🎼 Let's talk about Sex 🎼

 

A lil' 90s throwback 

The more things change, the more things stay the same...

One of the items that is most interesting to me in evolution is sexual selection. The extremes that some organisms will go to and the adaptations made simply for reproduction that would otherwise be detrimental are so intriguing to me. The male fiddler crabs, whose oversized claw can be half their body size, don't really use this claw for anything outside of attracting a mate a digging a burrow. The bright colors of birds will almost always put the boring red, yellow, brown, and black pigments of mammals to shame; with the exception of the Mandrill whose vibrant coloring is indicative of their reproductive success.

Image Credit

Though complex thought and free will are considered some of the hallmarks of humanity, I love taking a look at some of the pieces of evidence that show how little control humans have over who we choose as our sexual partners. We like to believe that our choice in mate is a series of calculations and unique situations, but after viewing The Science of Sex Appeal, my position was reframed.

More than Just a Pretty Face

One of the first characteristics we notice about someone is their face and the orientation of their facial structures. Some of the simple examples of this are when you notice that a person's eyes are slightly farther apart than what seems "normal." This is called the "Golden Ratio." The Golden ratio, related to the Fibbonacci sequence, creates a predictable pattern of what we find attractive. The following video has you try a little "Brain Game" to see if your results end up being the same as the participants.

More than just the Golden Ratio, facial symmetry also plays an important role in sexual selection for humans. Slight abnormalities or illnesses during fetal development can cause a slight shift in the structures of the face. This slight asymmetry might signal poor health or genetic weaknesses. The proof is in the pudding: Look at the two images below and determine which one you gauge as more or less attractive.

image credit

(a) is the original image, whereas (b) has been changed to amplify the amount of asymmetry in the image. Most people agree that the image on the left is more attractive than the image on the right.

Not only does the level of symmetry in a face impact it's attractiveness, but we can also determine an individual's hormone levels by simply looking at their face! In the same article as above, composites of males and females were made and then masculinized and feminized to determine if participants showed a preference for either composite. Which images do you find more attractive?

The composites on the left have been masculinized, producing more angular changes in the jaw and hairline. The composites on the right have been made more feminine with softer angles in the jaw and hair, as well as thinner eyebrows. Most individuals find the female (top) on the right and the male (bottom) on the left more attractive.

But Wait, There's More!

About 50,000 years ago, humanity went through a bottleneck event that almost ended our species. For that reason, humans are more similar to each other than virtually any other primate, especially as human populations travelled further from Africa. This lack of diversity makes the chances of inadvertent inbreeding very risky, so the ability to smell individuals who are more genetically similar to you would be an evolutionary advantage. If you reproduce with someone who has a complimentary major histocompatibility complex (MHC), then the chances of your children having a wider immune response is greater. Women seem to find the scent of individuals who are not closely related to them less repulsive, and even pleasant near ovulation.

Say what?

At the dawn of civilization, the selection of mates would have frequently been done around a fire, or during times when a person's facial features were masked. What's left is the sound of their voice. What may be surprising is the level of impact that voices have on whether we find someone attractive. In speaking with Hazda tribal communities, researchers found that the men with the deepest voices were also those who were most likely to have the highest number of children.

As a result, additional studies have been done that show men also have a preference for higher pitched voices in women. Even more interesting to me is that the women's voices changed throughout their menstrual cycle.

And Everything Falls into Place...

Each of these situations gives men, but more so, women, a glimpse into the biological health of their potential partners. We like to say that everyone has different preferences, but the evidence seems to contradict that premise. A symmetrical face indicates healthy development; testosterone and estrogen give visual cues as to how much of each hormone you received during puberty; scent helps maintain incest avoidance, and; voice gives another clue as to a male's testosterone level, as well as a female's fertility. These buried drives steer us toward the most desirable partner we can snag, and we have much less control over the choices we make than most would like to believe. 

I guess we're not so different from our Mandrill cousins.

Come What May!

 Are we still Evolving?

