How does the cosmozoan theory seek to explain life on Earth?
According to this theory, life has reached this planet Earth from other cosmological structures, such as meteorites, in the form of highly resistant spores. This idea was proposed by Richter in 1865 and supported by Arrhenius in 1908 and by other contemporary scientists. The theory did not gain any significant support as it lacks evidence. It is strongly linked to the ‘eternity of life’ theory of the origin of life on Earth.
In the nineteenth century, Hermann Richter put forward the idea that life has always existed in the Universe, propagating itself from one place to another by means of ‘cozmozoa’ (germs of the cosmos). In this theory, life has existed and will exist for all eternity across the Universe, and so there is no need for an explanation of its origin. Two other eminent scientists of the time – Lord Kelvin and Herman von Helmholtz – also took the same view.
In 1908, the Swedish physical chemist Svante Arrhenius put forward a new version of the cosmozoan theory, and gave it the name panspermia. Arrhenius’ contribution was a new theory of the mechanism by which life could be transported between planets; he proposed that bacterial spores were propelled through inter-planetary space by radiation pressure. Previous versions of the theory had assumed transport was by means of meteorites or by comets. However, the very high temperatures that meteorites create on entering the Earth’s atmosphere seemed to rule this out. In Arrhenius’ version of the theory, spores arriving at the Earth (possibly attached to grains of interstellar dust) could fall slowly to the ground without being subjected to high temperatures due to air friction.
One of the motivations for Arrhenius’ panspermia theory was that it also seemed to provide a solution to the disproof by Louis Pasteur’s experiments of spontaneous generation in bacteria. If there was no way in which the origin of life could be explained, it was reasonable to suppose that life was an inherent property of the Universe and had always existed. Arrhenius’ theory was dropped by most scientists when it became apparent that the bacterial spores would be subject to UV radiation and X-radiation, zones of charged particles, which would inevitably destroy them.
However, another version of the cosmozoan theory or panspermia does have some evidence to back it up. This version – called weak panspermia or pseudo-panspermia – is the theory that organic compounds arrived from outer space and added to the chemicals on Earth that gave rise to the fist life. In 1969 a meteorite landed in Australia that was 12% water and contained traces of 18 amino acids. This evidence points not only to the presence of organic compounds in outer space, but also to the capacity of such compounds to reach Earth. Also, complex organic molecules have been detected in star-forming clouds, further adding to the evidence for organic molecules in space.
How does the biochemical theory seek to explain life on Earth?
Th current ideas we have about how life may have evolved on Earth as a result of biochemical reactions (sometimes called abiogenesis) owe much to two biologists working early in the twentieth century:
• Aleksandr Oparin, a Russian biologist who fist put forward his ideas in 1924, and
• John Haldane, an English biologist independently put forward almost identical ideas in 1929
Thy both suggested that:
• the primitive atmosphere of the Earth was a reducing atmosphere with no free oxygen – as opposed to the oxygen-rich atmosphere of today
• there was an appropriate supply of energy, such as lightning or ultraviolet light, and
• this would provide the energy for reactions that would synthesise a wide range of organic compounds, such as amino acids, sugars and fatty acids.
Oparin suggested that the simple organic compounds could have undergone a series of reactions leading to more and more complex molecules. He proposed that the molecules might have formed colloidal aggregates, or ‘coacervates’, in an aqueous environment. Th coacervates were able to absorb and assimilate organic compounds from the environment in a way similar to the metabolism of cells. Thse coacervates were the precursors of cells and would be subject to natural selection, eventually leading to the fist true cells.
Haldane’s ideas about the origin of life were very similar. He proposed that the primitive sea served as a vast chemical laboratory powered by solar energy. As a result of all the reactions powered by solar energy, the sea became a ‘hot dilute soup’ of organic monomers and small polymers. Haldane called this the ‘prebiotic soup’, and this term came to symbolise the Oparin–Haldane view of the origin of life. But is there any evidence for the theory? In 1953, Stanley Miller conducted his now-famous spark-discharge experiment. In this investigation, he passed electric sparks repeatedly through a mixture of gases that were thought to represent the primitive atmosphere of the Earth. These gases were methane (CH4), ammonia (NH3), water (H2O) and hydrogen (H2).