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.