Category: Evolution

I joined the faculty at Cedar Crest College in August 2009, so the class of 2013 is, in some ways, also my class as they are the students who were new to Cedar Crest, and spent four years learning all about it alongside me.  I don’t teach classes that freshmen take, so I didn’t actually see many of them until they were sophomores or beyond.  One student who came in that year, Lauren Della, is someone I ended up working on research projects two years in a row with, during her junior and senior years.  One of the great privileges of teaching is to watch students grow as they learn.  Grow in knowledge, grow in confidence, grow in leadership.  And all of those were definitely true of Laurian.


A lovely gift from a wonderful student.

When she graduated this past spring, Laurien gave me a wonderful gift, a ceramic brain with a figure of a man with a cane walking along the parietal lobe.  I love it.  It’s sitting on my desk, though a piece of it fell off (from the inferior temporal lobe) and people coming into my office all had to comment on my broken brain.  It was part of her Art Therapy show, and I love that she integrated brain and behavior so beautifully and creatively.  One of the many things the parietal lobe is responsible for is helping to understand and navigate the space around us, so the figure walking across the surface is especially fitting.


The parietal lobe.

It seems appropriate, then, to make the parietal lobe the theme of this series of blogs.  The parietal lobe is located on top of the brain, slightly toward the back and center.  We began the course by looking at the role the parietal lobe plays in consciousness.  People who suffer from unilateral neglect fail to be aware of information coming in from the opposite side.  So if someone has suffered a stroke that damages their right parietal lobe they will ignore things on their left side.  They will write on half a piece of paper, draw half a house, or eat half a pancake.  It’s not that they are numb or blind on the left side;  their sensory capabilities are not diminished.  They simply are not conscious of things on the left, and act as if any objects to their left simply do not exist.


An individual with unilateral neglect drew the flower on the right.

As I mentioned earlier, the parietal lobe helps us to organize and navigate space, not only in terms of what’s out there, but also how we orient relative to objects in the spatial environment.  Also, the parietal lobe also provides feedback for how our body is oriented in space.  So it’s easy to see why damage to this part of the brain manifests itself in an inability to attend to spatial information to the left side of the body.

Most of the sensory information that comes into the brain goes through the back half first to be processed, then is sent forward to the front half to be acted upon.  In addition to kinesthetic feedback from the muscles and joints, the back portion of the brain, including the parietal lobe, also processes information from some of the other senses, particularly touch and vision.  For many of our senses, information from the sensory organs is topographically mapped onto the brain.  The best example of this is the somatosensory cortex, which is an area in the parietal lobe, just before the central fissure and running down from the center to the side of the brain along the surface.  This is the part of the brain that receives touch information from various places around your body.  The entire surface of your body is mapped onto this area of the brain.  It is a distorted map, because some parts of your body, such as your fingertips, are more sensitive and thus require more space in the brain to process the information coming from them, than other areas, such as your back or your thighs.

somatosensory cortex

The parts of our body are topographically mapped in our brains on the primary motor cortex (in red) and the somatosensory cortex (in green). The somatosensory cortex is located in the parietal lobe, just before the central fissure.

Recently, researchers have discovered that the parietal lobe also plays an important role in numerosity or “number sense” seen in many species. Numerosity doesn’t refer to counting because that involves associating specific quantities with a symbolic representation in the form of a number, which is a pretty advanced cognitive capability.  Instead, what our number sense allows us to do is to estimate quantity in a general way, such as when we’re offered a choice between two slices of pie and know that even though we might want the bigger piece, for the sake of our diets we need to take the smaller piece.  It’s the sense that allows us to make “greater than” or “less than” distinctions.

