Perhaps the biggest question in science is how life originated on Earth. The conventional explanation is that life spontaneously arose on the planet in a chemical soup about four billion years ago.
A radical alternative proposal, known as panspermia, holds that life was seeded on Earth from elsewhere in the cosmos. Panspermia is derived from Greek roots: pans meaning all, and spermia, meaning seed, or life everywhere.
Panspermia received a new lease of life in recent decades from the joint work of two astronomers, Sir Fred Hoyle and Professor Chandra Wickramasinghe. Sadly, Hoyle died last August, just before his colleague announced new evidence that life exists beyond Earth.
Our planet was formed almost five billion years ago, and there is evidence that life existed on Earth four billion years ago. The main-line scientific explanation is that conditions on the early Earth favoured the formation of basic building-block molecules of life that dissolved in the oceans to form a "primordial soup". The chemicals in the soup underwent random combinations and recombinations, and this chemical evolution culminated within a billion years in the spontaneous origin of the first living cells.
This explanation is confronted by serious probability problems. The simplest living cell is still quite complex. The conventional hypothesis proposes that this complexity arose by random shuffling of chemicals over a short period on the geological timescale. This implies a strong innate tendency in the laws of chemistry for the self-assembly of living structures, but I am unaware that such a tendency exists. Hoyle suggested the probability that the conventional proposal is correct is similar to that of a tornado roaring through a junkyard and spontaneously assembling a jumbo jet.
The belief that life can spontaneously arise from non-living material was widely believed until Louis Pasteur proved, in 1857, that life can arise only from other living things. Prompted by this principle, the German biologist, Herman von Helmholtz, proposed in 1874 that life began on Earth when it was seeded here from elsewhere in the cosmos - panspermia. The idea was elaborated in 1908 by the Swedish chemist, Svante Arhennius, who proposed that life in the form of bacterial spores could travel from planet to planet, propelled by the force of solar radiation.
Panspermia was dealt a temporary paralysing blow in 1924, when P Becqeurel argued that bacteria would be destroyed by ultraviolet radiation during long-distance space travel. We have since discovered that bacteria are much tougher than Becquerel thought, and NASA experiments have shown that bacteria could survive long-term travel through outer space.
In the 1950s, the spontaneous-generation-of-life concept was resurrected to explain the one-off origin of life on Earth. Harold Urey and Stanley Miller, at the University of Chicago, simulated what they considered to have been the early Earth's atmosphere and demonstrated the spontaneous formation of building-block biomolecules. It was expected that the elucidation of the mechanism of the spontaneous origin of life would quickly follow, but little progress has been made.
Panspermia was dismissed by mainstream science for most of the 20th century, but in the 1970s Wickramasinghe and Hoyle vigorously revived the idea. They did not dispute the notion of a spontaneous origin for life, but they proposed that such complexity could arise only in an incubator the size of the cosmos and over a time span encompassing the age of the cosmos.
Wickramasinghe and Hoyle proposed, on the basis of spectroscopic observations, that much of the material in the vast interstellar gas and dust clouds that roam the cosmos is freeze-dried bacteria - the cosmos is teeming with life.
Stars and planets are formed when interstellar clouds condense under gravity. Great heat is generated when the cloud coalesces into stars and planets, and bacterial life would not survive. But at the outermost regions of solar systems, the dust is also condensed into frozen comets. In the interiors of these comets, heat from the radioactive decay of certain elements melts the ice to form a lukewarm liquid interior in which bacteria can grow and divide. Even if only the tiniest fraction of bacteria survived the comet condensation phase, it would quickly reproduce to fill the comet's interior.
In the early life of a solar system, comets are regularly jarred out of their orbits and into the heart of the system, colliding with the planets and thereby seeding them with bacterial life. This is how Hoyle and Wickramasinghe explain the arrival of life on Earth. If space is teeming with bacteria, it should be easy to test this hypothesis. Hoyle and Wickramasinghe recently collaborated with the Indian Space Research Organisation to search for them. The organisation launched a balloon to an altitude of 41km, a height that wind-borne bacteria native to Earth normally do not reach, and took samples under sterile conditions. The samples were found to contain clumps of bacteria, different to the strains we are familiar with. Wickramasinghe announced the amazing results in August.
If the claim that bacteria are constantly falling to Earth from space holds up, it will be an enormous scientific advance that will revolutionise our view of life and the universe. If Wickramasinghe is correct, he can expect to hear from the Nobel Committee. On the other hand, Wickramasinghe may have shown that conventional earthly life extends further up in the atmosphere than anyone previously thought. Time will tell.
William Reville is a senior lecturer in biochemistry and director of microscopy at University College, Cork