The Four Principles

While working through our evolution unit, I will frequently get questions (sometimes with prompting) about whether or not humans are still evolving. I ask students to think about the four requirements needed for evolution to take place: Variation, Inheritance, Selection, and Time. They will bring up that even though humans share 99.9 percent of the same DNA, there are a number of different phenotypes on which natural selection could act. The concept that those phenotypes are generally inherited from parents to offspring is not usually argued, either. The amount of time required for evolution to occur varies, but for the sake of this argument, we are simply discussing whether under current circumstances, are humans continuing to evolve, so the amount of time is indeterminate, assuming that society continues in a relatively similar state that currently exists (specifically regarding industrialized countries for this argument). 

Selection?

That leaves the question of whether there is some sort of selection occurring. Most students cite that the survival rate for humans has been increasing over the last 70 years (crude death rate of 19 per 1000 to about 7.5 per 1000 from 1950 to 2020). The likelihood for an individual to die is low enough that natural selection would have little to no effect on the overall population. Furthermore, crude death rate accounts for all causes of death, including individuals who have already passed their genes on to the next generation. This leads most students to conclude that through vaccination programs and modern medical technology, that there is no selective pressure to act on any specific phenotypes. 

This is the point when I usually point out that avoiding being removed from the population is not the only way to be selected for. Many students overlook the role of differential reproduction. Though I don't show it to students, this discussion always brings to mind the (school inappropriate) 2006 film Idiocracy.

WARNING: The video uses profanity and some may find it offensive. A descriptive summary is below.

Essentially, the movie describes that those who are most able and thoughtful regarding the raising of their children, are also the most likely to have a small number of children, or elect to have none. Those who are having large number of children but providing little parental care are practicing r-strategist tactics, but with high survival rates of their young, the allele frequencies of these high success reproducers end up reflecting the general population more.

"We are Borg!"

The question of whether we will eventually "evolve" to incorporate technology into our bodies are no longer a question of "if," but rather "to what extent?" Here,  I am using the term "evolve" more generically as it is usually applied to technology rather than biological evolution. From wearing a FitBit to monitor your health to the eSight 3 to help people who are legally blind enhance their vision, the use of wearables is already commonplace. The physical connection and/or implantation of technology is also nothing new. Cochlear implants have helped the deaf recover at least some level of hearing. The ability to control prosthetic limbs with your mind allows entirely new levels of overcoming disabilities. These types of enhancements allow individuals to remove, or at least diminish, obstacles that would otherwise prevent them from living as full of a life. Jason Sosa describes a world where neural implants allow for watching movies as an immersive experience and taking vacations as a Total Recall style of adventure.

My concern with this is the ethics of providing access to these enhancements and how this could open or close doors, in much the same way as in Gattaca. Regarding the question as to whether this would be the next step in human evolution, I would have to say no, at least based on current technology. At the most, current trans-human developments could increase survival rates for those with a disability, but as discussed previously, their survival is already being significantly increased through modern medicine. The other major advantage would be the (potential) increase in access to mates. Many people would find it difficult to date someone who speaks a different language from themselves, so dating someone who is deaf if you do not know American Sign Language might be an obstacle. Use of a cochlear implant might remove that barrier and increase the possibility of reproducing. Since the implants themselves are not heritable, then I do not see the addition of implants as a selective pressure in humans. The presence or absence of implants will more likely be determined by wealth than by genetics. If anything, I see the use of implants changing humanity by broadening the gene pool and allowing a higher diversity of alleles since more people would theoretically be surviving.

What if...?

The main way that I could see the use of implants being the next evolutionary step for humans is if genetic engineering were used to create organic electronics/computers and wetware that could grow as part of us. Then there would be a phenotype that natural selection could act on. Until then, I think that the continued incorporation of technology will continue, but that it doesn't count as biological evolution for humans.

It's not just a hat rack, my friend!

 A large brain is arguably the greatest defining characteristic that sets humans apart from other animals and, specifically, from other apes. Only cetaceans, such as dolphins and other toothed whales, have similarly large brains. Though they also have a large neocortex, the prefrontal cortex of dolphins is much more underdeveloped than in humans, an area that accounts for decision-making, problem-solving, and acting with long-term goals in mind. A major question is how the human brain developed so quickly in its relatively short period of time.

(Image Credit)

Energy Hog!

