Renowned paleontologist Stephen Jay Gould remarked, “There’s been no biological change in humans in 40,000 or 50,000 years. Everything we call culture and civilization we’ve built with the same body and brain” (Templeton, 2010). Such ideas have led to the misconception that humans are not evolving, or that all the structures and genetic codes that make the human population “human” will remain the same for the rest of eternity. 

Gould is right in the sense that it is unlikely that human populations will develop new, complex structures in the coming millennials. Humans probably won’t start growing eyes on the back of their heads (though that would be nice at times, wouldn’t it)? This is an example of macroevolution, and the process doesn’t always occur at this level. Rather, evolution is occurring at the micro level, characterized by changes in the frequency of alleles (or versions of a gene) in a population’s gene pool. Evolution is driven by natural selection: variation in a population means that some phenotypes will contribute to an individual’s survival, affording those organisms greater reproductive fitness so that they could pass on those traits to their offspring, increasing the frequency of that allele in the population. Meanwhile, organisms with the unfavorable allele will die, unable to pass on their trait. Even though variation (and therefore, phenotypes unfavorable to our survival) exists in our population, humans have been able to modify their environment with technology, such as through inventing new medical treatments; for example, Type 1 diabetes can now be treated with insulin, allowing those people to reproduce and pass on any genes associated with Type 1 diabetes to their offspring. As a result, in developing countries, genes associated with the disease are no longer selected against (Understanding Evolution, 2021). 

Nonetheless, humans still face challenges to their survival and reproduction, leading evolution to look different now than it did twenty thousand years ago. People in densely populated areas face epidemic diseases in comparison to our hunter-gatherer ancestors (who didn’t have much human interaction), so natural selection has favored genes that protect against viral diseases and increase immunity. In fact, a study from Dr. David Enard at Stanford University uncovered that 30% or protein adaptations since humans’ divergence with chimpanzees has been driven by viruses (Genetic Engineering & Biotechnology News, 2016). In this way, humans are evolving at the micro level, which could potentially lead to changes at the macro level.

Demonstrating an example of this microevolution is the ability of about 70% of Europeans to tolerate lactose, a sugar found in milk. Currently, 68% of the world’s population is lactose intolerant (“Definition & Facts for Lactose Intolerance,” 2022). Those who are unable to digest lactose experience bloating, nausea and other gastrointestinal symptoms. Yet, many Europeans can drink milk without these symptoms. This is because they carry a regulatory change in the region of DNA that controls the expression of the gene that codes for lactase, the LCT gene. The change allows LCT to be switched on, and for lactase production to continue even as the amount of enzyme wanes; this change began to emerge in the population about five thousand to ten thousand years ago, which is when the domestication of milk-producing farm animals, such as cows, became more common. 

On the geographic time scale, ten thousand years ago isn’t that far back, considering that the first humans emerged two million years ago (Little, 2021). Yet, it took all those years for the change to become more prevalent in the European population. Thousands of years ago, lactose intolerance was more troublesome; milk was a staple, and there weren’t lactose-free alternatives like there are today. If people weren’t able to consume milk, then they faced malnutrition and the aforementioned symptoms, which would create a selective pressure favoring the regulatory mutation affecting the LCT gene. Now, lactose-free and dairy alternatives have allowed people to thrive even if they’re lactose-intolerant, diminishing that selective pressure as intolerance does not impact reproductive fitness. In regions where these alternatives aren’t as widely available, though, it is likely that such a mutation would be favored. 

Furthermore, this pattern of microevolution has enabled for the survival of humans at higher altitudes. As oxygen levels decrease as altitude increases, those living at sea level will experience faster breathing and a higher heart rate, a compensation method of the body.  Symptoms such as insomnia and nausea can occur as the altitude increases further. Yet, Tibetan highlanders are able to live 13,000 feet above sea level all year and Nepalese Sherpas (who are ethnically Tibetan) are able to climb the slopes of Mount Everest without the supplemental oxygen that many require. This is because they have evolved different physiological mechanisms. When lowlanders visit high altitude areas, their bodies begin to produce more red blood cells, which help transport available oxygen throughout the body; this may compensate for decreased oxygen levels, allowing breathing and heart rate to return to normal (Brophy, 2022). This is an example of phenotypic plasticity, or shifts in an object’s physiology or behavior dependent upon the environment it occupies, not upon a genetic change, or mutation (Fusco & Minelli, 2010). 

