Panspermia is a hypothesis that the seeds of life are prevalent throughout the Universe, and furthermore that life on Earth began by such seeds landing on Earth and propagating. The idea has its origins in the writings of Anaxagoras, but was first proposed in its modern form by Hermann von Helmholtz in 1879. 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 consensus view holds that panspermia - especially in its interstellar form - is unlikely given the challenges of survival and transport in space.

Sir Fred Hoyle (19152001) and Chandra Wickramasinghe were important proponents of the hypothesis who further contended that lifeforms continue to enter the Earth's atmosphere, and may be responsible for outbreaks of disease (pathospermia). Hoyle's advocacy is both a blessing and a curse: although he was a highly original thinker and won top scientific accolades, some of his principal ideas (such as steady state theory) have been largely shown to be false.

Panspermia 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. The mechanisms proposed for interstellar panspermia are theoretical 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 as terrestrial bacteria were shown to have survived in a dormant state on the Moon. Since then, however, NASA has implemented strict abiotic procedures to avoid planetary contamination.

Exogenesis is a related, but less radical, hypothesis that simply proposes life originated elsewhere in the universe and was transferred to Earth, with no prediction about how widespread life is. The term "panspermia" is more well-known, however, and tends to be used in reference to what would properly be called exogenesis, too.



Until a large portion of the galaxy is surveyed for signs of life or contact is made with other civilizations, the panspermia hypothesis in its fullest meaning will remain difficult to test. There is, however, circumstantial evidence for exogenesis:

Narrow time window for geogenesis

Missing image
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. Unless the Earth just happened to be the site of a large number of fortuitous coincidences (not so unlikely under the anthropic principle), this would imply that life appears in several hundred million years when conditions are favorable.

  • 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 (they are likely older). 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 sulfur 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. 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 produce sufficient quantities (Gilmour et al., 1997). This puts the earliest possible emergence of life in the Universe at ~12.7Ga, 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 - 3.5 Ga), most plausibly 400 Ma (3.9Ga - 3.5Ga), and possibly <100Ma (3.9Ga - 3.85Ga) if the Greenland (3.85Ga) isotope signal is correct. If life originated elsewhere, the window expands to ~9Ga. 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 3.5Ga.


Evidence has accumulated that some bacteria are more resistant to extreme conditions than previously recognised, 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.

  • Streptococcus mitis bacteria that had accidentally been taken to the moon on the Surveyor 3 spacecraft in 1967, could easily be revived after being taken back to Earth by the Apollo 12 astronauts 31 months later.
  • 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[1] (
  • Semi-dormant bacteria found in ice cores over a mile beneath the Antarctic - this lends credibility to the concept of sustaining the components of life on the surface of icy comets.
  • Bacteria which don't rely on photosynthesis for energy. In particular, endolithic bacteria using chemosynthesis found inside rocks and in subterranean lakes.
  • Deinococcus radiodurans is a radioresistant bacterium that can survive high radiation levels.
  • Dormant bacteria isolated from insects in amber 10s Ma old (Gilmour et al., 1997)

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.
  • Possible water oceans on Europa and perhaps other moons in the Solar system. Even moons that are now frozen ice balls might earlier have been heated internally by radioactive rocky cores. Bodies like this may be extremely common throughout the universe.

Evidence of extraterrestrial life

There is no definitive evidence of extraterrestrial life at the present time. However, a number of experiments have produced results which are ambiguous and are disputed by some parts of the scientific community. Several claims have been soundly shown to be unjustified. Finally, there has been at least one attempt to hoax the existence of extraterrestrial life.


Missing image
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. 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.
  • 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 (Wainright, 2003). 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 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.
  • 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 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.


  • 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.
  • Unpublished claims ([2] ( & [3] ( that spores detected in the red rain that fell near Kerala, India in 2001 were of cometary extraterrestrial origin, were subsequently debunked. As of 2003, it is now generally accepted that the rain was coloured red by a dust cloud from the Gulf region that contained terrestrial fungal spores (see [4] (
  • In 2002, the discovery of glycine (the simplest amino acid) in interstellar clouds was reported. [5] ( Subsequent investigation has refuted these claims.


  • 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 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 (109-1012particles/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 spread in many different environments throughout the Universe.
  • 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 Columbia space shuttle survived its crash landing from 63 km inside of 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. [6] ( The existence of Martian meteorites and Lunar meteorites in captivity suggests that transfer of material from other planets to Earth happens regularly.
  • Occam's Razor states that when multiple explanations are available for a phenomenon, the simplest version is preferred. From this perspective, geogenesis appears to be the default assumption when compared with panspermia or exogenesis: the former assumes a single a step - that life originated on Earth - ahead of the more elaborate idea that life formed elsewhere and was subsequently transplanted to the Earth biosphere. Given that an understanding of life's emergence remains partly speculative, however, the "simplest" explanation is not necessarily clear: geogenisis eliminates the step of transferring life across space, but requires a lot to happen in a relatively short time frame.

Directed Panspermia

A second prominent proponent of panspermia is Nobel prize winner Francis Crick, 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.

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.

Science Fiction

The theory of panspermia has been explored in a number of works of science fiction, notably Jack Finney's Invasion of the Body Snatchers (twice 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 favor 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.

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).

Fiction writer Dan Brown also includes panspermia in the novel Deception Point.

See also


  • Rhawn Joseph, "Astrobiology, the Origin of Life and the Death of Darwinism", University Press, 2001, ISBN 0970073380
  • 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 0708822355
  • Gilmour I, Wright I, Wright J 'Origins of Earth and Life', The Open University, 1997, ISBN 0749281820
  • Hoyle F, 'The Intelligent Universe', Michael Joseph Limited, London 1983, ISBN 0718122984
  • 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
  • Narlikar JV, Lloyd D, Wickramasinghe NC, et al. (2003) Balloon experiment to detect microorganisms in the outer space. ASTROPHYS SPACE SCI 285 (2): 555-562
  • Nisbet, E. (2000) The realms of Archaean life. Nature, 405(6787):625 -- 626.
  • Wainwright, M. (2003) A microbiologist looks at panspermia. Astrophysics and Space Science, 285(2):563 -- 570

External links

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