Wednesday, June 17, 2009

Has human evolution stopped?

The human family, the Hominidae separated from the apes around 5 to 7 million years ago and the modern human species, the Homo sapiens appeared in East Africa around 200,000 years ago. However, it is difficult to know whether we have evolved a great extent since then. We must consider the fact that evolution acts on many different levels; through natural selection, sexual and kin selection etc. Natural selection is mediated by the environment and is primarily for improved survival in the prevailing conditions of the time. Sexual selection however is exerted by other members of the population so that they can mate with individuals who they believe to have preferential traits while kin selection is a process sometimes directed by individuals towards their relatives, a kind of altruism which allows reproductive success of their relatives. These, especially natural and sexual selection are external forces ultimately dictating the speed of evolution by increasing or reducing allele frequencies on the basis of their reproductive benefit. However, they may be impeded by genetic drift which sees to alter gene frequencies on the basis of random distribution of parental genes in the offspring which introduces an element of chance. The effects of genetic drift are more pronounced in large populations due to the increase in the possible combinations of genes. Because evolution is a trade off between natural selection and genetic drift, one dominates over the other depending on how strong selection pressures on a particular locus are to cause gene frequency shifts.
One reason that has emerged to suggest that human evolution is stopping or slowing down is based on the huge increase of our population on the evolutionary scale and in recent times. Figures from February 2009 suggest that the world population has now reached 6.7 billion and for example, for a British child, there is a 99% chance that it will reach its reproductive age compared to 500 years ago when this was only 50%. Large populations have a higher stability than smaller ones as newly arising mutations will be swamped by the already established genes. Therefore, mutation rates of genes can be high but retention in the population will be more difficult. Another point is that although the acquisition of point mutations are fairly constant, DNA has repair mechanisms to allow retainment of its original condition.
Steve Jones, a geneticist at UCL, has argued that natural selection is no longer important for humans. Since it works by ensuring the survival on those who are more reproductively successful, through medical intervention, we have altered this so that nearly anyone in the population can have a child. He therefore believes that human evolution has indeed stopped. However, saying that natural selection is no longer significant among humans is not necessarily correct.
Since the transition from hunter-gatherer behaviours to more agricultural (and technological) ways, we have developed characteristics specific to this. For example, once people began rearing cattle, gaining milk derived nutrition spanning life, instead of it being halted at the infant stage, became advantageous which gave rise to lactose tolerance. Due to this, presently, the gene for lactose digestion appears in 80% of Europeans. However, for Steven Pinker, author of ‘How The Mind Works,’ the emergence of culture meant a move towards ‘non-genetic means to adapt’ for example through behavioural changes.
One of the greatest triumphs of our species was to develop consciousness. Consciousness is something very difficult to define however. When faced with a stimulus, we can retrieve information from past experience to relate to the present situation and make a conditioned response. However, other animals can also do this. We on the other hand can also register the event as being pleasant or not i.e.have an emotional response to it to trigger goal states. In this I mean we can then decide ourselves which course to take after this. Therefore, objects can become things of desire and increasing our experience of them may lead to an enhanced potential for survival and reproduction in the environment. This step in our reaction introduces the state of being self aware due to the level of control consciousness has allowed us to have. Self awareness is also a prerequisite for the understanding of others and manipulation of the environment, which is what we have achieved and also, the formation of culture. 50,000 years ago tribe formation because of this and new hunting techniques with the invention of tools because of increased intelligence allowed the number of tribes in an area to proliferate and in doing so increase opportunities for social interactions.
Humans have also developed left-right asymmetry of the brain which apes do not show. The importance of this is that besides the opposing sides of the body being controlled by the opposite hemisphere, the two hemispheres also differ in their functioning. For example often the left is the dominant which specialises (on average) in logical and sequential/mathematical reasoning. The right is related to more artistic abilities such as shape and colour recognition (e.g. faces) i.e understanding patterns. The development of these features has given a new course for selection pressures to act on. However, not just on single genes as intelligence and increased congition etc. are caused by interactions of a variety of genes. Selection pressures however can seek to form an optimal combination of genes. Therefore, many would argue that in fact human evolution is accelerating by cultural development through these means and creating behavioural change. Increase in urbanisation and technology through our ingenuity has meant as Ian Tattersall put in ‘Becoming Human: Evolution and Human Uniqueness’ that ‘we are being besieged by social stimuli’ so this is what we are adapting to.
In our evolutionary history, it has been found that instead of gradual allele frequency shifts (phyletic gradualism) causing change, there were periods of species stability interrupted by speciation, extinction and replacement –punctuated equilibrium, which was put forward by Niles Eldredge and Stephen Jay Gould in 1972. They found that a new species can be created from a pre-existing one when the selection pressures are large and the species is able to adapt, speciation occurred as a short-term process compared to in phyletic gradualism. Therefore, there is little evolutionary change during most of their history but when evolution occurs, it is rapid and short-term. If the pre-existing population is divided due to a physical barrier, caused by anything such as a seaway, the population begins to diverge and because smaller gene pools are less stable, speciation occurs even faster. Therefore, there is little evolutionary change during most of their history but when evolution occurs, it is rapid. However, in ‘evolutionarily recent’ times, we have found means to overcome physical barriers such as seaways and terrestrial barriers such as mountains etc. and therefore, we are becoming more uniform in our species with the human population being constantly mixed . It has therefore been put forward that we are tending towards a ‘uniformly brown-skinned population.’However, Robert Moyzis at the University of California at Irvine believes that due to the accumulation of various SNPs (single nucleotide polymorphisms, single base changes) in certain populations, we are diverging because of the differences between races.
Therefore, there are many theories as to what will happen in our evolutionary future. Many focus on cultural changes due to behavioural development and others on morphological change. However, it is very unlikely that human evolution has stopped. This would mean that the mechanisms for evolution no longer apply to us which separates us from the rest of the animal kingdom and seems to single us out as being special when in fact we arose the same humble way that other creatures have. We cannot say that we are improving either as this would commit evolutionary fallacy that each stage if evolution works to improve us. Evolution seems to act on many levels and it is difficult to predict what direction it is heading as it is essentially blind. Therefore we also may not be able to rely on our past evolutionary history as a means to put light on the future.
Becoming Human: Evolution and Human Uniqueness, Ian Tattersall, Oxford University Press, 2000
How The Mind Works, Steven Pinker, Penguin, 1998
Human Evolution; an Illustrated Introduction, Roger Lewin, Blackwell Publishing, 2005
Cells, Embryos and Evolution, John Gerhart and Maarc Kirschner, 1997

1 comment:

  1. You cover a lot of ground, but you have missed a few points, particularly well-known examples of recent evolutionary change such as lactose tolerance. See also John Hawks' blog (and work):