On the other hand, Tibetan highlanders have a change in a piece of DNA called EPAS1, which codes for a regulatory protein called HIF-2α, which senses oxygen and helps control the process of producing red blood cells, thereby playing a critical role in the body’s ability to adapt to changing oxygen levels (EPAS1 Gene: MedlinePlus Genetics, n.d.). This change causes Tibetans to produce fewer blood cells, as extra red blood cells can make blood thicker, preventing them from traveling through capillaries to oxygenate cells. Other traits of Tibetans, such as a higher breathing rate, and blood vessels which expand to allow better oxygen transport, may contribute to their altitude tolerance. Thus, mutations are still occurring in the genome; while humans are able to modify their environment, there are still selective pressures which increase the frequency of beneficial mutations in the population. 

Problems such as lactose intolerance and altitude sickness have been solved by humans. Yet, there is one major problem that has no cure: cancer. While many types of cancer, such as colon cancer and lung cancer, occur in older adults, recent studies have suggested that the cancer rates have increased in adults younger than fifty (Bever, 2023). Thus cancer, especially childhood cancer, directly impacts reproductive fitness. In the body, human cells mutate, giving rise to the disease; the process, within our own bodies, recreates the evolutionary process that allowed humans to adapt to their environment. For example, in the development of colorectal cancer, random mutations initially result in cells dividing out of control; in the hereditary form of the disease, mutations are found in the MSH2, MSH6, and MLH1 genes, which code for proteins involved in repairing mistakes made in DNA replication (National Center for Biotechnology Information (US), 1998). Chemotherapy or an immunotherapy targeting cells with a specific mutation may be administered; unfortunately, in many cases, some cancer cells may have randomly mutated a mechanism to evade therapies. In the case of colon cancer, resistant cells showed a desensitization of the plasma membrane, or where the plasma membrane had become less permeable to chemotherapeutic products (Colon Cancer: Overcoming Chemotherapy Resistance, 2022). The cancer cell population would then evolve as this trait is passed on to the next generation of cells. Thus, individual human cells are mutating, though in a way that has a detrimental impact on the human.  

However, it’s important to note that some tumors can accumulate millions of mutations; this creates significant variation in a population of cancer cells. All of these variations may not contribute to a greater chance of survival and are completely random; this leads to the neutral theory of evolution. This theory contends that many evolutionary changes are not caused by natural selection, but rather by genetic drift – the fluctuations of genotypes in a population due to random chance (i.e. individuals may pass away, leading to the eradication of an allele in that population). Whereas natural selection has a direction, genetic drift is random. Thus, the concept of natural selection may still apply to tumor cells, but definitely not in all cases (Neutral Theory: The Null Hypothesis of Molecular Evolution | Learn Science at Scitable, n.d.). 

Interestingly, recent research has suggested that humans may have evolved mechanisms to maintain the appropriate numbers of cells within tissues, as well as cell suicide and cell senescence  responses to inappropriate signals, such as those emitted by oncogenic mutations (Casás-Selves & DeGregori, 2011). Yet the fact that cancer remains in the population demonstrates that natural selection does not create the perfect organism; it favors organisms that can reproduce. For example, the tumor suppressor protein, p16, causes some rapidly developing cells to go into senescence, potentially curbing the development of cancer; yet, in a rodent study, the overproduction of p16 caused rodents to destroy insulin-producing cells, leading to aging and fatal diabetes. The p16 also creates a growing population of senescent cells, which could cause people to become vulnerable to cancer in older age – after they reproduce (Zimmer, 2008). Natural selection favors tumor suppressing mechanisms to a certain extent, as having too many of these mechanisms can cause health problems impacting reproductive fitness. Since cancer continues to eliminate members from the population, some suppressing mechanisms will be favored over others, leading to microevolution as seen so far. 

Thus, humans are evolving in minor ways. The direction of evolution may vary due to the interactions between people and their environments; nonetheless, there are still strong selective pressures, such as the prevalence of cancer and genetic disorders. This will lead to some phenotypes being favored over others. While it will take an extremely long time for drastic changes in the human structure, and even for speciation, to occur, small changes are underway. 

Examining biology from the lens of evolution can help us map these changes in not only humans, but also in other organisms – giving purpose to the subject of biology. Specifically, it can help us understand not only how biological systems are changing over time, but also why they’re changing: due to random mutations in the genome, a pressure stemming from the environment that favors certain variations, and natural selection which will eventually cause certain (and even new) traits to become more common in a population. This leads to change and therefore… evolution. The fact that the presence of all life on our planet results from this process is quite remarkable and unbelievable. 

Bever, L. (2023, August 16). Cancer among younger Americans is on the rise, new study shows. Washington Post. https://www.washingtonpost.com/wellness/2023/08/16/young-patients-breast-gastrointestinal-cancer/#:~:text=A%20study%20published%20Wednesday%20in,those%20age%2030%20to%2039.

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