I was recently involved in a debate with creationists, who reject the theory of evolution. Today I will briefly outline some of the main evidence for evolution and next week I will discuss the radically different position adopted by the creationists.
Science proposes that life arose and developed on Earth roughly as follows. The Earth is about five billion years old and life spontaneously arose from lifeless molecules about four billion years ago.
Evolutionists believe the myriad lifeforms on Earth are ultimately descended from that original simple form and are directly descended, with modifications, from previous living forms.
The theory of evolution proposes natural selection as the mechanism through which evolution works. There is genetic variation in all species which is maintained by spontaneous random genetic changes called mutations.
Those variants best suited to the environment thrive and procreate best and spread their kind at the expense of less well-suited variants.
Significant environmental changes can tip the balance in favour of a hitherto minority variant that now finds itself particularly well adapted to the new conditions.
In this way the environment naturally selects for individuals best suited to it. With various other factors assisting such as geographical isolation, new species gradually replace older types.
The evidence in favour is so overwhelming that evolution is accepted as fact in all open-minded circles. Perhaps the most compelling aspect of the scientific case is the way in which the evidence from different sources all points to the same conclusion.
Thus, the evidence from the fossil record, from the comparative anatomies of living species and from molecular details of living species all point to biological evolution.
Fossils of organisms that lived in past ages are preserved in the rocks of the Earth. These lie in layers atop each other, with the oldest layers lying deeper and younger layers successively piled on top of them.
Very few fossils are found in the oldest rocks. The oldest fossil found to date is of an ancient unicellular bacterium-like organism dated as approximately 3.4 billion years old.
As you come up through increasingly younger rocks, multi-cellular animals and plants appear roughly in the following order: fishes, terrestrial plants, amphibians, insects, reptiles, mammal-like reptiles, primitive mammals, more elaborate plants, dinosaurs, flowering plants, birds, modern mammals, humans.
The geological layers and their fossils were dated and characterised by geologists long before the theory of evolution was proposed in 1858. They concluded that these layers were laid down over many millions of years. They also noted that the great majority of fossils represent organisms that are no longer present on Earth.
These scientists were all creationists who believed that God created life as many separate species. Nevertheless, they also had the confidence to believe the history written in the rocks and they concluded that God had created life on many occasions.
The theory of evolution interprets the fossil record as a succession of life forms, each descended with modification from previous forms.
In some cases the gradual changing of one form through intermediate into a new form can be seen in the fossil record. One well-known example of an intermediate form between two different species is Archaeopteryx the winged reptile, an intermediate form in the evolution of birds from reptiles.
If evolution is true then living organisms should carry evidence of their historical development in details of their anatomy, body chemistry and molecular detail. And this is indeed the case.
For example, evolution holds that humans are descended from an ape-like ancestor. Correspondingly, at the base of every human spine is a structure called the coccyx which is the rudimentary vestige of a tail, indicating that we are descended from a tailed form. Many other species also show vestigial structures that have no current functions.
At first glance, there is little similarity between the forelimbs of a human, a dog, a seal, a crocodile and the wing of a flying fox. These are homologous organs, i.e. they fulfil corresponding functions in each of the species.
When you examine the anatomy of each of these organs, they are seen to share a basic five-digit structural design, modified in each species.
Evolution explains this common basic structural plan as descent from a common ancestor. By comparing the anatomies of different species it is possible to build an evolutionary tree grouping closely related species together, separating them from more distantly related species, and extrapolating backwards to where different branches diverged from common ancestors, and backwards finally to the original ancestor of all life.
Different species also have homologous molecules. If evolution is true, one would expect that the further apart two living species are in evolutionary terms (i.e. the longer ago they shared a common ancestor) the bigger the differences that would be apparent in the structures of homologous molecules.
This would be particularly true for homologous protein molecules whose structures are long sequences of 20 different amino acids built by genetic information.
Genetic information changes randomly with time (mutations), and the longer the time elapsed since a common ancestor the greater the opportunity for two species to diverge in the structure of homologous molecules.
And this is precisely what you find. Evolutionary trees drawn on the basis of anatomical evidence are broadly identical to evolutionary trees drawn on the basis of molecular data.
Creationists believe that God created all living species separately about 6,000 years ago when He formed the Earth. They completely reject evolution. I will discuss their position next week.
(William Reville is a senior lecturer in biochemistry and director of microscopy at UCC)