Heritability, Vg, Ve, and Cranial Morphology in Humans and Neandertals.

1. Both your interview and your article mentioned the complex factors involved in Neandertal brain/cranial morphology.  What is the h² of facial form (or facial size) thought to be?  What does this tell you about the V_E and V_G of cranial morphology?

 Both your interview and your article mentioned the complex factors involved in Neandertal brain/cranial morphology. What is the h² of facial form (or facial size) thought to be? What does this tell you about the V­_G and V_E of cranial morphology?

When considering any trait in an organism, evolutionary biologist must try and determine how much of a trait is dependant on heritability (h²). This narrow sense ability determines a ratio or percentage that can be attributed to genetic factors (V_G­) by diving the genetic variation by the total phenotypic variation.

For cranial size, I found three different numbers for heritability in humans, and they are as follows (taken from Martinez-Abadias’s “Heritability of human cranial dimensions: comparing the evolvability of different cranial regions.”):

Height: h² = 0.34

Length: h² = 0.32

Breadth: h² = 0.28

When these numbers are analyzed as percentages, it is easy to see that cranial size in humans has low heritability. In other words, about two-thirds of variability in human skull size is due to environmental factors (V_E); genetic makeup only accounts for about one-third of variability in human skull size.

Our paper discusses a couple different environmental factors that may contribute to the variance in skull morphology, and they are climate, locomotor behaviors of the species, and genetic drift.

Neandertals were thought to live in northern Europe, an area of mostly cold climate.  However, when comparing morphology of Neandertals to humans today, they were most similar to those skulls of sub-Saharan Africa and so this theory was not exactly supported as the main environmental factor. They do make mention of nasal features corresponding to morphologies found in high-altitude organisms, which could give some (if only little) credibility to that theory.

Dental loading is also another theory considered to explain skull morphology, because the anterior teeth have more wear than posterior teeth. This dental loading would have to be considered an adaptive trait on which natural selection would have acted for many generations. On investigation of bite forces though, modeling shows that Neandertal bites were not particularly high and therefore provides a problem for supporting this theory of adaptive dental loading.

Genetic drift seems to be the most supported hypothesis in his paper, as the author ran a simulation using quantitative genetics and was unable to reject any statistical results. Further, he argues that fossil records support the genetic drift theory even more by stating that skull features do not appear all at once but rather gradually accumulate over the years.

2. Two of the three hypotheses proposed to explain the evolution of Neandertal cranial features are adaptive (natural selection), and one is “neutral” (genetic drift).  Explain these competing hypotheses and why the authors support the latter, non-ultra-Darwinian view.

This article presents three hypotheses for the evolutionary explanation for Neandertal cranial features: adaptation to cold climates, adaptation to anterior dental loading, and genetic drift.  The first hypothesis, claiming adaptation to cold climates, is based on the observation that the geographic range of the Neandertals was centered  in northern Europe, which was considerably cooler at the time of the Neandertals.  However, just because they experienced colder temperatures does not require that their cranial features are adaptations to these colder conditions.  Since most of these type of hypotheses focused on the nasal region. Studies have been done on the internal nasal dimensions.  The narrow superior internal nasal dimensions, tall nasal apertures, and progecting nasal bridges are typically found in high latitude recent humans and could be adaptations to cold climates.  However, the Neandertal nasal region does not appear to be an adaptation to cold climates and if they were adapting to cold similarly to presen-day humans, then climatic adaptation is not a likely explanation for their cranial form.

Another characteristic of Neandertals is that they tend to have more worn anterior teeth than posterior ones.  In addition to that, these anterior teeth showed a high incidence of enamel chipping, microfractures, and microstriations on the labial surfaces which suggest that Neandertals used their mouths like a vise.  The second hypothesis, the anterior dental loading hypothesis, states that the facial form of the Neandertal are adaptations to dissipate the high mechanical loads produced by such behavior.  However, the article states that since the facial features of Neandertals appear early in development they cannot be direct mechanical responses to anterior dental loading.  The article goes on to say that instead, these would have to adaptations produced by natural selection after the species performed the behavior for several generations.  Another issue the author has with this hypothesis is that biomechanical modeling suggests that Neandertals were not able to produce high bite forces, at least not high enough to promote any resistance, and thus their cranial form cannot be adapted to resisting high bite forces if they were incapable of producing them.

The author favors the genetic drift hypothesis for many reasons.  First is that Neandertals and modern human populations became isolated from one another around 350,00 years ago.  This would have caused the two to diverge from each other, even in the absence of natural selection through events such as changing allele frequencies.  They tested this hypothesis and estimated that both groups diverged 311,000 to 435,00 years ago, which closely matches dates derived from ancient Neandertal and extant human DNA sequences, which they say is expected if genetic drift is responsible for the cranial divergence.  Another strong piece of supportive information the author brings up comes from fossil records.  He says that Neandertal features, like modern human features, did not appear all at once, rather, they gradually accumulated over hundreds of thousands of years.  This is the expected pattern if genetic drift were responsible.

