EXOGENESIS

An alternative to Earthly abiogenesis is “exogenesis”, the hypothesis that primitive life may have originally formed extraterrestrially, either in space or on a nearby planet such as Mars. Such ideas have had many eminent supporters over the years, including Francis Crick, the co-discoverer of the structure of the DNA molecule, and the astrophysicist Sir Fred Hoyle among others. These theories may go some way to explaining the presence of life on Earth so soon after the planet had cooled down, with apparently very little time for prebiotic evolution.

In the 1980s, Hoyle developed and promoted, along with fellow astronomer Chandra Wickramasinghe, the theory of “panspermia”. This is the idea that the origin of life on Earth must have involved cells which arrived from space, and that evolution on earth is driven by a steady influx of viruses arriving from space via comets. Hoyle calculated the chances of the simplest living cell forming out of some primordial soup as infinitesimally small, and described that theory as “evidently nonsense of a high order” (although others have argued that Hoyle's own line of reasoning incorporates a number of logical mistakes and omissions).

In the aftermath of supernova explosions and the process of star formation, many hydrocarbon molecules (those made entirely or mostly out of hydrogen and carbon), including formaldehyde, hydrocyanic acid and other so-called pre-biotic molecules, are spontaneously formed in nebulae. Such pre-biotic molecules appear to exist across the universe, and existed before the creation of the Solar System, although it is thought that about 9 billion years or so of star-making were required to produce the right conditions.

(Click for a larger version) Fossilized bacteria in the Murchison meteorite (Source: Cosmic Ancestry: http://www.panspermia.com/zhmur1.htm)

Some complex compounds found in outer space, such as glycoaldehyde for example, have been made to react in laboratories to make a sugar called ribose, a key ingredient of ribonucleic acid (RNA) which, with the removal of an oxygen atom, becomes DNA (deoxyribonucleic acid). It is generally agreed that these compounds are not themselves products of life, but form spontaneously by banal chemical reactions. They are, however, able to interact to form more stable, typical organic compounds, many of them similar to substances found in living organisms.

Spectroscopic analysis of radiation, meteorites and comets has revealed the presence of amino acids and other biologically significant compounds on celestial bodies, such as the famous Murchison meteorite (which fell on Australia in 1969), Halley’s Comet (a recurring visitor to the neighbourhood of Earth) and Saturn's moon Titan (the seas of which are believed to be made of hydrocarbons). The Murchison meteorite has been found to contain 411 different organic compounds, including 74 amino acids, 8 of which are found in the proteins of living organisms. In particular, recent analysis has shown that it contains some quite complex organic chemicals, called uracil and xanthine, which can form the self-replicating molecules that are the essential genetic ingredient of all known lifeforms, RNA and DNA.

Many comets are encrusted by outer layers of dark material, thought to be a tar-like substance composed of complex organic material and formed from simple carbon compounds after reactions initiated by ultraviolet light irradiation. Studies have shown a strong correlation between the abundances of hydrogen, carbon, oxygen, nitrogen and sulphur in living organisms and in material found in comets, and water (a prerequisite for life as we know it) has been identified as the most common triatomic molecule in the universe. Indeed, there seems to be ample evidence that organic matter is spread throughout the galaxy and the universe, in asteroids, comets, meteorites and even in the gas and dust in interstellar space. A rain of material from comets and meteorites could therefore have brought significant quantities of complex organic molecules to the primordial Earth.

(Click for a larger version) Artist's depiction of life falling to Earth on a meteorite, according to the panspermia hypothesis (Source: Internet Encyclopedia of Science: http://www.daviddarling.info/ encyclopedia/B/ballpans.html)

An alternative source of first life which has been suggested is our neighbouring planet, Mars. Being smaller in size, Mars cooled before the Earth (by several hundreds of millions of years), allowing prebiotic processes to develop there while the Earth was still too hot. Life, it is argued, could later have been transported to the cooled Earth when material was blasted off the crust of Mars by asteroid and comet impacts. Because Mars continued to cool faster (and eventually all but lost its atmosphere), the hypothesis continues, it eventually became hostile to the continued evolution, or even the existence, of life. Although the Earth may well be following the same fate as Mars, it is doing so at a much slower rate, which has allowed the establishment of much more complex lifeforms.

A meteorite found in Antarctica in 1984 (dubbed ALH84001) has been shown to have a chemical composition very similar to that on the surface of Mars, including bubbles of trapped gas consistent with the Martian atmsophere, and seems to have been blasted into space (and thence to Earth) by an asteroid or comet collision with Mars. Interestingly, the rock also shows evidence of microscopic wormlike formations, suggesting fossilized Martian bacteria, probably (athough not definitely) the first evidence of life from outside the Earth.

A recent experiment, led by Jason Dworkin, subjected a frozen mixture of water, methanol, ammonia and carbon monoxide to ultraviolet radiation, attempting to mimic conditions found in an extraterrestrial environment. This combination yielded large amounts of organic material that appeared to self-organize, and to form cell membrane-like bubbles when immersed in water. The bubbles were also found to glow or fluoresce when exposed to ultraviolet light (perhaps a precursor to primitive photosynthesis?), as well as providing a protective layer to diffuse any damage that might otherwise be inflicted by the ultraviolet radiation, which would have been vital in an early world without an ozone layer.

It has even been postulated that the organic molecules which made up early life on Earth originated in other star systems. In 2004, a research team detected traces of polycyclic aromatic hydrocarbons (or PAHs) in a distant nebula, the most complex molecules so far found in space, and a very large range of molecules, including cyanide compounds, hydrocarbons and carbon monoxide, have been detected in the formation of a Sun-like star. Such research is still at a very early stage, though.