidentified variables.

Directed panspermia

A second prominent proponent of panspermia was the late Nobel prize winner Professor Francis Crick, OM FRS, who along with Leslie Orgel proposed the theory of directed panspermia in 1973. This suggests that the seeds of life may have been purposely spread by an advanced extraterrestrial civilization. Crick argues that small grains containing DNA, or the building blocks of life, fired randomly in all directions is the best, most cost effective strategy for seeding life on a compatible planet at some time in the future. The strategy might have been pursued by a civilization facing catastrophic annihilation, or hoping to terraform planets for later colonization. Later, after biologists had proposed that an "RNA world" might be involved in the origin of life, Crick noted that he had been overly pessimistic about the chances of life evolving on Earth[9]. See: Francis Crick.

Other proponents of panspermia believe that life never evolved from inorganic molecules, but that it has existed as long as all other forms of matter. This is an extension of panspermia called cosmic ancestry.

Theoretically, by humans traveling to other celestial bodies such as the moon, there is a chance that they carry with them microorganisms or other organic materials ubiquitous on Earth, thus raising the curious possibility that we can seed life on other planetary bodies. The same can be said for unmanned probes manufactured on Earth. This is a concern among space researchers who try to prevent Earth contamination from distorting data, especially in regards to finding possible extraterrestrial life. Even the best sterilization techniques can not guarantee that potentially invasive biologic or organic materials will not be unintentionally carried along. So far, however, in the limited amount of space exploration conducted by humans, "terrestrial pollution" does not appear to be a problem although no concrete studies have investigated this. The harsh environments encountered throughout the rest of the solar system so far do not seem to support complex terrestrial life. However, it should be noted that matter exchange in form of meteor impacts has existed and will exist in the solar system even without human intervention. As evidence, some argue that anomalies found within Martian meteorite ALH 84001 indicate that bacteria could travel from planet to planet without intelligent help.

There exists speculation on a connection to the Titius-Bode Law, argumenting that earth may have received seeds of life by directed panspermia, because the extraterrestrial senders knew that earth belonged to a sun system with stable Titius-Bode structure. See: External Link "Directed Panspermia and Titius-Bode"

  • Panspermia is in the X-Files episode "Bio-Genesis".
  • The novel The Gripping Hand by Larry Niven and Jerry Pournelle mentions that panspermia is a commonly accepted theory in that Universe. Niven also extensively writes about both directed and non-directed panspermia in his Known-Space novels.
  • The film Panspermia by computer graphics artist Karl Sims features a world of complex and diverse species created by using "artificial evolution". It has become one of the most influential works in the fields of both computer graphics and artificial life. It was edited into the "Seeds of Life" segment of the computer graphics compilation Beyond the Mind's Eye.

Science fiction

The theory of panspermia has been explored in a number of works of science fiction, notably Jack Finney's The Body Snatchers (three times made into a film) and the Dragonriders of Pern books of Anne McCaffrey. In John Wyndham's book, The Day of the Triffids (also made into a film), the first person narrator, writing in historical mode, takes care to reject the theory of panspermia in favour of the conclusion that the eponymous carnivorous plants are a product of Soviet biotechnology. The book and film of The Andromeda Strain examines the consequences of a pathogenic extraterrestrial organism arriving on Earth.

In contrast, Stephen Baxter's Titan suggests the possibility of geocentric panspermia: the astronaut's final dying act is to release bacteria into a Titanian lake. The theme is continued as the resultant beetle-like civilisation purposefully colonise new star systems.

Some works of science fiction advance a derivative of the theory as a rationalization for the improbable tendency of fictional extra-terrestrials to be strongly humanoid in form as well as living on earth-compatible worlds (see Class M planet) and having similar levels of technology. In a Star Trek: The Next Generation episode ("The Chase"), the humanoid aliens, which include the humans themselves, are results of the genetic codes spread through the Universe by the Ancient humanoids and were placed by the Ancient humanoids themselves.

See also

References

  1. ^ [[1]]
  2. ^ [[2]]
  3. ^ [[3]]
  4. ^ http://archives.cnn.com/2000/TECH/space/11/24/alien.microbe.claim/index.html
  5. ^ http://www.sciam.com/article.cfm?articleID=000D499B-4662-1C60-B882809EC588ED9F
  6. ^ http://space.newscientist.com/article/dn725
  7. ^ http://www.lincei.it/pubblicazioni/rendicontiFMN/rol/pdf/S2001-01-04.pdf
  8. ^ M. Wainwright, N.C. Wickramasinghe, J.V. Narlikar, P. Rajaratnam. "Microorganisms cultured from stratospheric air samples obtained at 41km". Retrieved 2007-05-11.{{cite web}}: CS1 maint: multiple names: authors list (link)
  9. ^ "Anticipating an RNA world. Some past speculations on the origin of life: where are they today?" by L. E. Orgel and F. H. C. Crick in FASEB J. (1993) Volume 7 pages 238-239.
  • Rhawn Joseph, "Astrobiology, the Origin of Life and the Death of Darwinism", University Press, 2001, ISBN 0-9700733-8-0
  • Cohen, B., Swindle, T. and Kring, D. (2000) Support for the lunar cataclysm hypothesis from lunar meteorite impact melt ages. Science, 290(5497):1754 -- 1756.
  • Crick F, 'Life, Its Origin and Nature', Simon and Schuster, 1981, ISBN 0-7088-2235-5
  • Gilmour I, Wright I, Wright J 'Origins of Earth and Life', The Open University, 1997, ISBN 0-7492-8182-0
  • FITCH FW, ANDERS E (1963) ORGANIZED ELEMENT — POSSIBLE IDENTIFICATION IN ORGUEIL METEORITE. SCIENCE 140 (357): 1097
  • Hoyle F, 'The Intelligent Universe', Michael Joseph Limited, London 1983, ISBN 0-7181-2298-4
  • Kring DA, Cohen BA (2002) Cataclysmic bombardment throughout the inner solar system 3.9-4.0 Ga. J GEOPHYS RES-PLANET 107 (E2): art. no. 5009
  • Lepland A, van Zuilen M, Arrhenius G, Whitehouse M and Fedo C, Questioning the evidence for Earth's earliest life — Akilia revisited, Geology; January 2005; v. 33; no. 1; p. 77-79; DOI: 10.1130/G20890.1
  • Louis G, Kumar AS (2006) The red rain phenomenon of Kerala and its possible extraterrestrial origin, Astrophysics and Space Science DOI: 10.1007/s10509-005-9025-4
  • NAGY B, CLAUS G, HENNESSY DJ (1962) ORGANIC PARTICLES EMBEDDED IN MINERALS IN ORGUEIL AND IVUNA CARBONACEOUS CHONDRITES. NATURE 193 (4821): 1129
  • Narlikar JV, Lloyd D, Wickramasinghe NC, et al. (2003) Balloon experiment to detect micro-organisms in the outer space. ASTROPHYS SPACE SCI 285 (2): 555-562
  • Nisbet, E. (2000) The realms of Archaean life. Nature, 405(6787):625 -- 626.
  • SCHIDLOWSKI, M. (1988) A 3,800-MILLION-YEAR ISOTOPIC RECORD OF LIFE FROM CARBON IN SEDIMENTARY-ROCKS. Nature, 333(6171):313 -- 318.
  • Wainwright, M. (2003) A microbiologist looks at panspermia. Astrophysics and Space Science, 285(2):563 -- 570

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