3. In an evolutionary sense, why is it informative to study dental and cranial morphology in hyraxes or non-human primates?

                Studying dental and cranial morphology in non-human primates can be informative because it allows us to learn about and describe general aspects of the body mass, diet, and behavior of a fossil primate. Paleoanthropologists use the comparative method to understand morphological adaptations in fossil primates; meaning they base their inferences on the fossil primates by comparing them to studies of form and function of extant primates. For example, if the fossil primate and extant primate seem to have similar dental characteristics then you can infer that they may have eaten the same types of foods. Dental characteristics are related to more than just diet though, they also help determine the body mass of primates. Large-bodied primates tend to have relatively large teeth and paleoanthropologists hypothesize that correlates to body mass in extant primates also hold for extinct primate taxa. So, teeth can tell us about the size and weight of fossil primates and extant primates.

                As mentioned before, dental correlates to diet in fossil (and extant) primates, specifically enamel thickness and dental morphology. Enamel thickness reflects differences in the physical properties of foods eaten by primates. For example, species with a diet composed of hard, gritty food such as seeds tend to have thick enamel. Species such as folivores eat leafy plant foods and tend to have thin enamel. The anterior dentition of many primates serves in the initial preparation of food for chewing. The functional signal for incisor and canines is seen in primates that use their teeth on hard substances such as wood, bark, and fruit. The projections of the incisors can also tell you how the primate eats its food. On the other hand, a primate that eats leafy materials passes its food directly to its premolars and molars (back teeth). It’s easier to chew up leaves with your posterior dentition than with your front canines and incisors.

Pic on Left: Chimps are omnivorous (so they have characteristics of both plant eating molars and sharp canines and incisors) and their diet mostly consists of fruits and plants, insects, and sometimes small monkeys (baby bushbucks, young baboons) and baby mammals (meat is only 5% of diet, males hunt for meat more than females). ( http://www.buzzle.com/articles/chimpanzee-diet.html)

Australopithecus afarensis are one of the early human forms from 3.85 and 2.95 million years ago in Eastern Africa. They had apelike face proportions (flat nose, strongly projecting lower jaw) and braincase (small brain, 1/3 of modern human brain), long arms with curved fingers for climbing trees, small canine teeth, and lived both in trees and on ground which helped them survive for almost 1 million years as the climate and environments changed. They mainly ate a plant-based diet; the remains of their teeth indicate they ate soft, sugar rich fruits but their tooth size and shape suggest they could have eaten hard, brittle foods too (fall back foods for when seasons didn’t have fruit possibly?). (http://humanorigins.si.edu/evidence/human-fossils/species/australopithecus-afarensis)

There is some experimental evidence that lifetime behavior differences, specifically which foods are eaten, can influence cranial form. Experiments on rock hyraxes investigated the effects of food processing on cranial growth and form. Lieberman and colleagues found that the maxillary molars of rock hyraxes are positioned directly behind the orbits as in humans, which make them more appropriate models for human mechanical loading patterns than some other prognathic primates. They found that diet and perhaps certain behaviors can influence facial size.

 

Picture on the left shows lateral views of human (top), rock hyrax (middle), and baboon (bottom) adult skulls scaled to same length. The entire molar row lies beneath or posterior to the plane of the orbits (dashed line) in humans and hyraxes, but not in baboons (which are a particularly prognathic primate). (http://ars.els-cdn.com/content/image/1-s2.0-S004724840400051X-gr1.jpg)

                Cranial morphology provides insight on both diet and locomotion in fossil primates. Paleoanthropologists can gain information on the relative bite force involved in mastication, or chewing. Primates that feed on hard materials (i.e. seeds, fibrous plant parts) tend to have larger muscles and muscle attachments on their skull and mandible.

                Information on activity patterns and locomotion can be determined by looking at the relative size of the eye orbits compared to the overall size of the skull. Nocturnal primates tend to have relatively larger eye orbits compared to their skulls and diurnal primates have relatively small orbits compared to the skull. The location of the foramen magnum approximates body posture and locomotion. In tetrapods, the foramen magnum tends to be located more posterior on the head but in upright primates, it is located directly under the skull.

                   Studying dental and cranial information in non-human primates and hyraxes is beneficial and informative to understanding the behaviors and environments of early human forms and how they evolved into the modern human form that we know today. By studying changes in dental morphology throughout non-human primates, we can infer their diet and methods of eating and compare their ways to humans. We can study when these changes occurred and in what species and possibly link the changes to when there was a significant climate or environmental change that caused species to evolve and new species to form. The same goes for studying cranial morphology. By comparing skulls between non-primates and humans, we can see the differences and similarities and then try to figure out why the differences that are there exist and possible explanations for the differences.

                In our paper, Weaver explains that there are three main evolutionary explanations for Neanderthal cranial morphology including, adaptation to cold climates (most have been found in northern Europe), adaptation to anterior dental loading, or genetic drift. Neanderthal features didn’t all appear at once; they gradually accumulated over a period of 300,000 years. A similar pattern may explain the appearance of modern human features. Neanderthals could have been isolated by genetic drift and this could have led to our modern human form. Studying dental and cranial morphology allows us to figure out a time frame of when these changes could have occurred, why, and explain why we look and behave the way we do today.

