As the theory of evolution has become increasingly developed and tested by biologists, new questions have arisen. Not all problems have been solved, and there is still fresh debate around the old questions. In this extract from Evolution: A Very Short Introduction, Brian and Deborah Charlesworth describe some examples of biological phenomena that are difficult to explain. Some of these were dealt with by Darwin himself, others have been the subject of later research. Below are two problems which evolutionary research has left largely unsolved.
Two other major and largely unsolved problems in evolution, at the opposite extremes of the history of life, are the origin of the basic features of living cells and the origin of human consciousness. In contrast to the questions we have just been discussing, these are unique events in the history of life. Their uniqueness means that we cannot use comparisons among living species to make firm inferences about how they might have occurred. In addition, the lack of any fossil record for the very early history of life or for human behaviour means that we have no direct information about the sequences of events involved. This does not, of course, prevent us from making guesses about what these might have been, but such guesses cannot be tested in the ways we have described for ideas about other evolutionary problems.
In the case of the origin of life, the aim of much current research is to find conditions resembling those which prevailed early in the Earth’s history, which allow the purely chemical assembly of molecules that can then replicate themselves, just as the DNA of our own cells is copied during cell division. Once such self-replicating molecules have been formed, it is easy to imagine how competition between different types of molecule could result in the evolution of more accurate and faster replicating molecules, that is natural selection would act to improve them. Chemists have been very successful in showing that the basic chemical building blocks of life (sugars, fats, amino acids, and the constituents of DNA and RNA) can be formed by subjecting solutions of simpler molecules (of the type that are likely to have been present in the oceans of the early Earth) to electric sparks and ultra-violet irradiation. There has been limited progress in showing how these can be assembled into still more complex molecules that resemble RNA or DNA, and even more limited success in getting such molecules to self-replicate, so we are still far from achieving the desired goals (but progress is constantly being made). Furthermore, once this goal is achieved, the question of how to evolve a primitive genetic code that allows a short RNA or DNA sequence to determine the sequence of a simple protein chain must be solved. There are many ideas, but as yet no definitive solutions to this problem.
Similarly, we can only make guesses about the evolution of human consciousness. It is even difficult to state the nature of the problem clearly, since consciousness is notoriously hard to define precisely. Most people would not regard a newborn baby as conscious; few would dispute that a two-year-old child is conscious. The extent to which animals are conscious is fiercely debated, but pet-lovers are well aware of the ability of dogs and cats to react to the wishes and moods of their owners. Pets even seem to be able to manipulate their owners into doing what they want. Consciousness is thus probably a matter of degree, not kind, so that in principle there is little difficulty in imagining a gradual intensification of selfawareness and ability to communicate during the evolution of our ancestors. Some would regard language ability as the strongest criterion for possession of true consciousness; even this develops gradually with age in infants, albeit with astonishing speed. Furthermore, there are clear indications of rudimentary language abilities in animals such as parrots and chimpanzees, who can be taught to communicate simple pieces of information. The gap between ourselves and higher animals is more apparent than real.
Although we know nothing of the details of the selective forces driving the evolution of human mental and language abilities, which evidently far exceed those of any other animals, there is nothing particularly mysterious in explaining them in evolutionary terms. Biologists are making rapid progress in understanding the functioning of the brain, and there is little doubt that all forms of mental activity are explicable in terms of the activities of nerve cells in the brain. These activities must be subject to control by genes that specify the development and functioning of the brain; like any other genes, these will be liable to mutation, leading to variation on which selection can act. This is no longer pure hypothesis: mutations have been found which lead to deficiencies in specific aspects of grammar in the speech of their carriers, leading to identification of a gene involved in the control of some aspects of grammar. Even the mutation in its DNA sequence that causes the difference from normal is known.
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