Nucleic Acids, Cells And Intelligent Life ( Life In The Universe)

We have a basis for understanding the synthesis of elementary proteins. This is but the first rung on the ladder towards the evolution of life as we know it. As we climb this ladder, the system adapts itself more strongly to the terrestrial environment and so it is from here on that the divergences between life-forms on various planets will increase. There is a long way to go from the simple protein to homo sapiens, but if we are to get some feeling for the likelihood of there being an intelligent life-form elsewhere in the Universe, it is of interest to follow through the chain of events as we believe they happened on Earth.

Here then is the crux of the story of the emergence of intelligent life. The first stages of the argument are the most speculative, but as we move closer to the present the picture is clarified by the observations of fossils and other biological evidence on the evolution of life. The piecing together of diverse facts that go into the picture is one of the most fascinating examples of detective work in modern science. For example, there is good reason to suppose that all life on Earth is descended from a common ancestor. Further¬more, Man has spent such a short time on this planet that any intelligent life on other planets is likely to be so far out of step with us on the evolutionary track that we could never communicate with it.

The most important thing to begin with is an appreciation of what features are essential to an entity’s being classed as alive. We have already discussed this point at the beginning of this chapter, where we saw that the two essential features are the ability to metabolize and the ability to reproduce. Both features imply a complexity of organization that must derive from a sufficiently complex organic basis. On Earth, this basis is provided by proteins and nucleic acids. Nevertheless, the mere presence of proteins and nucleic acids is not in itself sufficient to ensure metabolism and reproduction. They must be organized into higher structures, and act in a coherent way.

Proteins are the basic constituents of the cell; they perform the numerous tasks by virtue of which we say the cells are alive. They are responsible for the structure and activity of the cell. The function of the nucleic acids (like DNA and RNA) is to manufacture proteins according to a specified pattern, and a typical cell may turn out thousands of protein molecules every minute. The nucleic acids are enormous structures resembling twisted ladders whose rungs are pairs of substances called NUCLEOTIDES. The twisted sides of the ladder make the famous DOUBLE HELIX. Only four nucleotides are used in the construction of a DNA MOLECULE, and the sequence of nucleotide pairs that form the ladder-rungs constitutes the GENETIC CODE. The genetic code is indeed a code in the usual sense; the plans for a protein molecule are expressed as linear sequences of organic molecules. Since proteins occurring in nature are made from only 20 amino acids, four nucleotides grouped in pairs are more than enough of an alphabet to describe the con¬struction of a given protein molecule. Interestingly, the first definite coding scheme was put forward by George Gamow in 1954. Although the details of Gamow’s theory were subsequently proved wrong, the importance of his contribution is generally acknow¬ledged by molecular biologists today.

The way in which protein molecules are made raises a fundamental question: which came first, the protein or the nucleic acid that contains the protein plan? This is analagous to the famous chicken and egg dilemma, and may of course be resolved similarly by saying that the present system is highly evolved. But what did it evolve from, and how ?

As to the how, it is generally felt that some sort of natural selection operated in favour of the development of our particular genetic code, though it is not yet possible to describe the selection in detail. As remarked earlier, this is a crucial point; the onset of natural selection in some primitive form is often considered to be the point at which life began. An interesting and possibly very important fact about the genetic code is its ubiquity on Earth. The DNA molecule contains the plans of proteins in a coded form where each amino acid of the protein is described by three successive rungs of the DNA ladder. What is striking is that the coding is
identical in all terrestrial life. A possible implication of this is that all life descended from a single ancestral system. If more than one ancestor were involved, we might reasonably expect to see considerable variations in the coding. The single ancestor theory is not the only possible explanation of the universality of the genetic code, but there are other peculiarities of the code which are also consistent with this particular theory. It should be remarked that this does not exclude the possibility of there having been other kinds of primitive life-form on Earth in the remote past. But it does imply that one of these (our ancestor) was more successful than the others and succeeded in taking over the Earth by eliminating them.

There is an independent piece of evidence also favouring the single ancestor theory. This concerns the optical activity of amino acids. In a laboratory, any complex amino acid can exist hi one of two forms which differ only in that they are mirror images of one) another. The forms are referred to as right-handed (dextro) and left-handed (laevo) in analogy with the gloves of a pair that are identical in all respects except that they are mirror images. In biological systems, all amino acids are left-handed. (This is not just a matter of semantics, a left-handed molecule rotates the plane of vibration of an electromagnetic wave one way, while a right-handed molecule rotates it the other way. Thus handedness can be measured using polarized light.) There is no obvious reason why both forms of amino acid should not coexist; if amino acids are made in a laboratory, both forms are produced in equal amounts. This is true of the Miller-Orgel type of experiment. One way of explaining the left-handedness phenomena is in terms of life being descended from a single event and place!

