Biological Models

 As the previous introductory outline indicated, the scope of human evolution is being considered within the context of past, present and future. However, it is clear that humanity has reached this point in its current physical evolution primarily based on biological mechanisms, driven by natural selection.

 Therefore, this first section of discussions will focus on the biological models thought to underpin evolution, first in terms of the genetic model of natural selection and then the possible mechanisms of epigenetics to influence the action of genes over much smaller time frames. In this context, the discussion of biological evolution continues the debate about ‘nature versus nurture, although there is now increasing evidence that both have important roles, when considered in terms of the genetic and epigenetic models to be outlined. However, as a starting point, evolution is often simply thought of in terms of Darwin’s idea of natural selection, where life slowly adapts to meet the challenges of a changing environment, while epigenetics might be seen to be a more controversial issue, especially if linked to Lamarck’s idea of acquired traits, which was thought to affect evolutionary processes over much shorter timeframes. By way of historic background, we might anchor the ‘nature’ side of this debate to two works by Charles Darwin, i.e. 'The Origin of Species by Means of Natural Selection (1859)' and 'The Descent of Man (1871)'. Of course, at the time of writing, Darwin knew nothing of modern-day genetics, which had to wait for later discoveries, e.g. the double-helix structure of DNA by Crick and Watson in 1953. From an even earlier historic perspective, we might anchor the ‘nurture’ side of the debate to the work of Jean-Baptiste Lamarck and the publication of 'The Inheritance of Acquired Traits (1809)’, which was also the year Darwin was born. Central to Lamarck’s work was the idea that evolution proceeded on the basis that individuals adapt during their own lifetime and transmitted the traits they acquire to their offspring. While Lamarck’s model was superseded by Darwin’s natural selection, hindsight tells us that both were operating on the basis of limited observational evidence and almost no knowledge of molecular biology, genetics and more recent developments in epigenetics.

So what is the present-day state of the evolutionary debate?

The addition of genetics to Darwin's theory of natural selection has become known as ‘modern evolutionary synthesis. As such, it seeks to explain how natural selection results in genotype and phenotype changes taking place at the cellular level via processes decoded into DNA structures, i.e. genes. In contrast, epigenetic effects are not directly encoded into the DNA sequence, but may affect the operation of DNA by mechanisms that switch genes on and off resulting in differing cell functions. So, in this respect, the nature versus nurture debate has now been transposed into the debate as to how far evolutionary processes require both genetics and/or epigenetics mechanisms. However, in order to proceed, it is necessary to provide some basic introduction of the terminology and ideas that surround any discussion of the structural mechanisms associated with evolution in terms of cells and the DNA contained within.