3.3 OVERVIEW OF HOMINID EVOLUTION

I. INTRODUCTION.

The classification and interpretation of fossil hominids is an ongoing debate. Explaining them is made more difficult by the variety of unfamiliar terms used to identify them and the many geographic sites in which they are found. Our discussion is deliberately superficial so that we may focus on the overall picture. There will be an emphasis on dental features.

Our organization in this unit is as follows:

(0) The great apes: chimpanzee, gorilla, and orangutan.

(1) The 'gracile' Australopithecines: anamensis, afarensis and africanus.

(2) The 'robust' Paranthropus group.

(3) Homo habilis 'handy man', often called 'early Homo.'

(4) Homo erectus 'upright man'.

(5) Neandertals, often called 'Archaic homo sapiens.'

(6) Homo sapiens 'wise man'.

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II. SITES IN AFRICA.

 

The 'gracile' Australopithecines, 'robust' Paranthropus group, and Homo habilis are known only from Africa. There are many sites, but we will consolidate them into two geographic locations: East Africa and South Africa. Each have their own unique geologic features.

East Africa sites are located along the Great Rift Valley, a site of tectonic and volcanic activity for millions of years. The most famous location is Olduvai Gorge where Louis and Mary Leakey worked for many years. Hadar, where Johanson found 'Lucy' is near the Rift Valley in Ethiopia.

 

Figure One Africa


The sites consist of sedimentary layers where fossils have been preserved over millions of years. There has been volcanic activity from time to time which has permitted dating of the various fossil finds by radiometric dating. The tectonic activity and weathering have exposed the layers. The rainy season each year brings new fossils to the surface.

South Africa sites are limestone deposits unearthed in quarries. Millions of years ago there were many caves, often exposed as sinkholes. Many animals, and a few hominids walked in, fell in, or were dragged in by predators. In time, rubble filled the caves and their contents became a tightly bound deposit known as breccia. Raymond Dart and Robert Broom studied South Africa sites.

Exposing these deposits is not a tidy affair. Dynamite is used in the mining process. Fossils must be painstakingly picked out of the breccia. There are no volcanic deposits, so radiometric dating is not possible. Dates are derived by faunal correlation with sites elsewhere where dates are known.

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III. THE GREAT APES

Why do anthropologists study them? They provide important clues about locomotion, behavior, adaptation, and lifestyle in our study of fossil hominids.

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The Orangutan is now known only from Borneo and Indonesia. They are large animals with considerable sexual dimorphism: males weigh about 200 lbs, females about 100 pounds. They tend to live solitary lives. They are primarily frugiverous (fruit eaters) and usually live in trees. These animals are on the brink of extinction in the wild. Orangutan, especially the adult male, tend to be solitary.

The Gorilla is the largest of living primates and are restricted to Africa. Males can weigh up to 400 lbs and females around 200 lbs. They are vegetarians and tend to live on the ground due to their size. Gorillas live in large family groups. They are an endangered species. Diane Fossey, who died in 1985, studied the endangered mountain gorilla.

Chimpanzees consist of two species, the common chimp and the bonobo. Male chimps weigh about 100 lbs; females about 80 lbs. Biochemical evidence suggest that they are our closest relatives amongst the primates. They live in large social groups. Social behavior is complex. Their facial expressions tell much about their inner emotional state. The bond between mother and offspring is life long. They share a dark trait with humans: they are capable of calculated murder.

Figure 2

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IV. GREAT APE JAW AND TEETH CHARACTERISTICS

Figure Two A and Two B

In figures two A and B are reproduced lateral profiles of the male and female gorilla. This portrays the dramatic sexual dimorphism between the sexes. Note specifically the sagittal crest and the overall robust character of the skull. The canine is dramatically larger in the male; it is much smaller in the female.

Next, compare in figure three the maxillary dental arcade in the gorilla, a gracile Australopithecine and the modern human. The striking difference in the size and form of the canine teeth is clear. The arch form in the gorilla has cheek teeth in parallel rows; in the human, the arcade assumes the form of a parabolic curve. In the gorilla, observe the diastema on each side anterior to the canine into which the respective lower canines fit. Similar diastemata exist in the lower arch to accommodate the maxillary canine--but those are distal to the lower canines. Such spaces appear in the deciduous dentition of about 50% of human children--a curious holdover from our hominoid past.

Figure Four Maxillary Dental Arcades

In hominids, the diastemata are lost and the dental arches are closes with all of the teeth in contact--with the exception of those documented in the earliest hominids. In afarensis, an early Australopithecine, the dental characteristics are intermediate between ape and human. The canine teeth are mildly sexually dimorphic. Small spaces are retained by afarensis anterior to the canine in the upper, posterior to the canine in the lower.