One of the first obstacles to overcome allowing for a large brain is the amount of energy required to support it. Even though the human brain only accounts for 2% of the mass of the body, it consumes 20% of the body's energy. In order for a large brain to develop, a species would need a consistent food supply in order to support such a power hungry organ. This could also explain the slowed adolescent development that we see in humans compared to other hominids. The high energy needs can be spread out over a longer period of time and the energy needs would not be as immediate. Also, the fact that the prefrontal cortex is one of the last regions to fully develop and isn't completed until the late teens to mid- twenties. This would explain many of the poor decisions made by teens and (at times) their inability to grasp the consequences of their actions.

But I digress. The incredibly fast evolution of the human brain may be controlled, in part, by noncoding regions of DNA. Specifically, the ZNF588 gene is a transcription regulating protein that is active in human, but not in chimpanzee forebrains.  Errors in this gene will typically lead to mitochondrial diseases, which makes me wonder if the activation of this gene allows more energy to be accessed at critical times of brain development resulting in a larger cerebral cortex.

(Photo credit: National Institute of Mental Health)

Gene Mutations

Like everything in evolution, the raw material for natural selection is mutations. As discussed above, a mutation that allowed for the expression of a transcription protein turned on the ZNF588 gene. Two other genes are related to the brain development of mammals and possess mutations that coincide with the large brain development of humans. The microcephalin (MCPH1) and the abnormal spindle-like microcephaly associated protein (ASPM) have mutated faster than normal in human ancestors and are prime candidates for increasing the size of the cerebral cortex. The MCPH1 gene evolved quickly in the lineages leading up to Homo sapiens, whereas ASPM saw its fastest evolution occur in the time after human ancestors split from chimpanzees. Other genes, such as ARHGAP11B, also have been shown to influence the development of the brain after the separation from chimpanzees. 

Selection for Big Brains

The real question is, "How could humans develop such big brains in such a short period of time?" I believe that the answer can still be seen within the context of natural selection, but the discrepancy can be explained because brain development is such an unusual characteristic to quantify the rate of natural selection. The rate of selection is proportional to the level of advantage that the mutation provides. In this video about Rock Pocket mice, the mutation of their coat color spread relatively quickly because the advantage was so great. 

It's hard to understate the advantage of having a brain that can do so much. Even in regards to the large amount of energy it requires, this enhanced organ allows for problem-solving and predicting outcomes. The ability to communicate with others and express complex ideas could help them survive. The social aspect of pooling resources and developing complex hunting strategies to take down much more physically impressive game far outweigh the drawbacks of increased energy consumption and prolonged development. I think that the addition of mutations and the extreme advantage that they provide can explain the fast rate of encephalization that we see in humans.

Only Skin Deep!

What is race?

The concept of race has been a difficult topic to come to terms with over the last few hundred years. In recent history, race has been largely associated with skin color, but does the evidence support this claim? Today, I'd like to walk you through the current evidence of what biology concludes about the existence of distinct races, specifically based on skin color.

Where's Your Hair?

Before we can talk about changes in skin color, first, we have to look at one of the characteristics that set humans apart from the other apes: our lack of body hair. Amongst chimpanzees, our closest living relative, the young are born with white or pinkish skin. Their skin color is usually white underneath a thick layer of brown or black fur, with the exception of the feet, face, and hands, which are black. This would indicate that the adaptation of having dark skin occurred after the divergence of these two branches of the ape family. 

But why might that occur? What possible benefit would there be to gain pigment? The most logical explanation is that the hair was lost in order to improve heat regulation. Walking on two legs is more efficient than walking on four, but prolonged hunting and longer distances to travel would have produced more heat for early hominids, requiring a way to dissipate that heat. Hair loss on the body would have allowed for faster evaporative cooling.

The timeline (above) from Nina Jablonski's 2021 paper demonstrates a qualitative change in the amount of hair and the color of the skin in early hominids to present. Would the environment have provided the selective pressure needed to lose this hair?

Role of Climate

The graph below shows the average Earth temperature along with occurrence of different hominins as well as important advances in their development. Going back 9 million years, the Earth's temperature was warmer, but starting around 6 million years ago began to fluctuate regularly. When hair loss began around 3 million years ago, the Earth was still relatively warm. Even though the average temperature had been cooling, equatorial zones only dropped around 6 degrees C in comparison. This is a significant drop and shows that the tropics do change, but northern Africa would have remained quite warm and efficient evaporative cooling would still be needed.

