In general, the term 'evolution' can imply a drastic or gradual change from a very broad perspective. Life on earth, the universe,galaxies, as also the earth in general have evolved through millions of years. In this essay we consider only one aspect of evolution emphasizing on evolution as a biological tool for change among species and consider fossil record as supportive of both evolution theories and also the other theories contrary to evolution. Evolution is the central unifying concept, a theory that successfully connects biology, palaeontology and other branches of science. Evolution is a gradual descent of organisms accompanied by changes that help the organisms to adjust and adapt to the surroundings. 'Descent with modifications' as Darwin contended implies changes in organisms in successive generations (Mayr, 1976). These changes are triggered by the derivation of new species and there is a change in the properties of populations of organisms and these properties tend to transcend the lifetime of any single individual. Newers pecies are modified versions of older species. Although, individual organisms do not biologically evolve,populations evolve when heritable genetic materials are transmitted from one generation to another. Biological evolution can range from very limited changes to drastic transformations on a large scale changing the entire special together and bringing in new forms. Evolution can thus be defined as inheritable changes in populations of species that are spread and transmitted over many generations (Zimmer, 2002). It is also more scientifically defined as changes in the frequency of alleles within a gene pool carried through different generations as understood in the Darwinian version of the theory(Dawkins, 1989). Evolution studies are supported by detecting changes in gene frequency within a population and the fact that the theory of evolution emphasizes on a common ancestor, only indicates that two or more species show successive heritable changes in populations since they are separated from each other as distinct forms (Allen and Briggs, 1989). Most popular definitions of evolution however highlight not the transmission of heritable traits and changes but the processes of diversity that has given rise to millions of species from the most primitive organisms. Here however we move on to the evidence for and against evolution theories and the role of fossil record in this context. Some researchers claim that the theory of evolution has been supported by four primary sources that serve as evidence (Zimmer, 2002; Allenand Briggs, 1989):
- The fossil record that tracks changes in early and primitive forms of life
- The anatomical and chemical similarities in the constitutions of different species.
- The genetic changes observed and recorded in several living organisms over several generations
- The geographical spread and distribution of species that seems to suggest a definite pattern, and
The Fossil Record
Fossils are buried in rock layers as indentations of dead plant and animal materials. The totality of these artefacts and their impressions on the rock formations is considered a fossil record. Fossil record as we have briefly mentioned is the primary source of evidence supporting the theory of evolution and the gaps in these records ironically also forms the bone of contention taken up by anti-evolution theorists. Fossil records are used by scientists to understand the process of evolution in general, and the subsequent changes in several species at several times of the earth's existence(Donovan and Paul, 1998). The Fossil Record seems to provide an important clue to the changes in primitive and even now extinct species and this definitely helps us to frame a conceptual graph on how evolution has taken shape. Fossil and rock record forms the primary source of evidence collected by scientists for nearly400 years and the consequent database obtained is mainly observational. The fossil record among all other evidence gives a large database of documented changes in past life on earth. The use of Fossil record to study life forms on earth dates back to pre-Darwinian times and the changes in life forms could be studied from a sequence of layers of sedimentary rocks and fossils of different groups of species were found in each of these successive layers (SA, 1982).Sedimentary rocks are found widely across the earth's surface and are formed when small particles of sand, mud or gravel, shell or other materials withered off by water or wind accumulates in sea beds and oceans. As these sediments pileup they bury shells, leaves, bones, and parts of living organisms. Layers of sediments are thus formed for every large period of time and all these layers become subsequently cemented to each other to become different layers of sandstone,limestone, shale and so on. Within these layers of sedimentary rocks the plant and animal remains become buried as fossils and are later revealed as fossil records (Allen and Briggs, 1989). From these fossil records several species have been identified, some of which are extinct and some of which have traits transitional between different major groups of organisms. Fossils of transitional forms actually give considerable evidence of species evolution over time. However there is not enough evidence through fossil records to conclusively prove evolution, as there are still talks of 'missing links' as very few and according to some, no transitional forms have been actually discovered. The Fossil record data available to us is incomplete and in conclusive at present. During the late eighteenth and early nineteenth centuries,William Smith, a British Engineer observed different assemblages of fossils preserved at different levels and different ages of rocks. These assemblages succeeded one another in a regular and determinable order (cited in, Wikipedia,2004). This was further bolstered by the fact that rocks collected from different locations showed similar fossil formations according to the different times they represented. Smith named this correlation of rock fossil data as the principle of faunal succession. The occurrence of faunal succession was one of the primary arguments of Darwin who used fossil evidence as supporting the theory of evolution. Various modern approaches to the theory of evolution have been recently developed. Mayr claims that the theory of Punctuation for instance has two basic points that
- most or all evolutionary change occurs during speciation events, and
- most species usually enter a phase of total stasis after the end of the speciation process (which involves formation of new species).