Examine the lower jaw of the gorilla and human in lateral aspect illustrated in figure four. In the ape, the teeth are used to crush and tear; hominid teeth are used to crush and grind the food. The cusps of the teeth remain sharp throughout life; in humans, the occlusal surfaces in time wear down to a flat plane. Hominid canines no longer protrude above the tooth row.

A question about function: how much side-to-side jaw movement would you expect with large, dagger-like gorilla canines? Mechanically, the elongated canines restrict side-to-side movement.

Figure Five Lateral View of Ape and Hominid Mandibles

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V. THE HOMINIDS.

The hominids split off from the apes about 5-7 mya. There are so far almost no fossils from that period to document the split. The time of the split is inferred by biochemical data.

As a group, including the 'robust' Paranthropus hominids in the next section, we can say the following: They are found only in Africa, they walked upright, they had small brains (about a fourth of ours in size), tool use was unlikely, and they had large protruding faces. They were highly dimorphic: males were about twice the size of females. The Australopithecines had a three million year career on Earth before dying out about a million years ago. In their time they were contemporaneous with Homo habilis and Homo erectus. Some were dead ends: they simply died out. The course to modern humans is not a single straight line.

 

The Taung child discovered by Dart had a deciduous dentition with the first molars just erupting. It is pictured in figure six. Dart concluded that it was upright walking based on the foramen magnum directed downward. You would suspect its age to be six years based on the eruption pattern, but that is not the case.

Apes develop more quickly than humans. Study of other Australopithecines reveals that their timing and pattern of eruption in more apelike than human-like. The Taung child's age is now estimated at three to four years

Figure Six Taung Child

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Afarensis, the most famous of which is 'Lucy', is earlier than Taung which is A. africanus. It is known only from East Africa.

Are all the names getting to you? Let us look briefly at the dental evidence for Afarensis.

The teeth and jaws of A. afarensis have several features that are intermediate between those of apes and modern humans. The arcade is omega shaped, intermediate between the box row in apes and the parabolic curve in humans. Afarensis had more dimorphic canines than humans. While the lower first premolar (Pm3 to paleontologists, remember?) in apes have only one cusp. Afarensis has acquired two. Check all of this out in figure six.

Mary Leakey uncovered remarkable evidence at Laetoli (East Africa) that afarensis was bipedal: a 75-foot long trail of footprints made by three bipedal individuals who crossed a bed of volcanic ash 3.5 million years ago. Pelvis, femur, and knee joint design is the anatomical evidence for upright walking.

Figure Seven A. afarensis

Last, look at the maxillary arch from A. africanus found in South Africa. It dates from 2.5 to 3.0 million years ago and represents the adult form of the Taung child, itself an africanus. Some suggest that africanus led to Homo habilis. As you look at the arch in figure seven, the dental anatomy of the individual teeth is clearly modern. The dental arch is still omega in outline, but is approaching the modern parabolic form. There is a trend toward more effective grinding in the design of the dentition.

Figure Eight A. africanus, maxillary arch

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These were first identified by Robert Broom in South Africa. They were sturdier, with heavier bones than their neighbors. They appeared about 1.8 Mya and disappeared about 1 Mya, the final chapter of the Australopithecines, the longest duration on Earth for any of the hominids. Their molars are enormous, the lower jaw is very large, and the entire skull has been reoganized to accommodate the massive chewing apparatus. There is a pronounced sagittal crest as in the male gorilla. The zygomatic arch is flared out to accommodate the massive temporalis muscle. Wear patterns on the teeth suggest a diet of very hard foods like seeds or nuts.

Figure Nine robustus and africanus

 

A. boisei, a robust from East Africa has molars with occlusal surfaces the size of nickels. Mary Leakey dubbed the creature 'Nutcracker Man!'

All of the robust forms are believed to be dead ends. They are not part of our heritage.

In contrast to the large cheek teeth, the anterior teeth in P. robustus are diminutive. Our maximum biting force is measured at 275 lbs. The maximum biting force estimated for 'Nutcracker Man' is 1700 lbs!

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called 'Early Homo.'

Fossils of early Homo, a hominid with a larger brain and more humanlike teeth, have been discovered in many sites in East Africa. It was announced by the Leakeys in 1964. Fossils assigned to H. habilis are quite variable. Like the Australopithecines, they are known only from Africa.

