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.

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 (10⁹–10¹² 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 spread in 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 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. http://news.bbc.co.uk/1/hi/sci/tech/2992123.stm 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, which is the "simplest" explanation is not always necessarily clear: geogenesis eliminates the step of transferring life across space, but requires a lot to happen in a relatively short time frame.