The Palaeozoic-Mesozoic boundary (250 million years ago) represents the greatest period of extinction in the entire fossil record. Marine invertebrates were especially affected. Whole groups became extinct, including the trilobites that had dominated the oceans for millions of years. Plant life was not seriously affected but 75 per cent of amphibians and over 80 per cent of reptile families disappeared. At present, it is estimated that four or five families disappear every million years. But at the end of the Palaeozoic, we had the disappearance of 75-90 per cent of all species. By such catastrophic events did the evolution of the species unfold. Yet this process of mass extinctions did not represent a step back in the evolution of life. On the contrary, precisely this period prepared a mighty step forward in the development of life on earth. The gaps left in the environment by the disappearance of some species gave an opportunity to others to rise, flourish and dominate the earth.
The factors that influence the distribution, diversity and extinctions of life forms are endlessly varied. Furthermore, they are dialectically interrelated. Continental drift itself causes changes of latitude, and therefore climatological conditions. Variations in climate will create environments that are more or less favourable for different organisms. Tolerance to temperature fluctuations and climatic conditions are key factors in this process, giving rise to diversification. We see that diversity usually increases as we get closer to the equator.
The break-up of continents, their separation and collisions, all these factors change the conditions under which the species develop, cutting off one group from another. Physical isolation produces new adaptive variations, reflecting changes in the environment. Continental fragmentation thus tends to increase the diversity of life forms. Kangaroos survived only because Australia was isolated from the other continents very early, before the explosive rise of the mammals that caused the disappearance of large marsupials in all the other continents. Similarly, the destruction of oceans produces mass extinctions of marine species, yet at the same time creates the conditions for the development of new land plants and animals, as was the case at the inception of the Pangaean land mass. Death and birth are thus inseparably linked in the chain of evolutionary development, where the mass extinction of one species is the prior condition for the emergence and development of new ones, better equipped to cope with changed conditions.
The evolution of the species cannot be regarded as an isolated self-contained fact, but must be seen as the result of a constant and complex interaction of different elements – not only the infinitely large numbers of genetic mutations within living organisms, but also the continual changes in the environment: fluctuations in sea-level, water salinity, the circulation of oceanic currents, the supply of nutrients to the oceans, and, possibly, even factors like the reversal of the earth’s magnetic field, or the impact of large meteorites on the earth’s surface. The dialectical interplay of these diverse tendencies is what conditions the process of natural selection, which has produced forms of life far richer, more varied and more wonderful than the most fantastic inventions of poetry.