With a suitable planet, the stage is set for the drama of life. However, the stage is all that is there initially -- a planet with oceans and an atmosphere which consists primarily of nitrogen, with a few per cent of water vapor and carbon dioxide, and small amounts of other gases such as methane, ammonia and sulfur-hydrogen.

The primordial soup

This was the starting point of life on Earth (and probably of other planets as well). At that time (more than 4000 million years ago), organic material was scarce. Interstellar matter contains organic molecules such as hydrogen cyanide, polyacetylene and formaldehyde; traces of these must also have been present on Earth. The bulk of the organic material from which life formed (which was, however, only a minute proportion of the organic matter that is there now!) was created on Earth by abiotic processes.

How this happened, was demonstrated by the chemists Harold Urey and Stanley Miller in the early 1950s. They irradiated a concoction of methane, ammonia and water vapour with ultraviolet radiation and had electrical discharges (i.e., lightning) set off in it. After a few days, they found a brownish ooze in the reaction vessel, the so-called tholin. This is a mixture of miscellaneous organic compounds, among them several amino acids. What had happened was this: the radiation and lightning bolts cracked up the gas molecules; the resulting fragments combined into more complex molecules. On Earth 4000 million years ago, the same thing happened in the atmosphere. Rainfalls washed the tholin into the oceans; the resulting aquous solution is popularly called primordial soup.

The first replicators

It is yet unknown how the first living organisms formed out of this `soup'. The `soup' was a gargantuan natural laboratory where the widest diversity of chemical reactions took place. At some time a molecule formed which catalyzed the production of itself -- the first replicator. Chemists actually have managed to build simple self-replicating molecules which are possibly similar to those with which the evolution of life started.

Photosynthesis

The first primitive organisms lived in a plentiful environment. The Urey-Miller processes produced lots of organic molecules which the primordial organisms could thrive on. But this didn't last forever. On one hand, the organisms became more and more numerous. On the other hand, ammonia amd other water-soluble gases were washed out of the atmosphere by rainfalls, and the remaining methane used up by the Urey-Miller processes.

This meant that life had to find other sources of nutrition to survive. Doubtlessly, this was the time the first predators hit the scene, organisms which lived off other living organisms, and the first scavengers, which fed of those which failed, died for some reason.

However, this was little help against the ongoing crisis. If nothing had happened, the young life would have used up all the nutrients and died out. This might have happened on serveral planets. But on Earth, some organisms began producing their nutrients on their own from simple, inorganic molecules. This was possible through photosynthesis. These organisms gathered sunlight to get hold of energy they needed to build carbohydrates from carbon dioxide and hydrogen; however, as there was no molecular hydrogen available, this in turn had to be gained from some kind of inorganic hydrogen compounds (which again involved sunlight).

The first photosynthetic organisms probably used comparatively easy-to-break-up molecules like ammonia and sulfur-hydrogen (some bacteria use the latter until today), but then another line arrived which was capable of using water for this purpose. Though water needs much more energy to break it up, these organisms had a huge advantage because water was, of course, grossly abundant.

The role of oxygen

This version of photosynthesis, however, put out a waste product: oxygen. This was a problem. Molecular oxygen is an aggressive gas which does not mix well with organics. It cleared the atmosphere off what was left over from the reducing gases with which the evolution of life started, and was deadly to most of the organisms.

So life faced itself with a crisis again. There might be planets where the evolution of life came to a grinding halt at that point. Earth was, of course, none of them. Some organisms developed a protection mechanism against the oxygen. The principle was to set aside a part of the carbonhydrate supply to react with the oxygen. But this turned out to be more than just a protection measure against a dangerous poison! This process provided the organisms with much more energy than they could ever pull out of their food through `conventional' means, and these aerobic organisms quickly took over.

Multicellar life

All these organisms were single cells of a primitive, bacteria-like design, so called procaryonts. At some point, perhaps 1500 million years ago, a new, much more complex type of cell, the eucaryont, evolved. This cell type, equipped with a nucleus, mitochondria and other functional structures, became the building block of the multicellar organisms.

The first multicellar life forms were little more than clusters of identical cells, but step by step the functions of these cells differenciated. This way, more and more complex (and larger) organisms evolved. The evolutionary lines of plants -- mostly non-mobile, photosynthetic organisms -- and animals -- mostly mobile organisms feeding off plants or other animals -- separated from each other.

There is speculation if this is a necessary development happening on all Earth-like planets with multicellar life, or if there could be mobile (or even intelligent) photosynthetic organisms. The answer is probably that, if there are self-moving organisms at all, they must be heterotrophic, i.e., live off other organisms. This is because mobile organisms of course need more energy than non-moving ones, and macroscopic animals just don't have enough body surface to gather the energy they need from sunlight. (It is, however, conceivable -- though probably not very likely -- that a planet is only inhabited by plants, not by animals.)

The conquest of the dry land

Until perhaps 500 million years ago, the continents of our planet were merely bare rock, while advanced multicellar life forms were already thriving in the oceans. Then, the first land plants appeared, to be followed by the first land animals.

The conquest of dry land was a major step in evolution which demanded special adaptations. Water-living organisms need not care about loss of liquid. They did not need a waterproof membrane, for example. Land-living organisms, however, need such a membrane, and they need to compensate for water losses. This is a difficult thing, and there might be planets where life, though as old as Earth or even older, never conquered the continents.

Intelligent life and technology

A sessile plant does not need intelligence. To a self-moving animal, however, a complex brain is very useful. An intelligent predator might fell prey that is physically superior by playing tricks on it. On the other hand, an intelligent animal can evade a physically superior, but dumber, predator.

Intelligence also allows for the usage of tools, which is the very first step to technology. Real technology, however, probably prerequisites the domestification of fire. This of course means that only land-living beings can develop technology. There is no reason to assume that marine animals cannot develop intelligence that matches, or even exceeds, human intelligence. Actually, dolphins are at least as intelligent as apes; some scientists assume that they are as intelligent as us humans. However, such marine intelligent life forms have no access to technology.

The question how many planets actually spawn a technological civilization is unresolved. From the point when the first land-living animals appeared, it lasted more than 400 million years until the first autochthonic technical civilization -- I am of course talking about us here -- appeared on Earth. On some planets, this might never take place. And it is the question how long such a civilization survives. We have the power to blow ourself out of existance, or wreak so much havoc upon the planet that we (and most other species) cannot survive any longer. It is possible that all civilizations annihilate themselves this way after a few thousand years, but it might also be possible that a civilization overcomes such problems and stays for millions of years.

What will the future bring?

There is no reason to assume that evolution on our planet has come to a halt. Unless we wipe out ourselves, we will probably continue our development. No-one can tell us whither the journey goes. We might evolve into a super-intelligent species, or we might degenerate gradually. We might even evolve into something we as for now can't imagine at all.

It might also be possible that this post-human evolution will put us on a path which leads us away from planetary life. It need not even be biological evolution. The path might lead from humans through cyborgs to intelligent, self-replicating machinery living in outer space. It is therefore possible that when searching for intelligent life in the vicinity of sun-like stars, we are searching in the wrong place: they once lived on a planet of such a star, but now don't need Earth-like planets any more and are now to find in the vicinities of other kinds of objects which are more useful to them -- and where we would never expect life, such as blue supergiants or pulsars.

A civilization that destroys itself might foul up its planet to such a degree that no life survives. This is especially likely if it comes to a nuclear war. Other extinct civilizations may leave behind a somehat changed and disturbed, but still habitable planet (or many such planets, if they are an interstellar civilization) on which an other sentient species might evolve later. Through the eons, a planet might therefore spawn several different civilizations subsequently.