Harvey, Klein, Petridou, and Dumoulin (2013) recently studied the number sense in humans specifically to determine whether or not there was a similar topographic organization in the brain for numerosity despite the fact that there were no specific sense organs associated with a number sense.  They presented stimuli containing different numbers of dots and measured brain activity with a high-field fMRI.  Different areas of the parietal lobe responded to different numbers of dots in the stimuli.  When the number of dots in the stimulus was small, they found the greatest level of activity in the most forward part of the parietal lobe.  As the number of dots increased, not only did the focus of neural activity move further and further back along the parietal lobe, but the total number of neurons activated by the stimuli decreased.  These findings help to explain  why we tend to be good at estimating small numbers of objects, but become less and less accurate as the total number of objects in the stimuli increases.  The authors suggest that our parietal lobe acts as a sort of internal abacus, which is an ancient calculator that represents numbers spatially (Lewis, 2013).

Though it may not seem so, this is an important sense.  For most species, their day is spent searching for and acquiring resources such as food.  The ability to estimate quantity, or even to judge one potential source of food as better in terms of the amount it can provide over another site helps to make the foraging process more efficient.  The less energy an organism has to expend in order to acquire resources to survive, the better, and even something as simple as being able to quickly judge the richness of a resource compared to another one can make all the difference.


Harvey, B.M., Klein, B.P., Petridou, N., & Dumoulin, S.O. (2013).  Topographic representation of numerosity in the human perietal cortex. Science, 341(6150), 1123-1126 doi: 10.1126/science.1239052

Lewis, T. (2013). Is “Numerosity” Humans’ Sixth Sense? 


Several years ago, a study by Ross, Owren, and Zimmerman (2009) made quite a splash.  Prominent scientists, including Jerry Coyne (who pens the excellent blog “Why Evolution is True”) wrote about it, and the study was even featured on news programs that appeared on the BBC.

The reason this particular study generated such interest is because it had long been hypothesized that laughter, some form of which is seen in most of the great ape species, was the result of evolution.  Many researchers, including Provine (1999) argued that laughter was likely present, probably in the form of some kind of panting behavior, in an ancestor common to all extant humans and great ape species.  It’s an idea that makes a great deal of sense.  We have known for a long time that chimpanzees will make a panting sound when tickled that sounds curiously like laughter.  Darwin even describes this behavior in his book on emotional expression (Darwin, 1872).  Orang utans and gorillas do the same.

So here is what Ross and her colleagues did to show that the behavior of laughter is similar across species of great apes, and it is quite stunning in its simplicity.

They tickled a lot of animals.

I mean, imagine yourself as a research assistant in this lab, and your boss comes and tells you that your job is going to be to tickle a bunch of infant great apes!  That sounds like the best job in the world to me.  Specifically, their job was to tickle 21 infant and juvenile chimps, bonobos, gorillas, and orang utans, and three infant humans.  And while they were tickling them, they recorded the sounds they made and subjected them to an analysis.

So, what did they find?

From Ross, et al (2009). This is the tree they constructed by comparing the laughs of several primate species.

As expected, they found some similarities in the acoustical structure of the laughter produced by the species under investigation. Using those similarities, they placed the species into a tree arrangement, which looks like this:

To construct this image, they analyzed the auditory profiles of the laughter from their test subjects.  Essentially, they found the greatest similarity in the auditory structure of laughter between bonobos and chimpanzees.  This is hardly surprising, since those two species are very, very closely related.  Their laughter was structurally similar to humans.  The other two great ape species, gorillas and orang utans, had laughter that was the least similar to human laughter (though it was still fairly close).

What is extraordinary about this particular chart is that it bears a striking resemblance to this:

This is the taxonomic tree for great apes.  In other words, Ross’s, et al (2009) laughter map matches the tree that is produced by analyzing the DNA of all these species, a genetic family tree, if you will, of the hominids.  It is important to note that none of the non-human species have a vocal apparatus that is capable of producing human-like speech, though there are structural similarities in the throat and larynx.

So, why did laughter (tickle-induced laughter, at any rate) evolve?  Provine (1999) suggests that laughter is all about social bonding.  The young of most mammalian species engage in rough and tumble play, and tickling is often an important part of this play.  The tickling produces mutual laughter, accompanied by the release of neurotransmitters that induce positive affect, and the resulting enhanced social bonding is obvious.  This behavior carries over into adulthood and permeates many social encounters. Informal research from Provine’s (1999) laboratory indicate that people laugh the most often in social encounters, many of which do not necessarily include any humor.