Speciation thus involves transformation of species in geological time (Erwin and Anstey, 1995). Formation of new species is explained either by phyletic gradualism or a gradual steady transformation of species by phyletic evolution highlighting the deficiency of the fossil records, or by sympatric saltational speciation that highlighted punctuational equilibria and branching of species rather than transformation as lineages as the real explanation for evolution (Mayr and Provine, 1998). Biologists like Gould and Eldredge have also supported punctuation theories. Richard Dawkins on the other hand stresses on the principle of gene multiplication where genes as replicators seems to be the focal point of defining evolution (Sterelny, 2001). In quite an important paper Volkenstein (1987) suggests that there can be no contradiction between punctuated equilibrium and phyletic gradualism if synergetics and theory of information are incorporated within the theory of evolution. Punctualism can be seen as phase transition maintaining the directionality of evolution. Volkenstein argues that Punctualism, non-adaptationism and neutralism form the triad of internally connected features of evolution.
Problems with Fossil Records
Of course at that point, the absence of a proper theory of evolution prevented Smith or other researchers from providing an explanation of the actual cause of faunal succession. The cause of faunal succession as is known today is mainly due to evolution of organisms and species that change,transform or become completely extinct, leaving behind their traces on earth as fossils. Age of rocks and the changes in species features are both determined by fossil record and faunal succession used as tools in bio stratigraphy. However fossil data show extremely few records of transitional species,organisms that can conclusively suggest how and when evolution of new and different species occurred (Donovan and Paul, 1998). Darwin himself suggested that the geological record itself is imperfect and incomplete and this is further strengthened by the fact that transitional species were short lived and had very narrow geographical range. Radiometric and Carbon dating have made it possible to identify fossils more than 3.5 billion years old and have indicated that animal species may have appeared abruptly, a phenomenon which Darwin himself found difficult to accept. Even though one or two forms of organisms which may be considered as transient have been identified, there are no records of transitional plants and thus an evolutionary plant history could not be drawn as of yet. Along with these issues it has also been seen that most of the fossils found are of species which have existing forms and are either similar to existing species or are completely identical. The intermediate temporary stages as serve to act, as links between two related species seems to have been completely downplayed by the fossil data obtained. Animals seem to have remained more or less unchanged through all these years. Despite the collection of a huge number of fossils,nearly all of them being fossils of presently existing animals have created problems for the theory of evolution. It is a general belief that based on fossil discoveries already made, there will be little or no evidence that evolution had actually occurred and continues to occur (Donovan and Paul,1998). If animals die a natural death, they are usually decomposed even before being fossilized. However during sudden catastrophes can bury the animals and embed them deep in the earth. Some rocks and organisms that transformed to show fossils for years and decades were actually deposited within a short period of time. Although Darwin based his arguments heavily on fossil record, most scientists now believe that fossil record is actually incompatible with evolutionary theory as no transitional links or intermediate forms have been discovered among this huge collection of fossils in all these years. This suggests that there is no real evidential data that the theory of evolution is in fact true. There is no evidence of partially evolved species or intermediate forms either in the past or in the present fossil record and the fossil record available is quite representative of all fossil data that will ever be collected. Evolution seems to point out towards more undefined and partially evolved species, fact completely undermined by available fossil record that shows well-defined organisms rather than gradual gradations. The incomplete fossil record is the primary bone of contention in the evolutionary debate and seems to give an edge to non-evolutionists.
Considering all the aspects of the debate and gaps in fossil records and weighing this against evolution theories highlighting either generational transformation of lineages or drastic changes and speciation at specific periods, we can conclude that available gaps in fossil record may be more indicative and supportive towards speciation and abrupt changes rather than gradual evolution through phyletic transformation.