Biology 220 - Human Evolution Lab A comparison of primate skulls D. Sillman, Penn State New Kensington
Homo sapiens ('wise man'), the only human species surviving today, is characterized among primates by upright posture (bipedalism) and an enlarged brain. A comparison of primate skulls enables us to visualize some of the modifications which allowed and accompanied the development of these 2 important human characteristics. Thus, while increased fitness may be accomplished through increased muscle mass and strength, sharper and bigger teeth and claws and general ferociousness, there are many other tools for evolutionary success as reflected in human evolution.
The goal of this 2 week lab is to carefully observe, measure and creatively speculate about certain features of some representative skulls of the order Primates. The skulls provided include:
• Homo sapiens - modern humans • Homo erectus - an extinct human species. Fossils indicate this species lived roughly from 1.8 million years ago up until 250,000 years ago. The specimen provided is a reconstruction of a fossil. • Australopithecus africanus - ancient hominid. Fossils indicate these early humans lived roughly from 4.4 million years ago up until 1.5 million years ago. • Pan troglodytes - chimpanzee • Gorilla gorilla - gorilla • Pongo pygmaeus - orangutan • Papio doguera - baboon
Some other useful but confusing terms:
"Monkeys" refers to both New World monkeys (i.e. spider monkeys, howler monkeys) and Old World monkeys (i.e. macaques, baboons, rhesus monkeys). "Apes" refers to the gorillas, chimpanzees, orangutans and gibbons while "great apes" refers to just chimps and gorillas. "Anthropoid" refers to monkeys, apes and humans. "Hominoid" refers to apes and humans. "Hominid" refers to humans, both ancient and modern.
Some anatomical terms you should know: Anterior - toward the front Posterior - toward the back Superior - toward the top Inferior - toward the bottom Medial - toward the midline of the body Lateral - away from the midline of the body
Goals of this exercise: You will become familiar with some anatomical features of the skull which differ among the primates and, in some cases, attempt to quantify these differences. As you work, relate the differences you see to two hallmarks of modern humans; bipedalism and a greatly increased cerebral cortex (that part of the brain concerned with higher brain functions). The Primate Skull (use the anatomy books provided in lab to assist you in locating these structures)
The skull consists of the cranial bones (cranium), which house and protect the brain and the facial bones, which form the face and support the teeth. The bones of the adult skull articulate (join) firmly with adjacent bones at immovable sutures. The only movable bone in the skull is the lower jaw (mandible).
Cranium - Frontal Bone (Label on Figures I and II) (forms forehead, anterior portion of top of skull and roof of orbit of eye) Note: Overall size and 'verticality' of this bone. Note and measure: A bony ridge ('brow ridge') which may or may not be present above the orbits. Record your observation of a brow ridge in the human skull and the other primate skulls indicated in Table I.
Cranium - Parietal Bones (Label on Figure II) (paired right/left bones which meet along midline on top of skull) Note: Sagittal suture - separates the two parietal bones (but does not show on either Figure I or II) Note and measure: Presence or absence of mid-sagittal crest - a bony vertical 'keel' of bone formed at the sagittal suture. Record your observations of the sagittal crest in the human skull and the other primate skulls in Table I.
Cranium - Occipital Bone (Label on Figure II) (forms posterior portion of skull and contains the foramen magnum) Note and measure: Lambdoidal crest, formed at the lambdoidal suture between the occipital bone and the parietal bones. Is this 'keel' of bone present, or is the suture between these bones flat? Muscles that help lift the head attach here (you can feel these on yourself as you lift and lower your head). Record your observations in Table I.
Table I - Evaluation of Some Bony Ridges of the Skull in Various Primates (indicate + + for very prominent, + for prominent or 0 for smooth or absent) Gorilla Gorilla Homo Homo Australop. Pan Pongo Pongo Papio ♀ ♂ ♀ ♂ sapiens erectus africanus troglodytes pygmaeus pygmaeus doguera gorilla gorilla
Brow Ridge
Sagittal Crest
Lambdoidal Crest
Reflect: Expanded bony ridges indicate sites of muscle attachment - the larger the bony ridge the larger the attaching muscle mass. What muscular actions might be associated with the ridges you have just examined? Cranium - Occipital Bone (continued) Note and measure: Foramen magnum on the inferior aspect of the occipital bone. This large hole allows passage of the spinal cord. To quantify the position of the foramen magnum and evaluate how centered it is in relation to the entire skull, measure the following on the human skull and the other primate skulls:
• distance from midpoint of foramen magnum to posterior edge of occipital bone (Record as Posterior Distance in Table II) • distance from midpoint of foramen magnum to anterior edge of upper teeth (Record as Anterior Distance in Table II) • calculate (do the division!) and record the ratio (anterior/posterior) in Table II.