I have said in class that the main function of the mammalian brain is to help us navigate the environment.  Some argue that the main function of the human brain is to solve social problems.  I think they are one and the same–the main things we need to navigate in our increasingly complex environment are social relationships of one kind or another.  Our livelihoods, by and large, revolve around successfully navigating relationships with our families, our friends, our teachers, our bosses, our neighbors, and even strangers we encounter in our everyday lives (I consider driving a highly social behavior, for example, even though it doesn’t appear to be that at first).

The other thing that the Ross, et al (2009) study makes quite clear is that laughter in great apes is a distinct and clear behavior that likely serves some purpose (probably social bonding).  In other words, we are not anthropomorphizing the behavior when we hear it.  It really does seem to be laughter in the way we, as humans, understand it.  Though we can never know exactly what a non-human animals is experiencing, we can correlate the behavior that occurs with the laughter.

I want to close out this blog entry with a personal story.  I did quite a lot of research with my mentor at the Smithsonian’s National Zoological Park, in Washington, D.C.  They have several orang utans there, and my mentor was engaged in a research project with them.  The experiments we were doing, which were to look at the cognitive capabilities of the orangs, used touch screens so the orangs could make their responses.  When we were first starting out the project, we had to do a lot of troubleshooting.  This usually involved wheeling the apparatus up, putting a very obvious picture up, such as a big, red leaf, and trying to figure out ways to get the orangs to reach out through the mesh of their habitat to touch the apparatus.  This wasn’t such a problem with the females, since they have smaller hands and could just reach out.  There were other problems with the females, since the first time we rolled the apparatus up to one, named Bonnie, she reached out and punched the apparatus so hard she broke it.

This isn’t Junior, but I wanted to show you what their big, banana-fingers look like. The original picture appears at

The real problem turned out to be the male, Junior.  Male orang utans have very large hands, and these enormous, banana shaped fingers.  We though if we put the apparatus up close enough, Junior could poke his fingers out and make his responses.  So we started training that.  My advisor would often stand near the apparatus, and occasionally would reach in and try to guide his fingers to the stimulus to touch it.  After a few trials of this, he started moving her fingers around whenever she reached in, rather than letting her move his fingers.  We knew he was perfectly capable of touching the screen on his own, so we couldn’t figure out what he was doing until we let him just move her hand around, and he used her hand to touch the stimulus.

At this point, he sat back on his haunches, and started that odd panting laughter that they do.  It was clear to us that he thought this was hilarious.  Of course, once we realized what he was doing, and the fact that he was laughing over it, it made us laugh, too.  Aside from being a great story, his behavior also raises a tantalizing question about a sense of humor in species other than humans.  We know we have it, though humor is highly subjective, and highly complex in terms of behavior.  It’s incredibly difficult to understand what humor is to another species, though I think we got a very clear glimpse of it in the male orang we worked with.

There was a lot of laughter on that research project–orangs are really a joy to work with.


Darwin, C. (1872). The expression of emotion in man and animals. London: John Murray.

Provine, R. (1999). A Big Mystery: Why Do We Laugh? Retrieved 10/9/2012

Ross, M.D., Owren, M.J., and Zimmermann, E. (2009). Reconstructing the evolution of laughter in great apes and humans.  Current Biology, 19, 1106-1111 doi: 10.1016/j.cub.2009.05.028

As an undergraduate I took a seminar on feeding behavior in my senior year.  We covered a variety of topics during the semester, including some of the physiological mechanisms involved in feeding behavior, and emotions evoked from feeding.

