“The ultimate objective of an organism, if we can phrase it in these terms, is to contribute as many genes to future generations as it can. In principle, an organism can do this by reproducing itself, or by helping relatives that share the same gene(s) to reproduce more successfully, or by some combination of the two” (Dunbar [1988] in Plotkin [ed]). To make a brief evaluation of theory and research of the central topics of human evolution it is necessary to describe the basic processes of mating behaviour and parental investment, with reference to humans. .
Cartwright’s statement of the assumptions of his book Evolution and Human Behaviour (2000) provides a concise summary of human evolution: that humans have evolved from ape-like ancestors; that homo sapiens appeared about 200,000 years ago and that if the agricultural period began only 10,000 years ago, the genes present previous to this had a much longer time to entrench themselves. Thus, the genes we operate under now are a result of adaptation to the Palaeolithic period rather than modern times.
So, it would seem that our genes have been heavily influence by our Paleolithic ancestors and for this reason, evolution of human mating behaviour can be described in terms of costs and benefits to these hunter-gathering ancestors (Cartwright, 2000). Physiologically, males are capable of reproducing at a far greater rate than females. If reproductive success is the objective of humans, then why do men not try to fulfill their reproductive potential?
One reason is that deserting is that a ‘philandering’ male will face uncertain paternity and thus will be unsure as to his genetic legacy. Another way men are (or were) prevented from increasing their reproductive capacity is by women concealing their ovulation. These are just two examples of ways in which reproduction is balanced by males and females in their efforts to secure reproductive success.
An important aspect of mating behaviour is mate selection. As Miller (1997, p. 71 in Ciba Foundation Symposium) asserts: “Evolutionary psychologists have successfully combined sexual selection theory and empirical research to compile lists of sexual attractiveness cues used in human mate choice”. This list includes height, intelligence, walking speed, facial symmetry, sense of humour, waist-to-hip ratio, degree of genetic relatedness, full lips, political status and sexual foreplay skills (e.g. Buss, 1994, Ridley, 1993, Wright, 1994 in Miller, 1997). An example of this research is the systematic surveys carried out by David Buss (1992 in Gleitman, 1999).
His studies have shown for example that men rate attractiveness in their partners more highly than women do. This can be explained in evolutionary terms by the fact that attractiveness would have indicated health and in particular reproductive health, to our ancestors. The same reason would explain why younger women would be preferred. The observation that women’s criteria for selecting a mate include social and financial status is born out by cross-cultural studies (Buss, 1993 in Gleitman, 1999).
A further factor in mate selection is the investment of time in parenting, as Darwin asserted in 1871 (in Geary, 2000): “When parental investment (by either parent) is a feature of the species’ life history, it is inextricably tied to the dynamics of reproduction, that is, to sexual selection”. The female focus on parental investment in mammals is said to be between 95 to 97%. This is due to the time investment of internal gestation and obligatory postpartum suckling (Clutton-Brock & Vincent, 1991; Trivers, 1972 in Geary, 2000). Thus it can be said that there was almost no paternal involvement in ancestral mammalians.
Paternal investment involves trade-offs (and this has been modeled mathematically) between reproductive and survival related costs and benefits (Trivers, 1972; Westneat & Sherman, 1993 in Geary, 2000). Thus for species where offspring will be significantly endangered if the father does not make investment, selection favours individuals who show strong signs of paternal investment. Through evolution this trait will become stronger and eventually the species will show high levels of paternal investment regardless of proximate social and ecological conditions (Westneat & Sherman, 1993 in Geary, 2000). This is the case for humans, evidence for which will be shown below.
Ever since Darwin asserted that there was “no fundamental difference between man and the higher animals in their mental faculties” (1871, p. 446 in Cartwright, 2000), ethological research has been carried out and generalised to humans. Modern research has focused on the much more specific questions of, for example, comparative cognitive ability and evolution of brain size (Sherry in Ciba Foundation Symposium, 1997). In terms of empirical validity this approach can be seen as a marked improvement on earlier comparative psychological research.