Panspermia

Panspermia is the 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 themselves. 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.

Evidence

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

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 favourable.

Related Topics:
Precambrian - Fossil - Anthropic principle

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

• 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 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. 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 ~12.7 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

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Extremophiles

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. http://news.bbc.co.uk/2/hi/science/nature/4235979.stm
• 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

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 some fraction of the speed of light, and so take such a long time that many scientists think it unlikely that 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. Some remain unexplained. While such sightings were mostly ignored by scientific community in the last half of the twentieth century, a few peer-reviewed scientfic journals have published reports assessing physical evidence associated with a few of these sightings, for example, the Journal of Scientific Exploration (http://www.jse.com/).

Related Topics:
Special theory of relativity - SETI - UFO

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Disputed

• 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.
• 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 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.
• 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.

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.
• Unpublished claims (http://arxiv.org/abs/astro-ph/0310120 & http://arxiv.org/abs/astro-ph/0312639) 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 http://web.archive.org/web/20041028040513/www.indiaexpress.com/news/regional/kerala/20030619-0.html).
• In 2002, the discovery of glycine (the simplest amino acid) in interstellar clouds was reported. http://physicsweb.org/article/news/7/8/7. Subsequent investigation has refuted these claims.

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.