Handedness in Humans and Geladas

I thought I’d share a paper I wrote from last semester’s Comparative Psychology class on handedness in humans and geladas. Debating if I should actually follow through with this preliminary report and do some data collection. There is a YouTube video that accompanies this paper, which will appear on the bottom of the page.

Handedness can be defined as the unequal distribution of fine motor skills between the left and right hands. Simply put, handedness is the preference to use left, right or both hands when performing tasks. Humans are mostly right-handed across cultures. Here, I compare handedness in humans (Homo sapiens) to handedness in geladas (Theropithecus gelada).

Right-handedness is predominant across cultures in humans. About 85% to 90% of humans report themselves as right-handers (10). In studies of great apes, comparative analysis indicates that chimpanzees and bonobos exhibit population-level preference in right-handedness while gorillas and orangutans do not exhibit population-level preference in handedness (10). However, right-handedness seems to be uniquely human as no other primate species has such a clear bias (1, 13 & 14). The objective of this preliminary study is to elucidate whether geladas exhibit preference in handedness.

A male gelada at The Bronx Zoo.

Geladas are Old World Monkeys and can be found in the high plateau of North Central Ethiopia and Eritrea (9 & 2). Geladas are sometimes referred to as “bleeding heart monkeys” due to the presence of a naked, pink patch of skin on the chest of males and females that looks like an hourglass (2). They exhibit sexual dimorphism; males have manes and are slightly larger than females. The maximum lifespan of a gelada is about 19 years in the wild (5) and well over 30 years in captivity (9). Geladas are gramnivores; they are unique among primates because grass is their main food source but is occasionally supplemented by seeds, roots, and bulbs (3).

There are many definitions of handedness. Here, I define handedness as the preference of left, right or either hand when performing tasks. In humans, those who have a preference for using their right hand are right-handed while those that prefer using their left hand are left-handed. Those that prefer to use either hand when performing tasks are ambidextrous.

Approximate location of Broca's area and Wernicke's area. Photo from Wikipedia.

Handedness is due to the lateralization (asymmetry) of the brain hemispheres. Those that are right-handed have a more dominant left hemisphere, while those that are left-handed have a more dominant right hemisphere. In most humans, the left hemisphere of the brain is more dominant. The left brain hemisphere is also involved in language and is where Broca’s area and Wernicke’s area are located. Broca’s area is involved in speech production while Wernicke’s area is involved in language comprehension.

Right-handed preference is deep-seated in hominid evolution. Analysis of stone tools from Lower Pleistocene sites in Koobi Fora, Kenya and Middle Pleistocene sites in Ambrona, Spain showed that there is a consistent pattern of tools being produced by right-handed Homo habilis and Homo erectus (15). Preference for right-handedness may have existed as early as 1.4 to 1.9 million years ago, and might hint that the hominid brain had lateralized and was well on its way to becoming more specialized for different functions (15). Analysis of scratch patterns in incisors and canines from archaic humans (Sima de los Huesos in Atapuerca, Spain) and European Neandertals (Sima de los Huesos in Atapuerca, Spain and Vindija, Croatia) also showed that there was a persistent pattern of right handedness (6 & 7).

The prevalence of right-handedness in humans has a linear correlation with left hemisphere dominance (12). Since the left brain hemisphere plays an important role in language, brain lateralization leads to right-handedness. Thus, the prevalence of right-handedness in humans is a byproduct of human language.

The left-handed Ned Flanders from The Simpsons. Photo from SF Weekly.

The subjects for this study are from a captive population of geladas at The Bronx Zoo (Wildlife Conservation Society). There were 7 individuals present during the observations and all were male. Observations were done on two separate days between 12 P.M. and 3 P.M. The geladas were recorded using a video camera, and the recordings were later analyzed.

Geladas spend a significant amount of their time foraging (8). Site studies show that geladas spend 35.7% to 81.6% of their time foraging (8). There is a positive correlation between altitude and time spent foraging; more energy is burnt at higher altitudes, therefore more time is spent foraging (11).

A male gelada foraging with both hands.

My observations showed that geladas use either hand while foraging. They use either their left or right hands to pick up bunches of grass to eat. As my observation progressed, I became concerned that their repetitive motion of picking up grass and putting it in their mouths was not a sufficiently varied behavior to indicate handedness. Therefore, I also observed the geladas grooming.

A female gelada grooming herself.

