About our writer
Rosie enrolled for the Cambridge Immerse Biology Programme in 2015. She is now studying Natural Sciences at Emmanuel College, University of Cambridge.
Human evolution is a subject that I really enjoyed studying in my first year. It is also a common subject for interview questions. A-levels prepare you well to answer more generalised questions about evolution but humans are different. Has culture and technology buffered us from the selection pressures that once drove us to evolve? Are we now stuck in an evolutionary rut?
When Darwin first described natural selection, he stated that three conditions must be present for selection to take place. Firstly, variation must be naturally occurring, secondly, there must be a heritable replicator to transfer the traits between generations and finally, there must be a differential “fitness” and therefore survival according to the traits inherited and their interaction with the environment. The question with humans is: “what is the heritable replicator?” Genes or Culture?
Our typical understanding of natural selection involves interactions between the natural environment and selection on genetic variation through differences in survival and reproduction. This is a very slow process that can take thousands of years. There are, however, faster mechanisms than natural selection to deal with this stress. The fastest way of adapting to a new environmental stress would be to adapt the physiology and development of the organism. This can lead to plastic responses to stresses that allow us to accommodate to stress during the lifespan. An example of this includes tanning to protect your skin from UV radiation if you live in a hot country (or even just go on holiday). These changes are not heritable but can occur quickly to protect us from immediate stresses.
The other mechanism is culture and technology. With sufficient resources, culture and technology can mediate almost all stresses immediately. If it is fast and adaptive in response to stress, the other mechanisms of adaptation are not necessary and hence we could say that human evolution is over.
Cultural transmission describes the passing on of behavioural strategies and inventions to other groups and generations. One individual invents the innovation and it is gradually taken up by others who teach it to their children and so on. Cultural transmission of information is often thought to be unique to humans, but it is in fact documented among many non-human primates as well. Cumulative cultural change on the other hand does seem to be unique to humans. This is when innovations are applied to another invention – i.e. innovation upon innovation, passed on to other individuals.
Culture can be both adaptive and non-adaptive. Many functional traits increase efficiency, safety and reproduction and will therefore be transmitted between generations. Other poorly adapted traits can also be passed on. An example of this is the QWERTY keyboard.
Studies have shown that other keyboards (e.g. the Dvorak keyboard) are much more efficient as a higher percentage of keystrokes take place on the home row.
For the Dvorak keyboard, 70% of keypresses are on the home row of the keyboard whereas only 31% of keypresses are on the home row with the QWERTY keyboard. However, before the invention of the Dvorak keyboard, the QWERTY keyboard was adopted by makers of the typewriter. Since everyone was used to this way of typing the QWERTY keyboard was later adopted onto almost all computers in the English-speaking world, despite there being much more efficient, faster ways of typing. This is an example of historical contingency. Culture can therefore be adaptive or contingent.
The advent of agriculture was a major turning point in the evolution of humans. It represented a key shift in human control over the environment and led to technological development, industry and population expansion. Humans abandoned their traditional hunter-gatherer culture and lived together in one place while growing crops and keeping cattle. It also led to the first property ownership and this consequently led to competition and war over land and resources for the first time. There was also a spike in the number of diseases. The poor hygiene and proximity to the animals led to a number of zoonoses (diseases that can be passed from animals to humans) while the new diet led to nutritional deficiencies. However, by increasing the environmental productivity per area of land, agriculture increased fertility and ultimately longevity and health by buffering environmental variation through a surplus of food. The new stresses produced by this new lifestyle were ultimately alleviated by further cultural development of things like trade networks, exchange and commerce.
The origins or agriculture (it emerged in many places over the world at a similar time) led to the domestication of animals for the first time. Over many generations almost all animals show similar changes in response to domestication. These include: smaller body size, smaller cranial size, tooth size reduction, reduced aggression and increased fat storage.
Interestingly, these changes also occurred in humans around the time of the adoption of agriculture. Did we possibly manage to self-domesticate ourselves?
Evidence for continuing evolution
While humans are unique in being able to shift most of our adaptation onto faster “cultural” mechanisms, there is evidence that this is not enough in some cases and this leads to residual cultural stress. There is clear evidence for some evolution since the advent of agriculture. For example, the allele causing sickle cell anaemia increased in frequency over areas plagued by malaria as a way of providing some protection to individuals who were heterozygous for this allele.
The new culture of growing crops and keeping cattle led humans to start drinking milk. All humans have the enzyme lactase as a baby – this is what allows us to digest our mothers milk. Once you stop drinking milk it is normal for this enzyme to stop being produced. However, soon after the new agriculture-driven society was created, a mutation that caused this lactase to persist into adulthood quickly spread throughout Europe. This lactase persistence allowed humans to breakdown milk from cows and other animals, creating a new food source that has since been adopted into a wide range of dairy products.
Since the evolution of the first Homo sapiens the genetic variation within the species has gradually increased. Genetic variation within a population is beneficial because it increases the likelihood that some people would be able to survive if there was a sudden environmental change. Analyses of genetic information show that there has been a recent increase in genetic variation in human populations. This might simply be due to demographic expansion or it could be adaptive evolution.
To consider whether humans are still evolving we need only to ask whether Darwin’s mechanisms are still relevant. Yes, there is heritable variation that is distributed across adaptive systems and yes, there are differences in reproduction and survivorship. Our knowledge and control of the world around us means that culture can buffer us from most environmental changes and hence there are few selective pressures which act on humans. Evidence from recent genetic selection suggests that some genes are still under the control of natural selection. The genes that are selected for help us to adapt to benefit further from the dietary and ecological changes caused by the cultural change after the origins of agriculture.