Evolution

Simple cells are thought to have evolved 3.5 billion years ago. The earth is approximately 4.6 billion years of age. The simple life forms were algae like micro-organisms and later, cyanobacteria which were responsible for the great increase in oxygen towards the end of the first era known as the Achaean. Actual algae occurred much later at the dawn of the second Proterozoic era around 2.5 billion years ago. 400 million years ago, during the Palaeozoic era, plants colonised the land. Evolution may be defined as the process by which species undergo transmutation over geological time. Evolution is a core theme of biology, all life on earth today comes from a common ancestor and incorporates genetics, systematics, palaeontology, developmental biology, behavioural psychology and ecology. The mechanisms of evolution by natural selection are variation within a population, superfluous fecundity, limits on population growth and heredity success. Individuals with the most favourable combination of characteristics are most likely to survive and reproduce therefore adaptive traits accumulate in a population. New species arise by divergence. This may due geographic, behavioural-social or physical limitation on interbreeding between populations. Comparatively the anatomy of two different species may share homologous features they have inherited from a common ancestor. The similarities between Abolocetus and the modern whale is an example of divergent evolution. Similarly two species may look very similar but only have a very distant common ancestor. By convergent evolution they have adapted to similar niches in the specific ecosystem. The fusiform shaped shared by whales and fish as result of the high density of water is an example of analogous or homoplastic features.