FOR several centuries past it was widely believed that life was liberally sprinkled throughout the universe.
Widespread scientific probing since the 1960s has established, almost for certain, that no life exists now on any other planet in our solar system, although there remains a slim possibility that a primitive life form has arisen on one of the moons in the solar system. Our understanding of the nature of life on Earth strongly indicates that life has also arisen elsewhere.
In order to think intelligently about the possibility of life elsewhere, it is essential to understand the nature of the only form of life we know of life on Earth. Life on Earth is based on the element carbon, i.e. the backbones of all the important biological molecules are made from strings of carbon atoms.
These carbon back bones combine with a few other elements, principally hydrogen, oxygen and nitrogen, to form the spectrum of important biological (organic) molecules.
Of the 92 natural elements, carbon is uniquely suited to act as the basis for life. It is very versatile and forms a greater variety of compounds than all of the other elements combined.
Each carbon atom can form four stable bonds with four other atoms, including carbon atoms. This allows the formation of large, stable molecules with complex three dimensional shapes. Carbon is also one of the most abundant elements in the cosmos. It is likely therefore that, if life has arisen elsewhere in the universe, it is also based on carbon.
The cell is the fundamental unit of life. Each cell contains genetic material bearing the information that determines its day to day activities. This information is copied and duplicates are passed on to both daughter cells when the parent cell divides. The carbon based molecule in which the information is encoded is DNA.
The information is coded in the form of linear sequences of units called nucleotides. The information in the DNA effects its influence in the cell by directing the formation of agents called enzymes.
Enzymes are a special class of protein molecules. Each enzyme is a catalyst, that is, it greatly speeds up a chemical reaction that, in the absence of a catalyst, would proceed extremely slowly. Each of the myriad chemical reactions in the cell is catalysed by an enzyme.
Enzymes are composed of units called amino acids joined together in long strings. There are 20 different amino acids in proteins, and their precise sequence in a particular molecule determines the activity of that enzyme. The information encoded in DNA is translated into the various precise sequences of amino acids that uniquely characterise the thousands of enzymes in each cell.
Another characteristic of life is the ability to adapt to changes in the environment, i.e. evolution. Any living system must be capable of evolution, or sooner or later it will become overwhelmed by a hostile environment.
The basis for evolution in biological organisms is that genetic variations arise naturally, some of which confer a selective advantage on the bearers who are therefore chosen by natural selection to multiply to a greater extent than their fellows. In this way new favourable characteristics spread through the population.
Life is therefore a genetic system capable of evolving and adapting to its environment. Given time, such systems grow in complexity and develop the appearance of "purposefulness" that we recognise as characteristic of life.
There is a scientific consensus that life spontaneously arose on Earth about 3.5 billion years ago. At that time, the atmosphere contained no oxygen and was composed of a mixture of gases. No ozone layer enveloped the Earth, which was bathed in strong ultraviolet light from the sun. Electrical storms raged in the atmosphere.
Such conditions favour the formation of a wide variety of organic molecules. These dissolved in the oceans, which became rich chemical soups. Over millions of years, chemical evolution took place in the oceans, the basic building blocks of life were formed, and, eventually, the first living cell arose.
At first glance this scenario sounds improbable and fanciful. However, it can be demonstrated in the laboratory that at least the first steps in this process occur spontaneously.
An experiment was carried out by Harold Urey and Stanley Miller in the 1950s. They filled a glass tube with a mixture of the gases present in the early atmosphere. Some water was added, the tube was sealed and electrical discharges were passed through it for a week. A condensate of material formed and the chemical composition of this substance was analysed.
Such experiments have shown that at least 12 of the amino acids that occur naturally in proteins are formed, as also are the nucleotides found in DNA, i.e. the basic building block molecules on which life is based.
Furthermore, it is now known that these building block organic molecules are widely and spontaneously formed throughout the universe. For example, the chemical composition of meteorites has been examined. Some, the carbonaceous chondrocites, contain up to 20 percent water and 10 per cent organic matter. Ten of the 12 amino acids that were formed in the Urey Miller experiment have been detected in such meteorites.
Water is another basic requirement for life. It forms the background environment of the biological cell.
THE structures in the cell stand out as islands in this background watery ocean through which vital chemical communications swiftly pass.
The water molecule is ideally suited to supporting a carbon based life. Some of its desirable properties include the following: water is a liquid over a wide temperature range; it can dissolve a wide variety of substances; water expands when it freezes, which means that bodies of water usually do not freeze entirely solid. Only the surface layer freezes, allowing life to continue in the liquid phase beneath.
The elements of water, hydrogen and oxygen, are very abundant in the universe and it is likely that water is commonly found throughout the universe. Therefore, it is likely that life elsewhere in the universe will also use water as a solvent.
A world that supports life must also have an atmosphere. Life is a dynamic process that recycles matter via the atmosphere. Thus, green plant life removes carbon dioxide and water from the atmosphere, synthesises carbohydrate and releases oxygen into the atmosphere.
Animals take oxygen from the atmosphere, use it to burn carbohydrate, and release carbon dioxide to the atmosphere. Life elsewhere would similarly depend on some analogous recycling of matter via an atmosphere.
So, based on what we know about life on Earth, and the composition and activity in the larger universe, it is clear that life could arise and evolve in many locations throughout the universe. In part two of this article next week, I will examine where and how often this might have happened.