I remember my professor once talking about cravings.  We of course asked her if there was any truth to the adage that pregnant women have strange cravings.  She told us that that was probably not the case, that everyone has strange food cravings from time to time, and that people just notice the cravings of pregnant women more.  This actually made a lot of sense to me.  We pursued the subject with her, and learned that research suggests that salt is the only biologically driven craving.  What this means is that when you crave something salty, your body actually needs salt.  Given what you know about how the nervous system works, this should hardly be surprising.  But we were kind of amazed at the idea that other cravings, like a craving for oranges, were probably not caused by a deficiency in vitamin C, but more likely the product of an aversion to a specific diet.  In other words, you do not crave grapefruit because you need vitamin C; you crave it because you are tired of all the stuff you normally eat.  I always meant to pursue the question of what cravings actually are, since I think they are very interesting, but, as always, it is hard to find the time.

As I was thinking about this blog entry, however, I decided to do a literature search on cravings, and discovered a 2002 article authored by the very professor I took the feeding behavior seminar with, Dr. Marcia Pelchat, who is now at the Monell Chemical Senses Center in Philadelphia, PA.    Marci played an integral role in my life in terms of my career, so it was a pleasant surprise to come across that, and a few others from her on the very topic long in the periphery of my interests.

In this particular article, Dr. Pelchat reviewed the similarities between food addictions and drug addictions, with emphasis on physiological substrates.  A craving is defined as an intense desire for a specific food or a drug, and, according to Dr. Pelchat, it is no accident that the word links two apparently different types of cravings.  Her research has shown that food cravings, while often viewed  negatively, may actually promote more variability in the diet, and thus serve an adaptive function.

Many links between food cravings and drug cravings are apparent.  Preferences for sweets and the self-administration of certain drugs are also linked.  Alcoholics prefer more concentrated sweeteners than non-alcoholics, as well as exhibit a preference for sweets, and treatment programs for people attempting to stop smoking involve chewing various types of gum.  Opioid neurotransmitters, dopamine, and serotonin are also involved in cravings.  Endogenous opioids and dopamine are most likely involved in rewarding addictive behavior (be the addiction for food or drugs) and serotonin may affect cravings for carbohydrates.  Indeed, activity of the orbitofrontal cortex is implicated in obsessive-compulsive disorder, and this area is linked to the reward circuitry of the midbrain.    Though this area has not, to date, been directly implicated in food and drug cravings, this area does receive significant sensory information associated with food intake, notably gustatory and olfactory information.  The hormone leptin may also play a role in cravings for alcohol in individuals going through withdrawal.

In general, then, cravings, regardless of what is actually being craved, all activate the same architecture in the nervous system.


Pelchat, M. (2002). Of human bondage:  Food craving, obsession, compulsion, and addiction.  Physiology and Behavior, 76, 347-352

As a graduate student, one of the first courses I was required to take was a proseminar on animal behavior, taught by Howard Topoff.  As an undergraduate, I had gotten a smattering of evolution, but probably like many of you, I did not really have a good grasp of what exactly it was.  That was soon to change in Howard’s course.  We spent quite a lot of time discussing reproduction and evolution, and how those came into play in animal behavior.  I recall one lecture where he showed us a species of fish with a giant eye spot near its tail, very similar to this guy here, the foureye butterfly fish.

The front end of this fish (eyes and mouth) are actually on the right side of the picture.  That giant spot you see is near the tail, on the left.  It gives this fish a very disorienting appearance, which is exactly the point.   Imagine being a predator looking for a quick meal, and seeing that big ugly eye flashing at you.  I’d be intimidated, too!

Hopefully, you can see the value in this giant eye spot.  However it arose (as a random mutation, or it served some other purpose, it is hard to say), those fish that had it tended to survive long enough to produce offspring.  In doing so, they passed this trait along.  It stays in the species as a feature whether it continues to be useful or not.  As long as it keeps predators away, it is useful.  If all the predators that used to come after it somehow disappear, the trait stays because there’s nothing selecting against it (i.e. there’s no pressure from the environment to do away with it because fish without it survive longer and produce offspring).