The graph above from the Smithsonian Institute shows the average change in temperature, but doesn't demonstrate the significant drops in temperature that we would have seen in more recent times. Below is a diagram of global temperatures along with identified ice ages and the existence of various hominins over the last million years. The lack of body hair would have been a drawback rather than advantage in a cooling world, especially when competing with better cold adapted hominins, like Homo neanderthalensis. However, tool usage dating back to between 90,000 and 120,000 years ago shows the scraping of leather for clothing that could have maintained body temperature in these cool climates.

Selective Pressure

As the layers of hair continued to recede (as described by Jablonski), exposure to UV light would have opened the possibility for new adaptations to take hold. A common misconception, the risk of skin cancer or other burns would most likely not impact reproductive success enough to spread this mutation.

However, the production of two important chemicals would be enough to influence reproductive success. Vitamin D is important in maintaining bone density, and severe cases of deficiency can result in rickets, a malformation of bone structure. UV radiation is needed in order to produce sufficient levels of Vitamin D. Folate is also an important vitamin that is needed to make red blood cells. A deficiency of this vitamin could result in anemia, or during pregnancy can cause severe birth defects, such as spina bifida. Both of these outcomes could decrease reproductive success. Vitamin D is produced by UV exposure, but folate is broken down by folate exposure. This would cause a balancing act to produce enough melanin to maximize the amount of Vitamin D being made, while also minimizing damage to folate.

However, as Homo sapiens moved to new regions, the balance would shift. Fewer UVB rays meant less of a need for melanin to protect folate, but a greater need to synthesize Vitamin D. This results in a variety of skin colors across the globe, each being a best fit for a particular environment. The rendition below, created by Gail McCormick based on Jablonski's work, shows the predicted skin colors based on UV exposure.

The predictions made by Jablonski are supported by mutations in the human genome and tend to be found in native peoples of these regions. The image by Sarah Tishkoff from the University of Pennsylvania traces mutations of four major genes dating back nearly 1 million years. These mutations show some of the gains and losses of pigment in the history of our species

How does race fit into this?

Frankly, it doesn't. Many would argue that Tishkoff's image above shows that races are genetically different. When looking at the data closer, it shows that Africa has more diversity of skin color than all of the other continents combined. Robin Hammond shows this by capturing the beauty in the shades that exist across the land.

Photographer: Robin Hammond

Historically, there was a widely accepted belief that there were 5 races (mostly identified by skin color), and each of the members of those races were very genetically similar to each other. For a visual perspective, the similar colors in the image below are intended to indicate similar genetic background.

This popular concept would indicate that those of European descent are all genetically similar, whereas those of African descent would be genetically similar. More-so, that each "race" is genetically distinct from other races. Current genetic testing has shown how incorrect this popular concept actually is. A better representation rather than race, would be to organize by region. In the image below, the similar colors still represent similar genetics, but the dashed lines indicate a geographical region, rather than a distinct "race."

Though each region does tend to have similar genetics, there is a significant amount of variation within the region.  However, the small amount of variation between the regions blurs the lines of what one might consider "race." Only 7.4% of over 4000 genes were found only in a single geographic area.The following case study illustrates this concept. Drs. James Watson, Craig Venter, and Kim Seong-jin volunteered to donate DNA samples and have them compared. Colored bars represent alleles of a gene. Bars of the same color indicate the same allele. Though both Watson and Venter are of European descent, they each share more genetic similarities with Kim, who is of Asian descent, than they do with each other.

Does race mean anything?

Knowing a person's genetic ancestry can help medical professionals predict and/or diagnose conditions. Should medical professionals be keeping closer track of people's ancestries rather than a simple white male vs. black female to record drug efficacy or incidence of disease?  

Duana Fullwiley reflects on how her race changes as she travels from the United States, to France, and to Senegal as her skin color and features are interpreted differently in each country. The concept of race is multi-faceted and its importance will vary depending on who you ask. 

The history, practices, and culture of a people are important characteristics that help us define who we are.  The practice of basing that "race" off of skin color alone is indefensible.