Reference: "Primate Origin", http://www.pearsoned.ca/highered/showcase/lehman/media/lehm_ch05.pdf

4. Define autapomorphy and compare and contrast it with symplesiomorphy and synapomorphy, citing a Neandertal skeletal example of each.

Autapomorphy is one type of an uninformative character as a derived character state that is restricted to a single terminal taxon in a data set.  While synapomorphic traits are also derived character states, synapomorphies are shared traits and are used to define monophyletic groups.  In other words, synapomorphic traits are seen in decedents of a common ancestor, but not in the ancestor itself.  Autapomorphic traits are used to distinguish organisms from others who are closely related, with a recent common ancestor, while cladistics uses synapomorphies to help group together taxa sharing a recent common ancestor as they tell us about relationships within a group.   Thus, autapomorphic and synapomorphic traits are informative occurrences, while symplesiomorphic character states are primitive features that are shared between a common ancestor and its descendants.  Symplesiomorphic traits are therefore phylogenetically uninformative and do not indicate details of relationships.

 The article, The Meaning of Neandertal Skeletal Morphology, summarized these trait classifications and their importance in cladistics and evolutionary character state analysis, “the appearance of derived features in the fossil record can pint to the action of directional natural selection or genetic drift, and the retention of primitive features can indicate stabilizing natural selection (Weaver and Klein).” A Neandertal skeletal example of an autapomorphic trait is that Neandertals show a greater convexity of the infraorbital plane, or in other words, the absence of infraorbital concavity (Weaver and Klein) which is indicated by the a more posterior placement of the inferior orbital margin.  This convexity appears to be unique to the Neandertal as Homo erectus shows a flat infraorbital region and modern humans are defined by a concave condition of the infraorbital plane (http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=12&ved=0CDgQFjABOAo&url=http%3A%2F%2Fpages.nycep.org%2Fed%2Fdownload%2Fpdf%2FNMG%252024.pdf&ei=ySlxUcGCKoeMyAH_vYGACw&usg=AFQjCNEYQpwU_rJQpUtR44ZI-GCgMOzdtw&bvm=bv.45373924,d.aWc).

An example of a symplesiomorphic or shared ancestral character state is the absence of a mental eminence, in other words the absence of a chin (Weaver and Klein).  The absence of a mental eminence is a character state is a shared by the Neandertal and its ancestors, unique to modern humans.  Upright walking is a synapomorphic character trait of Neandertal and its close relatives with respect to Homo erectus as the recent common ancestor.  

from https://www.google.com/search?client=firefox-a&hs=XIW&rls=org.mozilla:en-US:official&q=monophyletic+group+of+neanderthal+and+homo+sapiens&bav=on.2,or.r_qf.&bvm=bv.45373924,d.aWM&biw=1525&bih=756&um=1&ie=UTF-8&hl=en&tbm=isch&source=og&sa=N&tab=wi&ei=EDVxUbORMsLr2AXwrID4Dg#imgrc=2vcjL2wJhQYR1M%3A%3BLUgAtbm3K3K-OM%3Bhttp%253A%252F%252Fafarensis99.files.wordpress.com%252F2012%252F01%252Fape-phylogeny.jpg%3Bhttp%253A%252F%252Fafarensis99.wordpress.com%252F2012%252F02%252F19%252Fancestral-hominin-or-stem-hominid-part-one%252F%3B500%3B352

            5. What does Neandertal cranial morphology have to do with cognitive neuroscience in modern humans?

As a relatively new scientific field, cognitive neuroscience can gain a lot of understanding from the study of Neandertal cranial morphology, gaining insight into the events that lead to our great specialization and cognitive abilities. ‘Paleoneurology is the only available tool to analyze and understand human brain evolution (www.emilianobruner.it/pdf/Paleoneuro03.pdf), therefore  understanding Neandertal cranial morphology is the key to insight in the development of cognition and resulting abilities.

 By investigating the cranial morphology from Neandertal to modern humans, there is the possibility of gaining insight into how evolutionary modifications of anatomical traits altered the functional abilities of the brain.  Palaeoneurological data of Neandertals might provide insight into the relationship between internal brain organization and brain functions as the differences in the structure of the cortex seen between that of modern humans and Neandertals could indicate the reasons for our increased cognitive ability. 

By studying the differences in Neandertal and modern human brain anatomy, research may be able to correlate an increasing number of relationships between anatomical regions and function.  Providing insight into function-location relationships, can aid in the diagnosing and potential treatment of neurological disorders and/or cognitive impairments including degenerative  and functional diseases.  For example, modern humans have increasingly specialized language associated with the left hemisphere (http://psycnet.apa.org/?fa=main.doiLanding&doi=10.1037/0033-295X.96.3.492). Morphological changes between Neandertal left hemisphere and the modern human left hemisphere could provide insight into anatomical structures crucial to language, and perhaps could provide information for the restoration of language lost due to damage of the brain.