There are two types of grooming in geladas: autogrooming (self grooming) and allogrooming (social grooming). Grooming is not only important for hygienic reasons but also important for strengthening social bonds (4). A study by Dunbar suggests that the major reason for grooming is the satisfaction of physical contact between the groomer and the individual being groomed (4).

From my observations, allogrooming starts with the individual being groomed initiating eye contact with the groomer. Then, the individual lies down and presents the area that needs to be groomed. Sometimes, a groomer simply approaches the individual they want to groom and starts grooming them. My observations showed that there is also no preference for which hand is used by geladas during grooming. The geladas used either hand for both autogrooming and allogrooming. The decision to use their left or right hand seemed to be determined by the area of the body that needed grooming.

From my observations, it seems that the geladas use either hand for foraging and grooming. The geladas at The Bronx Zoo seem to exhibit ambidexterity because they have no preference for their left or right hand, unlike humans who are predominantly right-handed.

Handedness is correlated with brain hemisphere dominance. For most humans, the left brain hemisphere is dominant and is also involved in language. Thus, the prevalence of right-handedness is a byproduct of human language. None of the geladas I observed at The Bronx Zoo exhibited preference for handedness. This is probably due to the fact that geladas do not have language; therefore, their left brain hemisphere is not dominant and does not lead to right-handedness. Besides foraging and grooming, more behavioral repertoire should be used in future analysis to further elucidate the preference in gelada handedness.









 References

 1. Cashmore, L., Uomini, N., & Chapelain, A. (2008). The evolution of handedness in humans and great apes. Journal of Anthropological Sciences, 86, 7-35.

 2. Dunbar, R.I.M., &; Dunbar, P. (1975). Contributions to Primatology: Social Dynamics of Gelada Baboons (Vol. 6). Basel, Switzerland: S. Karger AG.

 3. Dunbar, R.I.M. (1984). Reproductive Decisions. Princeton, New Jersey: Princeton University Press.

 4. Dunbar, R.I.M. (2008). The social role of touch in humans and primates: Behavioural function and neurobiological mechanisms. Neuroscience and Biobehavioral Reviews, 34(2), 260-268.

 5. Falk, D. (2000). Primate Diversity. New York and London: W.W. Norton & Company.

 6. Frayer, W.D., Fiore, I., Lalueza-Fox, C., Radovčić J., & Bondioli, L. (2010). Right handed Neandertals: Vindija and beyond. Journal of Anthropological Sciences, 88, 113-127.

 7. Frayer, W.D., Lozano, M., de Castro, J.M.B., Carbonell, E., Arsuaga, J.L., Radovčić J., Fiore, I., & Bondioli, L. (2011). Laterality: Asymmetries of Body, Brain and Cognition, doi:10.1080/1357650X.2010.529451

 8. Gron, KJ. (2008, September 3). Primate Factsheets: Gelada baboon (Theropithecus gelada) Taxonomy, Morphology, & Ecology. Retrieved May 11, 2011, from www.pin.primate.wisc.edu/factsheets/entry/gelada_baboon

 9. Hiller, C. (2000). "Theropithecus gelada" (On-line), Animal Diversity Web. Retrieved May 11, 2011, from www.animaldiversity.ummz.umich.edu/site/accounts/information/Theropithecus_ gelada.html

10. Hopkins, W.D. (2006). Comparative and Familial Analysis of Handedness in Great Apes. Psychology Bulletin, 132(4), 538–559.

 11. Iwamoto, T., & Dunbar, R.I.M. (1983). Thermoregulation, Habitat Quality and The Behavioural Ecology of Gelada Baboons. Journal of Animal Ecology, 52, 257-366.

12. Knecht, S., Dräger, B., Deppe, M., Bobe, L., Lohmann, H., Flöel, A., Ringelstein, E.-B., & Henningsen, H. (2000). Handedness and hemispheric language dominance in healthy humans. Brain, 123 (12), 2512-2518

13. McGrew, W.C., & Marchant, L.F. (1997). On the other hand: Current issues in and meta-analysis of the behavioral laterality of hand function in nonhuman primates. American Journal of Physical Anthropology, 104(25), 201-232.

 14. Steele, J. & Uomini, N. (2009). Can the Archaeology of Manual Specialization Tell Us Anything About Language Evolution? A Survey of the State of Play. Cambridge Archaeological Journal, 19, 97-110.

15. Toth, N. (1985). Archaeological Evidence for Preferential Right-Handedness in The Lower And Middle Pleistocene, and Its Possible Implications. Journal of Human Evolution, 14(6,) 607-614.

