came into existence. This equation was later fine-tuned by Drake and Carl Sagan. They estimated that there could be up to a million intelligent civilizations in our own Galaxy alone. These estimates were based on such things as the number of stars with planetary systems, the number of planets per star that might develop life, the fraction of these planets where intelligent life evolves. But are these estimates in any way accurate?
If the authors of "Rare Earth: Why Complex Life is Uncommon in the Universe" are correct, then intelligent life is unique to earth, a sort of cosmic fluke. Paleontologist Peter Ward and Astronomer Donald Brownlee suggest we might well be alone in this universe. While they suggest that single-celled microbial life such as bacteria might be prevalent throughout the universe, the conditions necessary for the evolution of higher life forms (like ourselves) are so complex and rely on such a long chain of events many of which happened by chance, that they are unlikely to arise elsewhere.
But not all Scientists agree. Evolving from microbes to intelligent life need not be such a long drawn out process everywhere, as it was on earth. While the tendency of evolution is to push organisms towards greater complexity, Earth's history shows periods of rapid evolutionary changes followed by periods where very little changed. But by removing these long periods of inactivity the process of evolution to intelligent life could happen more rapidly and there is no evidence to suggest that these periods of inactivity are necessary for the process - instead, they just happened by chance.
Factors limiting evolution on earth were;
- cataclysmic events - such as asteroid or cometary impacts
- massive volcanic activity
- lack of oxygen - as we know very little free oxygen was available until about 2 billion years ago when photosynthetic life forms began producing it. Oxygen also was vital for the development of the ozone layer which protects life on the surface from the sun's harmful UV radiation.
But other planets may have had more free oxygen in their early life and have been more fortunate or equally as fortunate with cataclysmic events.
We must also note that the sun is only 5 billion years old whereas our galaxy is about 13 billion years old. So earlier star generations with their planetary systems and eventually intelligent life must surely have evolved billion of years ago within our own galaxy. With a few billion years head start on us surely their technology would be advanced enough to have explored and colonised the galaxy and be aware of our presence. Maybe they are aware! Maybe they are amongst us! Maybe they did visit us in the past (remember Von Daniken)! Or maybe life is after all unique to earth!
It's probably reasonable to assume that microbial life is common since the 'ingredients' for life familiar to us, namely, organic material, water, energy, rocky planetary bases, lots of time, are widespread. A 'channel' or 'method' does exist to go from microbial life to intelligent life as humans have proved, so surely this can happen elsewhere! Then again why were the dinosaurs not the first 'radio-telescope builders' if all the necessary ingredients were available to them along with 150 million years as the 'numero uno' animal!
We also have to ask could something we would recognise or define as intelligent life develop using 'ingredients' different to what created us, and in environments we would consider hostile. We do not fully understand the process which kick-started life on earth so we cannot rule out other types of processes elsewhere! Let's not miss opportunities by focusing on 'life as we know it'!
