Given that other stars have planetary systems, what is the likelihood that the planetary system resembles our Solar System ? If we can answer that, we may be able to decide whether the evolution of life on Earth has any relevance to life elsewhere in the Universe. The only planetary system we have any knowledge of is the Solar System, so we must discover how this system originated, and see whether any exceptional circumstances went into its formation.

According to the best available solar models, our Sun is at least 4.65 billion years old. The oldest dated rocks on Earth are between 3.3 and 3.6 billion years old, and some Moon rocks have been dated at 4.1 billion years. The first life-form that we have any evidence of was present 2.7 to 3.2 billion years ago although this does not preclude the existence of life at even earlier dates. These numbers indicate that it took the Earth 0.5 to 1 billion years to form and solidify, and that rudimentary life-forms existed within the following 500 million years. This period is one about which we have little knowledge, but it is nevertheless possible to advance a number of plausible hypotheses based on our ideas of how the Earth formed, and on our understandings of simple biological processes. It is this period that is crucial to the origin of life on Earth, and we must understand it as well as possible if we are to attempt an assessment of the chances of finding life in other terrestrial environments.

The currently held view of the origin of the Solar System goes back to the Solar Nebula theory of Laplace. According to the modern view, the Sun condensed inside an extensive dust cloud. If the interstellar clouds we see now are anything to go by the primitive solar nebula out of which the planets formed would be cold and already contain organic molecules like methyl alcohol (CH3OH), acetaldehyde (HCOCH3), methyl cyanide (CH3CN) and methylacetylene (CH3C2H) to name but a few, besides the familiar carbon monoxide (CO), water (H20), and ammonia (NH3) inorganic molecules. The cloud collapses and becomes hotter and denser in the centre, until a point is reached where nuclear reactions start up there. A star is then born. By this time the process of slowing down the rotation of the star will be well under way, and the outer regions will have settled into a spinning, flattened, disc. The energy radiated by the newly-formed star will be enough to boil away the lighter molecules in the central regions of the disc. At the same time, the refractory materials (iron and silicates) condense out nearer the star, while the less refractory materials (ices of carbon, nitrogen and oxygen) condense at greater distances. Thus the basic com- positional difference between the inner and outer planets of the Solar System can be understood at a very simple level, and this leads us to suspect that such a division may not be uncommon in other planetary systems. The planets are thought to have been built gradually through the collision and coalescence of the condensed solid particles into planetesimals, and then/into planets. The force of gravity would help this process along and at the same time sort out the orbits of the protoplanets into a fairly regular and well-spaced configuration. Although all the details of this scenario have not yet been completely worked out, there are a number of very impressive (and almost equally convincing!) theoretical discussions of various stages of the process. In this regard, the computations have shown the build-up of planets and the organization of their orbits in a way that leads one to believe that other planetary systems may bear considerable resemblance to the Solar System.

Let us now turn to consider the evolution of one of these protoplanets. As the protoplanet contracts under its own gravitation, the centre heats up and melts. A crust is formed through which the lighter gases like methane (CH4), carbon dioxide (C02), carbon monoxide (CO), water vapour (H20) and ammonia (NH8) can escape to form an atmosphere. It is generally believed that there was no free oxygen at this stage. The primitive atmosphere will certainly contain some of the simple organic molecules that existed in the pre-stellar cloud, together with some of the more complex molecules formed during the period when the solid material first condensed out of the solar nebula. The lack of oxygen is an important factor in the origin of terrestrial life for a number of reasons. There would have been no ozone (03) to protect the surface from the Sun’s powerful ultraviolet radiation. The solar ultraviolet radiation could therefore have played an important role in the synthesis of organic molecules at the earliest times Moreover, the presence of significant quantities of free oxygen would have seriously inhibited the formation of the organic molecules that are necessary to life. We have direct evidence for the absence of oxygen two billion years ago in rocks of that age which were weathered without being oxidized. Indirect evidence for there being no significant amount of oxygen around before that time comes from the observation that the Earth has had liquid water since 3.3 to 3.6 billion years ago. (The oldest rocks show evidence of having solidified under water.) The Sun was colder in the past, and the water would have been frozen if the Earth’s atmosphere had contained as much oxygen as it does now. On the other hand, a reducing atmosphere, containing a small amount of ammonia, could act as a greenhouse and keep the Earth’s surface at a higher temperature. (A similar phenomenon is observed on Venus now.) One of the uncertainties in this latter argument arises because we do not know how much heat the Earth could have produced internally.

The possible existence of complex organic compounds at times before the Earth even formed is an important point as regards the very first steps towards the building of the highly complex organic structures that are characteristic of terrestrial life. The evidence of this comes from the observation that certain kinds of meteorite, the CARBONACEOUS CHONDRITES, contain natural carbon compounds of very high molecular weight. The bulk of these compounds resemble crude oils and tarry substances found on Earth, but there is also a small component of amino acids, purines and pyrimidines present. These latter compounds contain the essential ingredients of terrestrial life: the amino acids are the building bricks of proteins, and purines and pyrimidines are the code-units of DNA, the genetic molecule. It appears that some of these meteorites may have ages in excess of four billion years. Thus we have direct evidence that extensive synthesis of extremely complex organic compounds may have taken place even before the planets formed. Just how much organic material could have been made in this way is impossible to estimate. It is possible that the continual rain of meteorites on the Earth might have been a major source of organic material. The most important point, however, is that we have direct evidence that it is easy to synthesize complex organic molecules during the early history of the Solar System.

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