Modern Humans Are Still Evolving But Will Modern Men Get Wimpier?

Two interesting articles that went into my inbox today: Modern man a wimp says anthropologist and Darwin Lives! Modern Humans Are Still Evolving.

A cover illustration from Australian anthropologist Peter McAllister's new book entitled "Manthropology" and sub-titled "The Science of the Inadequate Modern Male." Photo from REUTERS/Hachette Publishing/Handout.

Modern man a wimp says anthropologist  from Reuters, summarizes Peter McAllister's book Manthropology: The Science of the Inadequate Modern Male. Using various data from Neanderthals and ancient aboriginal populations, McAllister concludes that modern men are inferior than their predecessor in running, jumping, and even sheer brute. "Any Neanderthal woman could have beaten former bodybuilder and current California governor Arnold Schwarzenegger in an arm wrestle", McAllister said. John Hawks from John Hawks Weblog has a lot to say about this in his post Is modern man a "wimp"? I think Hawks is spot on with his post.

The Time article, Darwin Lives! Modern Humans Are Still Evolving, is about a study in a contemporary Massachusetts population led by Stephen Stearns and his team of scientists from Yale University. Using correlations between women's physical characteristics such as height, weight, blood pressure, and cholesterol levels with the numbers of offspring produced, they found that "stout, slightly plump, but not obese" women tend to have more offspring as oppose to women with very low body fat count, as well as women with lower blood pressure and lower cholesterol levels.

Stearns explains that women with very low body fat count, low blood pressure and low cholesterol levels do not ovulate. Ovulation is, of course, when the matured ovarian follicle ruptures and discharge an ovum (or the egg). While human females ovulate about once a month, female chickens ovulate once a day. Of course, we refer to chicken ovum as chicken eggs.

Stearns and his team thinks that the characteristics for producing maximum offspring (stout, slightly plump, higher blood pressure and cholesterol levels) were passed down from mothers to daughters. Separating social and cultural factors using statistical analysis, Stearns and his team were able to conclude that these characteristics were passed down genetically. "Variations in reproductive success still exist among humans, and therefore some traits related to fertility continue to be shaped by natural selection" Stearns says. So women who have more children are more likely to pass down these traits to their offspring.

It explains the notion that modern humans are still evolving because variation in reproductive success means that there are selective pressure that favor certain traits. Interesting to note that while fertility is being shaped by natural selection (as per Stearn and his team's study), artificial selection is also shaping the width of female pelvis and the average brain size of infants being born. Mothers with a narrow birth canal (smaller pelvis size) puts both her and her infant in jeopardy during childbirth. The infant will be stuck in the birth canal due to restricted space. An infant with a larger than average brain size will also get stuck in the birth canal. Both scenarios will likely kill the infant and mother. However the advent of Caesarean section negates the restriction of a narrow birth canal and allows infant with larger brain size to be born.

So, we can see that modern humans are evolving through variation in reproductive success, female pelvis size and average infant brain size is also evolving through artificial selection though it is too early to say which direction the selection is favoring.

Garden of Eden: The Origin Of Modern Humans

A new genetic survey by Dr. Sarah A. Tishkoff of the University of Pennsylvania and her team has found that modern humans originated in an area between Namibia and Angola, on the coast of southwest Africa. Read the New York Times Article: Eden? Maybe. But Where’s the Apple Tree?

However, this harsh and inhospitable area is nothing Eden-like, they said. The origin of a species, like modern humans, is generally pinpointed to a place where its individuals show the highest genetic diversity. After comparing genetic data from populations around the world, the researchers pinpointed the population with the highest genetic diversity - somewhere on the coast of southwest Africa near the Kalahari Desert. The Bushmen or San people currently call this area their home.

The researchers have also calculated the exit point of modern humans that left Africa about 50,000 years ago. The exit point lies in an area of the African coast of the Red Sea. If the "Out Of Africa" theory holds true, this tribal group of about 150 people that left Africa at that exit point displaced other archaic Homo sapiens populations from different continents, giving rise to the current modern human populations.

"Whence Homo floresiensis? Clues From The Brain." By Dean Falk, Florida State University

During the 7th Human Evolution Symposium, Hobbits in the Haystack: Homo floresiensis and Human Evolution at Stony Brook, Dean Falk presented her lecture entitled "Whence Homo floresiensis? Clues from the Brain". She concluded that Homo floresiensis is a species of its own, LB1 did not suffer from microcephaly and Homo floresiensis is not a diminutive form of Homo erectus or Homo sapiens.


