Extraterrestrial Civilizations - Isaac Asimov [86]
Although the blue-green algae made use of the energy of sunlight to form their tissue components, they thereafter made use of chemical changes similar to those the bacteria used. These chemical changes did not supply much in the way of energy, so that the growth and multiplication of living things—to say nothing of its evolution into various more advanced species—was extremely slow. The reason for this is that the chemical changes that yield considerable energy to living things on our Earth of today all involve the utilization of molecular oxygen, and in the early days of life on Earth there was virtually no oxygen in the atmosphere.
The blue-green algae did produce small quantities of oxygen in the course of their photosynthesis, but the sparse distribution and feeble activity of the tiny cells made those quantities very small indeed.
But even though evolution progresses very slowly, it progresses. About 1,500,000,000 years ago, when Earth had been the abode of life for over 2 billion years, the first cells with nuclei appeared. These were large cells of the type that exist today, with more efficient chemistries, that were capable of conducting photosynthesis at greater rates than before.
This meant that oxygen began entering the atmosphere in perceptible quantities and carbon dioxide began declining. By 700 million years ago, after Earth had been the abode of life for nearly 3 billion years, the atmosphere was some 5 percent oxygen.
By this time, those forms of burgeoning animal life, still made up of single cells, which like bacteria made use of chemical changes rather than sunlight as a source of energy, began to develop means of making use of the free oxygen of the atmosphere. Combining organic compounds with oxygen releases twenty times as much energy for a given mass of such compounds as does the breakdown of organic compounds without the use of oxygen.
With a flood of energy at their disposal, animal life (and plant life, too) was able to move more quickly, live more briskly and efficiently, reproduce more copiously, evolve in more different directions. It could even make use of energy in what would have been a wasteful fashion by earlier standards. It evolved into organisms in which cells clung together and specialized. Multicellular organisms were developed, and rigid tissues had to be formed to support them and serve as anchors for muscles.
Such hard tissue was easily fossilized and thus by 600 million years ago, it seems (from the fossil record) that out of nowhere multicellular life, advanced and complex, was flourishing.
It was not until Earth was 4 billion years old, with a third of its life span gone, that such complex life forms existed.
If this, by the principle of mediocrity, is characteristic of completely Earthlike planets in general, then one-third of them are too young to have anything more than one-celled life. Conversely, two-thirds of them possess complex and varied multicellular life.
That gives us our tenth figure:
10—The number of planets in our Galaxy bearing multicellular life = 433,000,000.
LAND LIFE
However complicated and specialized a life form becomes, it doesn’t interest us in connection with the subject matter of this book unless it is intelligent.
It cannot become intelligent unless it develops a large brain (or the equivalent—except that, on Earth at least, we know of no equivalent) and this, it would appear, cannot be done without the development of manipulative organs of some sort and of elaborate sense organs of considerable variety.
It is the flood of impressions entering the brain from the outside Universe, and the questing manipulative organs that respond to these impressions, which stretch the brain’s resources to its capacity and beyond, and lend survival value to any increase in the brain’s size and complexity. If a small brain is already sufficient to handle the coordinating needs of the information an organism collects, a larger