The final stages, from proteins and nucleic acids to cells and then to plant and animal life, represents a long and complicated story. Let us tackle the formation of cells first. This requires a combination of two processes: a high concentration of protein material and a subsequent organization of the protein molecules to form large molecular structures. An interesting idea has been put forward to explain the high concentrations that are needed. As a solution of protein molecules gets more and more concentrated, (through evaporation, for example), the polymers form and grow until they reach a certain critical size. At that point, very concentrated droplets separate out of the solution. These droplets are called COACERVATES. They can selectively absorb substances from the external medium and can grow bigger and more complex. A kind of natural selection will operate to favour the continued existence of those droplets whose structure is such that they can benefit from their environment. Such droplets could be the sites for the origin of a genetic code.

What about the formation of organized macromolecular structures ? It is believed that once protein molecules come into contact they will arrange themselves automatically into a special macro-molecule. Quite how this works we do not know, but it is certainly a well-observed phenomenon. In some experiments, tissue made of protein is dissolved and the solution stirred up. This destroys the elaborate macromolecular structure of the original tissue (such structure can be seen with an electron-microscope). If the dis¬solved molecules are then precipitated out of the solution, it is observed that the structure of the original tissue reappears spontaneously. The order present in large aggregates of protein molecules is apparently inherent in the structure of the protein itself.

From here the story is one of continual building and natural selection. It may, for example, be favourable for survival for an agglomerate of protein molecules to be surrounded by a membrane. In this way. primitive cellular structures may develop. Later on, the cells may develop a nucleus and could resemble living cells more closely. It is interesting that we have some fossil evidence on the development of cells. The oldest known fossils are bacteria and calcareous sedimentary structures called STROMATOLITES associated with blue-green algae. These are dated at 3.2 billion years of age. What is of inportance about these life-forms is that their cells are especially primitive in comparison with the cells of animals and plants. The bacteria and blue-green algal cells have no organized nucleus. Such life-forms are grouped under the heading of
PROCARYOTES and they form the expected precursor to the present-day cellular structures (EUCARYOTES) which have a cellular nucleus.

If life on Earth evolved by a process of growth, modification and selection, and if it is descended from a single ancestor, we should be able to identify the times at which the various innovations to the structure of life appeared. This is the grandest of all family trees! The tree can be constructed by modern investigation of the protein structures of various life-forms. In this way we learn for example that plants and animals separated 1.1 billion years ago, and we can trace the evolution of the great biological families (phyla).

It has clearly been a long and difficult route to the evolution of man. So many things could have happened on the way that it is probably safe to say that extraterrestrial intelligence will differ considerably from us, if, indeed, it exists. In that context, it is important to understand what distinguishes homo sapiens from other animals it is certainly more than mere intelligence.

‘Man’ has created a world of culture by documenting his intellectual efforts. Animals, however intelligent, do not have this, though many undoubtedly have emotions, senses, memories and even perhaps creative imagination. Man can acquire knowledge about the world from his fellows, whereas an animal’s knowledge of the world is gained only from personal experience. The world of culture was created by man and the development of language must have been of paramount importance in this process. Man achieved selfconciousness through communication with his fellows.

There is a danger of putting ‘Man’ on too high an intellectual plane. What we believe to be objective knowledge arises as a result of the processing of our sense data by our brains. It is entirely conceivable that we are being fooled, and that another superior intelligence might come to differing conclusions about the same sense data. The human brain has evolved considerably even though it has only doubled or tripled in size in the two million years since our ape-like ancestors walked the Earth. What might a further ten million years do? It should be noted that there are stringent physiological constraints on the future development of Man. One reason for this is in the way humans are born: the infant’s head must pass through the pelvic girdle at birth, and this puts an upper limit on the size of the human brain at birth. The fate of our species may therefore be a rather stagnant one like the spider, in which any further growth of the brain would constrict the ingestion of food. There may also be social constraints on further development such as found in ants which seem to have used their intelligence to create a homogeneous and almost static society.

It has been remarked that language and our understanding of the world have developed together. The link between our under¬standing of the world and the language we use may be such a powerful one that our knowledge is actually constrained by our language structure. So even in mathematics, which one might have thought of as existing quite independently of the real world, we can identify only a few basic structures like topological structures and algebraic structures. Is this because that is all there is to mathematics, or is our mathematical knowledge constrained by the way it was acquired via language ? Our mathematical knowledge may even be constrained by the simple structure of our brains that formulate both the language and the mathematics. Life-forms evolving completely different means of communication, and having different brain structures might have different mathematics, and so their technologies could’ be inconceivably different from ours! We must be humble about our present position and beware of intellectual chauvinism.

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