Falk commented that LB1 had a small-sized but "fancy" brain and that there was a "global reorganization" of the brain. Comparing LB1 with 9 modern human microcephalic skulls, Falk came to the conclusion that LB1's skull is nothing alike from a microcephalic and that LB1 was not a microcephalic.

A computer image depicts the brains (red) of a microcephalic modern human (left), and the fossil specimen of the Homo floresiensis (right). Microcephalics are also called "pinheads" due to the "pin-like" shape of their frontal lobe. Photo from National Geographic.

The shape of LB1's brain matches closely to those of Homo habilis. Falk thinks that LB1 might share ancestral history with Homo habilis and australopithecines. This is due to the characteristics that are found in LB1's brain.


Falk ended her lecture by talking about "insular dwarfism". Brain size and relative body size shrinks proportionately in the case of insular dwarfism. In the case of Homo floresiensis, its brain size did not shrink proportionately with its relative body size when compared to Homo erectus or Homo sapiens (based on the argument that Homo floresiensis is a diminutive version of Homo erectus or Homo sapiens)

Karen Baab and Kieran McNulty: Modern Procrustes

I found this while searching for news articles on "The Hobbits" or "Homo floresiensis". Sometimes taxonomy gives me headaches. I don't know why humans are so obsessive compulsive about putting things in categories. I guess that same reason is why I put labels on my blog posts.

Anthropologists Karen Baab from Stony Brook University Medical Center and Kieran McNulty from University of Minnesota published "Size, shape, and asymmetry in fossil hominins: The status of the LB1 cranium based on 3D morphometric analyses" in the Journal of Human Evolution last July. Click for .pdf version of the article.

Photo from the Science Museum

Using principal components analysis in shape space and in Procrustes form space, they shrank skulls of modern humans, fossil hominins and apes proportionate to their body so that they can use these new simulated models to compare LB1's skull. They found that LB1's skull closely resembles those of the genus Homo, too different to be those of modern human but close enough to be related to Homo erectus or something more primitive.

"You can't just make things bigger or smaller," McNulty explains. "When things change size, they need different mechanical properties." Or, there could be certain spatial requirements for the brain. A host of factors determine the proportions of the "shrunken" skull.

They concluded from their study that LB1 was more primitive than those of the Asian Homo erectus and perhaps LB1 (Homo florensiensis) was a descendent of a hominin population that predates Asian Homo erectus which underwent a process of size reduction.

Read the press releases from University of Minnesota and Stony Brook University.

I think a lot of people see LB1 as modern human because the skull looks like a shrunken modern human skull. They try to explain why LB1 is so small in stature, from microcephaly to dwarfism. But sometimes looks can be deceptive. LB1 might look like a modern human but it's not. I think that Homo floresiensis is a species of its own and it seems that more and more evidence is surfacing to prove so.

Questioning The "Out-Of-Africa" Model

The "Out-Of-Africa" Model is something that all Biological Anthropology students get drilled into their heads in Anthropology 102. Some Homo erectus migrated out of Africa to different regions. Archaic Homo sapiens then evolved from Homo erectus. Archaic Homo sapiens from Africa migrated out and displaced other archaic Homo sapiens from different regions. These archaic African Homo sapiens then evolved and became anatomically modern humans (Homo sapiens sapiens) Academically it is also call the recent single-origin hypothesis (RSOH), the Replacement Hypothesis or the Recent African Origin (RAO) model. These hypotheses came from the discovery of mitochondrial DNA, a kind of DNA that's only passed down by females to their offspring. Tracking this mitochondrial DNA, we can all draw our lineage back to a single female aptly named Mitochondrial Eve. Read about the "Out-Of-Africa" Model from a wiki entry.

The "Out-Of-Africa" Model. Illustration from Raritan Valley Community College

Well all that is fine and dandy. We were even told that the "Multiregional" Model is pretty much not accepted in academia nowadays. However, this dude begs to differ.

The "Multiregional" Model. Illustration from Wikipedia.

Akhil Bakshi, an Indian "explorer" led a multidisciplinary Gondwanaland Expedition across the 17 countries of South Asia, West Asia and Africa. He questions the "Out-Of-Africa" Model and in favor of the notion that anatomically modern humans were the byproduct of interbreeding between archaic Homo sapiens across different regions. He is actually in favor of the "Multiregional" Model.

Read his article here (text or .pdf). What do you think?