In 1858, Charles Darwin and Alfred Russel Wallace presented papers at the Linnean Society in London in which they jointly announced a theory of evolution by natural selection. In its simplest form, natural selection is a process involving reproductive success and the environment. If a particular inherited trait provided individuals an advantage in adapting to a particular environment, then these individuals would have a greater chance of survival and reproducing. Over time, the population would have more and more individuals who had inherited the trait, eventually resulting in biological evolution.
With advances in understanding genetics, paleoanthropology, and other disciplines, the mechanisms of natural selection are better now better understood. Evolution, and more specifically, the theory of evolution through natural selection, is currently seen as the basis for understanding the biological sciences.
In 1859, the scientific theory of natural selection as the mechanism driving evolution was made available to the general public when Charles Darwin published his book Origin of Species. While the book did not focus on humans, the implications were easily seen: since humans, Homo sapiens, were biological entities, then natural selection must have played a role in human evolution as it had in the other species. In his short entry on Charles Darwin in The Oxford Companion to Archaeology, Brian Fagan writes:
“The book brought Darwin enduring fame and provided not only a fundamental general principle for biology, but also a biological theory for the antiquity of humankind.”
With regard to the implications of Darwin’s theory, Matthew Johnson, in his book Archaeological Theory: An Introduction, writes:
“It implied that Homo sapiens was an animal like any other. Humans were no more ‘unique’, in principle, than any other species. The methods of study that were appropriate for the natural world, then, were logically applicable to the human world also.”
In 1871, Charles Darwin followed up his successful Origin of Species with Descent of Man and Selection in Relation to Sex. This book examined the possible role of sexual selection as a mechanism for human evolution. In his book Undeniable: Evolution and the Science of Creation, Bill Nye writes:
“Sexual selection is the second fundamental idea in Darwin’s theory of evolution, ranking next behind natural selection. Sexual selection is the process by which organisms of the same species select genes to be passed to subsequent generations.”
With sexual selection, evolution is less random and less determined by environmental adaptation. In other words, with sexual selection, humans exerted some influence on the direction of biological evolution.
In his book Nature via Nurture: Genes, Experience, and What Makes Us Human, Matt Ridley writes:
“Darwin considered it just as important as natural selection, perhaps more so in the case of human beings, but sexual selection spent most of the twentieth century in scientific exile.”
Darwin envisioned evolution by natural selection to be a relatively slow, gradual process. With sexual selection he felt that the process of evolution was faster.
Human evolution through natural selection is chiefly change in response to environmental conditions, while human evolution through sexual selection entails human involvement. In other words, early humans selected as their mates humans who had characteristics which were considered attractive or advantageous.
There are several key features of modern human anatomy which appear to have evolved due, at least in part, to sexual selection. These features would include the large and unusually shaped penis, female pubic hair, large female breasts, and large female buttocks. Since all of these are soft tissue structures, the fossil record provides no clues as to when this evolution may have occurred. These features may have evolved by the time of the first humans—Homo habilis—or they may have evolved much later, perhaps with Homo erectus or even Homo heidelbergensis.
With regard to the role of sexual selection and female breasts, Bill Nye, in his book Undeniable: Evolution and the Science of Creation, writes:
“Human males seek out females who appear to be highly fertile, which is a likely reason why human females are permanently enlarged breasts—unlike almost all other mammals.”
After bipedalism, one of the primary traits seen in human evolution is the increased brain size: during a period of 3 million years, brain size increased 350% and cerebral blood flow increased 600%. This suggests that human evolution selected for brain metabolism: increased blood flow is an indication of increased brain activity. The increased blood flow was most likely caused by increased synaptic activity and interconnection among neurons. In noting the rapid and rather dramatic increase in brain size during the 300,000 years just prior to the emergence of anatomically modern humans, Robin Dunbar, in his book Human Evolution, reports:
“This suggests that there was intense selection pressure for larger brains throughout this period, and this has significant consequences for the kinds of tools and other material culture we find in the archaeological record.”
For many paleoanthropologists, increased brain size may be an indication of increased cognitive ability which manifests itself in such things as complex tool-making, language, and abstract thought (vital to art and to religion). In looking at the Acheulian handaxes made by Homo erectus, Genevieve von Petzinger, in her book The First Signs: Unlocking the Mysteries of the World’s Oldest Symbols, writes:
“Double-edged hand axes have more aesthetic appeal and are also more efficient than single-edged cutting tools. Preplanning the steps for shaping a hand axe is a longer, more complex process, which means these ancestors had a greater capacity for memory, imagination, and spatial visualization. The toolmaker would have kept a clear blueprint of the finished tool in his or her mind, both before and during the making of the tool.”
The evolution of larger brains has resulted in some speculation over the role of sexual selection in the evolution of behavioral traits such as language and tool-making. Did early humans, such as Homo erectus, select mates who demonstrated the cognitive abilities to create stone tools? Genevieve von Petzinger writes:
“By one million years ago, some oversized hand axes began to appear that were too large and unwieldy to have been useful, but which had been created with care and precision. Possibly they were some sort of teaching tool, or had ceremonial function, or conveyed status; they may even have been used to attract mates (i.e., my hand axe is bigger and more symmetrical than his hand axe!).”
In a number of sites, the Achuelian hand axes associated with Homo erectus show no obvious signs of use, which has led some researchers to suggest that they may have simply been an impressive emblem for attracting mates. Christopher Seddon, in his book Humans: From the Beginning, writes:
“One interesting theory proposes that the axes were made to impress prospective mates. When a female saw a large, symmetrical axe, she might have concluded that its maker possessed the right attributes to father successful offspring. The axe, having served its purpose (or not) would then be discarded. This proposal has inevitably become known as the ‘sexy hand-axe’ theory.”
It should be noted that while many researchers assume the stone tools were made by men, there is no definitive proof that this was actually the case.
Whether the maker of the hand axe was male or female, if the ability to make this stone tool was a factor in mate selection, then the result would be evolutionary sexual selection for the cognitive ability to conceive and make these complex tools. As a result, those genes would have become more prevalent in human societies and would lead to greater intellectual capacity.
One of the human characteristics that is associated with increased brain size and cognition is language. In his book When Languages Die: The Extinction of the World’s Languages and the Erosion of Human Knowledge, David Harrison reports:
“Linguists believe, based on a large body of research, that humans come genetically pre-programmed with elements of language, allowing them to learn any language to which they are exposed.”
The human capacity for language is the result of biological evolution. Was the genetic mutation which led to language enabled by sexual selection?
It has often been pointed out that in many cultures, leadership is determined by the ability to use language and that male leaders usually have more wives and more children. Thus, the ability to use language may result in greater reproductive success. While this may be an interesting idea, it is based on the false assumption that leaders are always men.
The idea that sexual selection played a role in the evolution of human cognition and the emergence of language has resulted in some interesting hypotheses. It should be kept in mind, however, that these are only hypotheses—possibilities based on the interpretation of current data on evolution.
More Human Origins
Ancient Africa: Australopithecus
Ancient Africa: Homo Naledi
Early Homo Sapiens: A Brief Overview of DNA
Ancient Africa: Early Homo Sapiens at Pinnacle Point
Ancient Humans: A Short Overview of Homo Ergaster
Ancient Africa: A Short Overview of Homo Habilis
The Ancient World: An Overview of Homo Erectus
Ancient World: A Short Overview of Homo Heidelbergensis