Humans, as a species, have been debating the validity of both sides of the nature vs. nurture argument for centuries. Much of what we do and who we are can be attributed to characteristics of our nature or our nurture. The theory of natural selection in evolution can be attributed to characteristics resulting from either nature or nurture that were selected as advantageous. Much of human development and evolution stems from the idea that our brains are far more developed than those of coexisting species. While the concept of evolution asserts that these species originated from the same common ancestor as ours, genetic mutations and natural selection have worked to divide our species by emerging characteristics, including brain size and function. However, despite the generic genetic ‘code’ of our human species, mutations and exceptions continue to occur frequently and result in genotypes and phenotypes that are atypical to the characteristics that have defined us as a species.
One strong example of this is microcephaly, a disease contracted in utero that causes underdevelopment of neuronal tissue in infants. These children are born with both smaller brains and skulls. Microcephaly first appeared as a result of incestual cousin marriages between Kashmiri migrants in Bradford, England and was found to be caused by a mutation in the ASPM gene which controls the neuron production in the brain. This was an important discovery for both geneticists and evolutionists that supported the idea that evolution is a difference of degree, not kind. While humans have greater brain tissue than other species, our genes for this phenotype are no different; we just have more repetitions of the same DNA motif that creates the proteins that build our neurons. These repetitions are directly correlated to the number of neurons in our brain compared to other species and are a strong indication of IQ.
A virus that has much of the world in crisis right now is the Zika virus, which is transmitted through mosquito bites and, in infected mothers, causes microcephaly in the early stages of fetal development. The direct route between a Zika infection and microcephaly has not yet been discovered, the two have merely been linked. The Zika virus has been identified as a single RNA strand that, having infiltrated a host cell, can multiply and spread, causing infection symptoms. When reflecting on the greater picture of evolution, and the idea that humans are characterized in part by our ability to grow more neurons than other species, viruses like Zika call us to question: Where on the scale of nature vs. nurture does microcephaly linked to the Zika virus come into play?
Some would say the Zika virus could be categorized under an emerging field in genetics: epigenetics. This would assert that the virus is working as an environmental factor, outside of the human genome, and somehow altering the expression of genes like the ASPM gene, resulting in an altered phenotype. Perhaps it even affects the same ASPM gene discovered by Geoffrey Woods’ Kashmir migrants back in 1967. If this is true of the Zika virus, the epigenetic form of it’s infiltration overrides any debate on nature versus nurture and demonstrates the harmonious way in which the environment can change the physical, natural aspects of our being.
Evolutionarily, we have uncovered that genetic mutations in distant ancestors led to larger brain sizes and over time, helped to distinguish the human species from our non-human primate relatives. We have also discovered that genetic mutations occurring randomly within our species can cause smaller brain sizes in infants with microcephaly. However, Zika demonstrates that there are external factors that may or may not affect our genes, that lead to the same results. Therefore, can we define these as nurture, our environment, playing into the expression of our nature?
Helpful video for understanding how Zika works:
Ridley, M. (2004). The Agile Gene: How Nature Turns on Nurture. New York: Perennial/HarperCollins.