The authors say that we can expect life to occur throughout the universe, because the basic fabric of life is the same.
The authors say that the biochemistry of life is far too complex a phenomenon to have occurred on the primitive earth, and that the habitat of the primordial earth was far too inhospitable for spontaneous generation to have taken place.
Zero humidity, absence of oxygen and low temperatures are the conditions of space. These conditions are highly favourable to the survival of
bacteria and seeds retaining their germinating power. Thus, bacteria and viruses are well equipped to make long journeys in space.
There are a large number of comets in the solar system. And they have a relative abundance of elements such as hydrogen, oxygen, carbon and nitrogen - the very constituents of life.
Planets could have received the seeds of life already present in these comets. In order for bacteria and viruses to survive solar radiation, they need a protective covering. Given the right temperature and the presence of the seeds of life, the number of living cells could have grown explosively. The multiplying capacity of bacteria is enormous; a bacterial population can double in a few hours. Once life begins, it could have spread very quickly into space and beyond.
It may be assumed that bacteria are an important constituent of interstellar dust. It is estimated that over a thousand tons of space debris enters the earth’s atmosphere every year; even if a fraction of this material consists of bacteria, we still get a large amount! The bacteria that find a suitable niche in the terrestrial habitat would be the ones that succeed. Just because the other planets are different from the earth does not mean that life is unique to the earth. Given that bacteria are widespread throughout space, it would be astonishing that they evolved only upon the earth.