Important: These measurements should be made as straight, flat lines. It will be necessary to carefully remove the lower jaw (mandible) first and you may need to position the tape measure above or to the side to keep it flat.
Table II - Relative Position of the Foramen Magnum in Various Primates Gorilla Gorilla Homo Homo Australop. Pan Pongo Pongo Papio ♀ ♂ ♀ ♂ sapiens erectus africanus troglodytes pygmaeus pygmaeus doguera gorilla gorilla Posterior Distance ( c m ) Anterior Distance ( c m ) Ratio of anterior/ posterior
Reflect: Is there a trend in the ratio? What would the ratio be if the foramen magnum were centered exactly? How is the position of the foramen magnum different in hominids vs monkeys and apes?
Cranium - Temporal Bone (Label on Figures I and II) (forms side of skull around and above ears) Note and measure: Prominence of the mastoid process, a roughened 'bump' of bone which you can easily feel behind your ear. The sternocleidomastoid muscle, which turns the head from side to side and flexes the neck, attaches to the mastoid process. Indicate your observations of the mastoid process in Table III. Label this process on Figure II.
Note and measure: Presence of the styloid process, a thin 'spike' of bone anterior and medial to the mastoid process. Several muscles involved in swallowing and speech attach to the styloid process. Indicate your observations of the styloid process in Table III. Label this process on Figure II.
Table III - Evaluation of Bony Processes of the Temporal Bone in Various Primates (indicate + + for very prominent, + for prominent or 0 for small or absent) Gorilla Gorilla Homo Homo Australop. Pan Pongo Pongo Papio ♀ ♂ ♀ ♂ sapiens erectus africanus troglodytes pygmaeus pygmaeus doguera gorilla gorilla Mastoid process Styloid
Process
Reflect: Is there a trend in prominence of these bony processes? Why might they be more prominent in hominids?
Facial Bones - Zygomatic bone (Label on Figures I and II) (paired right/left bones which form the cheekbones and the lateral surface of the orbit of the eye) Note: Zygomatic arch - a thin branch formed by the temporal and zygomatic bones. You can feel this process on either side of your face. Chewing muscles attach to this bony arch. Label this arch on Figure II.
Facial Bones - Maxilla (Label on Figures I and II) (paired right/left bones which form the upper jaw and house the upper teeth. Note and measure: Prognathism - a measure of how much the upper and lower jaws project forward. To help you visualize prognathism, imagine a flat-faced dog (a pug or bulldog - low prognathism) vs a collie (long muzzle! high prognathism). To quantify prognathism (jaw protrusion), make these measurements of the maxilla and record in Table IV. You must carefully remove the mandible first, and perform this measurement from the inferior aspect.
• height of maxilla - distance from inferior border of orbit to base of middle molar. Move the measuring tape as necessary to keep it straight and flat. Record as 'height' in Table IV. • length of maxilla - distance from inferior border of orbit to anterior edge of front incisors. Again, keep the measuring tape flat. Record as 'length' in Table IV. • calculate (do the division!) and record the ratio of length to height in Table IV.
Table IV - Prognathism (jaw protrusion) in Various Primates Gorilla Gorilla Homo Homo Australop. Pan Pongo Pongo Papio ♀ ♂ ♀ ♂ sapiens erectus africanus troglodytes pygmaeus pygmaeus doguera gorilla gorilla Maxilla Height ( c m ) Maxilla Length ( c m ) Ratio of Length/ Height Reflect: As prognathism decreases, what happens to the ratio? What is the trend in prognathism as you compare modern man to other primates? to earlier hominids?
Facial Bones - Mandible (Label on Figures I and II) (the lower jaw - the only movable bone of the skull, it houses the lower teeth)
Note: the ramus (plural='rami') - the vertically projecting 'branches' on each side of this bone. Label the ramus on Figure II. Note: condylar process - the rounded knob which articulates with the temporal bone at the temporomandibuloar joint ("TMJ") Note: the body - the curving portion of the mandible excluding the 2 rami - forms the 'chin'.. Note and measure: To quantify the shape of the mandible on the human skull and the other primate skulls indicated in Table V, we will compare the width and length using these distances:
Remove the mandible carefully and measure from the inferior aspect:
• width of mandible - distance between inner edges of condylar (condyloid) processes. Record as width in Table V. • length of mandible - distance between condylar process and anterior edge of body of mandible. Record as length in Table V. • calculate (do the division!) and record the ratio of length to width in Table V.