So this was the kind of stuff I was learning in graduate school.  But Howard was quick to caution us against “telling stories,” as he put it.  Hindsight is 20-20, and it is easy to come up with what seems like a plausible explanation for something to fit the adaptationist paradigm without any real evidence.  So, while it is a useful exercise, to consider why particular traits might have been selected, it should be done with a few caveats in mind.

This is the thesis of the scholarly article,  The Spandrels of San Marco and the Panglossian Paradigm:  A Critique of the Adaptationist Programme by Stephen Jay Gould (pictured here) and Richard Lewontin, which my professor had undoubtedly read by the time I took the proseminar in animal behavior from him.   Gould and Lewontin, in this 1979 article which is at least in part a critique of E.O. Wilson’s theory of sociobiology, argue that adaptationists and evolutionary psychologists sometimes go too far in attempting to explain every trait of an organism in terms of function.  According to them, organisms must be regarded as a whole organism, rather than as a conglomeration of individual traits that all evolved to solve a particular function for the organism.  By focusing on traits, Gould and Lewontin say, you lose sight of the whole.

They use an architectural metaphor to make this point.  The feature in question is a small triangle created when arches come together, known as a spandrel, pictured here.  This feature, often found in cathedrals and other large structures with arched ceilings, is usually filled with elaborate decorations.  We would, however, never analyze the spandrel before us, when we stand in such places, and think that they are anything more than a by-product of the arch placement, used to great effect by painters and mosaicists.  They are not the starting point from which the arches then become necessary. We have no trouble seeing the by-products in a non-biological system here, but when it comes to traits in organisms, we often engage in “telling stories” as my professor, and Gould and Lewontin, suggest.  This comes from the perspective that all the traits that make up an organism, from the shape of their limbs to the brain that gives rise to specific behaviors, are the product of and therefore constrained by the mechanisms of natural selection.  There is no place for the biological equivalent of a spandrel in this perspective.

If we stop and think about it, the idea that there could be by-products of natural selection, properties of the nervous system that emerge unexpectedly and unpredictably from a collection of traits, for example, makes a lot of sense.  I have an entire lecture on evolution and its role in behavior, in which I argue that everything that our nervous system does, evolved to help us navigate the world more efficiently and effectively.  I do not think this perspective is wrong, but I did allude to two properties of the nervous system in those introductory lectures, that fit into Gould and Lewontin’s perspective of emergent properties perhaps better.  Those two topics were mirror self-recognition and the appreciation of a beautiful sunset.  I think I could “tell a story” about each of these to show how they’re adaptive (and I think I even did).

But, lets take a step back and look at each of those as an emergent property of the nervous system rather than a trait that represents a particular adaptation to the environment.  So, the appreciation of a beautiful sunset arises in much more of a gestalt sense, from certain features of the nervous system, such as color vision and the experience of emotion.  With these two things in place, our big and powerful brains took these capabilities and used them in a way that went far beyond the sum of their parts, giving us an aesthetic sensibility that other species likely do not possess.  Similarly, the ability to recognize yourself in a reflective surface did not evolve for a specific reason; it is an emergent property of a cognitively and physiologically complex brain.  It is no surprise that species with larger brains tend to show this trait, and it develops in human children right after a substantial increase in the number of neurons and synapses in their brains.

So, a little food for thought. I could not resist the temptation of “telling stories,” because it is a fun exercise to look at a particular behavior and see into the past for a few moments.  But sometimes, it is equally important to take a step back from that and look at a behavior in a much broader context.

Next week, I will be posting about an article on ketamine, glutamate, and anti-depressants.


Gould, S.J. and Lewontin, R.C. (1979). The Spandrels of San Marco and the Panglossian Paradigm: A Critique of the Adaptationist Programme. Proceedings of the Royal Society of London, Series B. 205(1161), 581-598

Reiss, D., and Marino, L. (2001). Mirror Self-Recognition in the Bottlenose Dolphin: A Case of Cognitive Convergence. Proceedings of the National Academy of Science, 98(10), 5937-5942. 10.1073/pnas.101086398