The Neo-Darwinian theory maintains that life has been subject to a process of gradual transformation that allowed it to move from simple forms to ever more complex ones in small steps. Early life consisted of tiny unicellular organisms living in water, and every other form, extant or extinct, is connected by an unbroken chain of intermediate species to these first ones. This is not exactly the picture that one would get from the available fossil evidence. If evolution had been gradational, there should be greater variations between fossil specimens reflecting every small step in the process. But, this does not seem to be the case. Although there are an abundance of fossils of fully formed species, there are few contenders for their transitional forms (hence the phrase ‘missing link'). For example, there are no traces of the evolutionary ancestors of the trilobites in the rock layers beneath where the trilobites are found. It seems that trilobites, with their sophisticated optical systems, appear in the geological record relatively suddenly. These occurrences cannot be fully accounted for by the incompleteness of available data. Mounting paleontological evidence suggests that ‘speciation' (the emergence of new species) is a rapid process. Species change in relatively swift bursts, without leisurely transition periods. These episodes of fast speciation are separated by fairly long spans during which no significant alterations can be detected. In other words, species appear abruptly, often in entirely different forms, and remain substantially unchanged for millions of years - a condition of stasis at odds with Darwin's model of continuous change. Then, just as quickly they become extinct and are immediately followed by other very different species. The fossil record demonstrates abundantly that each episode of extinction was followed by a period when new forms proliferated, filling the ecological niches emptied by the old. Not only individual species but entire genera make their appearance in relatively short time. One example is the so-called Cambrian explosion about half a billion years ago, the sudden emergence, in the span of a few million years, of a great variety of the bigger animals that now populate the earth. The rapid evolution of mammals between 60 and 65 million years ago is another instance of this recurrent phenomenon. It is significant that every new cycle is not made of species at the same level of complexity, but more advanced ones.
This does not refute the continuity of the evolutionary process and certainly does not imply that an external force directly interferes with it, as the creationists (or the proponents of ‘Intelligent Design') would like to believe. Slow, continuous change (within species) may be the norm during periods of environmental stability, while rapid speciation may occur during periods of environmental stress. When the milieu changes and the existing niches disappear, some species die out. Then the ‘peripheral isolates' (species that live in relatively small numbers) invade the centres of dominance and take over as the new main species. Also there are some creative solutions. For example, a link between prokaryotes (cells without organelles) and eukaryotes (cells with organelles and other structures) has not been found. The difference between these single-cell species is striking, and yet there are no intermediate stages between them. There are many living samples of each, but none of the intermediate stages. One imaginative possibility, put forward by the biologist Margulis, is that eukaryotes could be the result of a symbiosis of two different prokaryote species.
However, even when the above hypotheses are taken into account, conventional Darwinian mechanisms do not seem sufficient to explain the stops and starts observed in the fossil record (why species appear so abruptly and why they persist so long without changing.). These punctuations are too radical to allow for Neo-Darwinian interpretation. The problem is not only to explain the sudden burst but also, as a science writer Richard Kerr puts it, ‘what would maintain the equilibrium... keeping the new species from evolving in spite of environmental vagaries' (1995, p.1421-1422).
Intriguingly, growing evidence suggests that extinctions follow relatively regular periodic patterns. The statistical chance of these patterns being a random occurrence is very small. Some of them may have been caused by physical factors (e.g. slight variations in the Earth's orbit over long periods, leading to a climate change). Nevertheless, it is conspicuous that new, and as a rule, more complex life always follows relatively soon after.
All the above makes it hardly plausible that new species could have arisen gradually by purely accidental transformation from one species into another.
- . Although it had its precursors, so-called punctuationism or punctuated equilibrium brought these facts to wider attention in the 1970s. It caused quite a stir, especially among dogmatic Neo-Darwinists, for fear that it could be used as a weapon against the theory of evolution as a whole.
- . Of course, ‘immediately' only in geological terms. For instance, Denton writes that ‘the evolutionary pattern was one of millions of years of stasis interrupted by periods of no more than 100, 000 years of rapid and sudden change' (1998, p.297).
- . Punctuationism, strictly speaking, is not ‘saltationism' (radical changes from one generation to the next or discontinuous appearance of new species), so it does not contradict the theory of evolution. It only adds weight to the argument that the traditional Darwinian mechanisms may not be the only factors.
- . The figure of 2.5 million years seems significant in this respect. Paleobiologist Sepkoski also suggests 26 million years, but according to Muller and Rohde, a 62 million year pattern is even more striking.