Table V - Size and Shape of the Mandible in Various Primates Gorilla Gorilla Homo Homo Australop. Pan Pongo Pongo Papio ♀ ♂ ♀ ♂ sapiens erectus africanus troglodytes pygmaeus pygmaeus doguera gorilla gorilla Width of Mandible ( c m ) Length of Mandible ( c m ) Ratio of Length/ Width
Reflect: Is there a difference in the overall robustness of this bone in Homo as compared to other primates? Is the ratio bigger or smaller in Homo as compared to other primates? Does this fit with the change in prognathism illustrated by the maxilla? The Teeth (Label on Figure II)
Old World monkeys, apes and humans all possess the same dental formula - number and arrangement of teeth. Begin with the top front tooth, right side and move laterally to identify the following teeth:
• 2 incisors • 1 canine • 2 premolars • 3 molars (only 2 if 'wisdom' teeth missing)
Note and measure: Based on this dental formula and assuming all teeth are present, record the total number of teeth in Table VI. Note and measure: Description of the canine tooth - is it large and pointy or is it similar to the incisors? Record in Table VI. Note and measure: Presence of the diastema - a gap between the canines and the incisors. Indicate present/absent in Table VI.
Table VI - Observations of Teeth in Various Primates Gorilla Gorilla Homo Homo Australop. Pan Pongo Pongo Papio ♀ ♂ ♀ ♂ sapiens erectus africanus troglodytes pygmaeus pygmaeus doguera gorilla gorilla
Total Number of teeth
Description of Canine Tooth
D i a s t e m a present/absent
Reflect: Canines obviously function as flesh tearing devices in carnivores. Might they have other 'behavioral' adaptations? Relate this to their significant reduction in size in modern humans.
Measuring Cranial Capacity
Note and measure: A crude measure of cranial capacity, which directly reflects size of brain, can be achieved by filling the cranial space with beans and then quantifying this volume. Use the beans and scoops provided and carefully fill the cranial space through the foramen magnum. Be careful to hold the skull in a position to prevent the beans from spilling. When the space appears full, carefully pour the beans out through the foramen magnum into a large, graduated cylinder, using the funnel. The measurement you will obtain will be in milliliters, which translate directly to cc's - the appropriate unit to use for volume. Record the cranial capacity in cc's in Table VII.
Table VII - Measurement of Cranial Capacity in Various Primates Gorilla Gorilla Homo Homo Australop. Pan Pongo Pongo Papio ♀ ♂ ♀ ♂ sapiens erectus africanus troglodytes pygmaeus pygmaeus doguera gorilla gorilla Cranial Capacity 1000 430 (cc's)
Comparing 'Brain' to 'Face'
As we compare the primates in this lab, we see "more brain, less face". You have made several measurements to corroborate this statement, including cranial capacity and prognathism. In an attempt to summarize the comparison between these two skull components, view each skull in profile and estimate a ratio of cranium to facial region. Record in Table VIII.
Table VIII - Relative Sizes of Cranial and Facial Regions in Various Primates Gorilla Gorilla Homo Homo Australop. Pan Papio ♀ ♂ Pongo ♀ Pongo ♂ sapiens erectus africanus troglodytes pygmaeus pygmaeus doguera gorilla gorilla Ratio of Cranium to Facial Region 3 to 1 in Side View
Study Guide - Human Evolution Lab There will be a 35 point practical exam covering this material. For this exam: **be able to identify the genus and species of each skull studied in lab **be able to identify the following bones and structures on each of the 7 different primate skulls you observed in lab.
Bones Structures frontal brow ridge (if present) parietal sagittal suture sagittal crest (if present) occipital lambdoidal suture foramen magnum temporal mastoid process styloid process (if present) zygomatic zygomatic arch maxilla teeth: incisors, canines, premolars, molars, diastema mandible ramus, body and condylar process of mandible teeth: incisors, canines, premolars,molars diastema (if present)
Study Questions - Part of next Lecture Exam Use your scientific creativity to hypothesize about the evolutionary, anatomical and ecological significance of the differences you observe in the primate skulls.
1. Which skulls have prominent sagittal crests and brow ridges? Do these traits differ with gender? Relate this gender-specificity to possible behavioral significance of these bony features. Which skulls show a reduction in these bony structures? What might be the evolutionary significance of this reduction? Remember, the size and shape of bony ridges and bumps often reflects the muscle attachments to those bony landmarks. 2. Which skulls have raised ridges present at the lambdoidal suture? Which skulls are flat at this suture? Can you relate these differences to differences in posture between primates? 3. How has the position of the foramen magnum changed in modern humans as compared to other primates? Can you also relate this to posture? 4. Hypothesize on the significance of the differences you see in the size of the mastoid and styloid processes in different primates. 5. Rank the skulls observed in terms of extent of prognathism. In your own words, what does this mean has happened to primate faces? 6. Which bones have changed the most to reflect the change in prognathism you describe in the previous question? 7. Describe the differences in dentition you observe among the primate skulls. Hypothesize on the significance of the differences you observe. 8. Is it reasonable to directly compare (the actual cc's) cranial capacities of these primate skulls as we have done in lab? What is faulty about our procedure and how could you correct this so as to make better comparisons between skulls?