Most significant is the association of stone tools with H. habilis. The earliest of these date to 2.5 mya. Those stone tools are known as Oldowan because of their discovery at Olduvai Gorge by the Leakeys.

They are made by striking several flakes off of a rounded stone to give it a rough cutting edge. Both the core tool shown in figure ten and the flakes could be used. Such simple tools would have many uses: skinning an animal, for cutting or sharpening wood implements, to name a few. The tools imply foresight, abstract though in planning, and manual dexterity. Above all they represent intentional modification of the environment, a trait certainly evident in us.

Figure Eleven Oldowan Stone Tool

The teeth as a whole, but particularly the posterior teeth have undergone a reduction in H. habilis when compared to A. africanus, the most likely predecessor to H. habilis. The molar teeth present a more squared outline and are more identifiably modern than those in Australopithecines.

The canines in particular are reduced. The third premolar, which in Australopithecines tends to be as large as or larger than the fourth premolar has now become smaller, anticipating the modern condition.

The grinding teeth, the molars and the premolars, have generally lost their complex enamel wrinkling ('crenulation') in H. habilis. The function of the crenulation in australopithecines was probably to increase surface area, particularly in younger individuals where jaw strength was less. With advancing age the crenulations wear down to a flat surface area.

Figure Twelve Mandible of Homo habilis

Another significant feature in H. habilis is the increased brain size: 630 ml as compared to approximately 400 ml, about what is found in the chimpanzee.

Figure Thirteen A. africanus and H. habilis shown in frontal, lateral, and superior view

This has many implications for cognitive ability. The increase in the size of the brain and the diminution of the teeth is reflected in the reorganization of the skull. The cranial vault is now higher. The maxilla and mandible are decreased in size and bone thickness because the teeth are decreased in size.

Because the dentition is not as large, the muscles of mastication which move the teeth are not as heavily developed. Thus, in Homo habilis the bony face protrudes less (is less prognathous and more orthognathous), the area medial to the cheek bone (zygomatic arch) is smaller, the mandible is lighter in construction and the temporal lines reveal a smaller temporalis muscle.

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The evolutionary trends toward relatively greater cranial capacity, orthognathy, dental reduction, and greater body size which had begun in H. habilis, continued in its descendants. The classic diagnostic features of the skull cap are the thick cranial vault, prominent brow ridges, and the sagittal keel.

Homo erectus was a wide ranging species who lived on earth for over a million years. It originated in Africa and was the first to move out of Africa. The following are the established 'firsts' for erectus:

--First hominids out of Africa, going into Europe and Asia

--First to use systematic hunting

--First 'home base' camps

--First systematic toolmaking; a tool kit including the Acheulian biface, the 'Swiss Army Knife of the Paleolithic'

--First use of fire

--First extended childhood; sexual dimorphism now only 20-30%

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Figure Fourteen Left: top of H erectus and H sapiens skulls; Left: rear of H. erectus and H. sapiens skulls

Overall, the brain case of H. erectus is larger (about 1,000 ml) than that of H. habilis, but is smaller than modern H. sapiens (about 1350 ml). The skull is lower and the face still protrudes more than modern humans. The frontal and temporal lobes of the brain were apparently less developed.

The jaws and teeth of H. erectus are still large compared to those of modern humans but smaller than those of earlier hominids. In particular, the sizes of the back teeth of H. erectus are decreased compared to Paranthropus or the Australopithecines. The anterior teeth of erectus are larger than those of modern humans. The upper central incisors are distinctively shovel-shaped. Electron-scanning microscopic examination of erectus teeth reveals a pattern of extensive meat eating.

Figure 15 Mandible of H. erectus

One noticeable characteristic of H. erectus is the development of the large brow ridges above the orbits. They are not apparent in H. habilis and they are much smaller in modern H. sapiens (ourselves). What is their purpose?

The brow ridges are an evolutionary reversal. Wolpoff has proposed the following explanation: Changes in diet reduced the amount of force needed for the back teeth, and increased use of the incisors as tools led to an increase in the force exerted at the front of the jaw. Changes in overall brain and face size produced differences in the orientation of neck and chewing muscles. The crest at the back of the skull indicates the attachment of strong neck muscles. On the face, the various forces exerted by chewing and neck muscles meet above the eyes. The brow ridges of H. erectus are a structural adaptation to strengthen the face at this critical juncture. I have often thought of them as analogous to a football player's helmet and faceguard. What do you think? Could they have been sun visors like those found in automobiles of the 1950s?

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Anthropologists have a general idea when they came from, but bitterly disagree about what happened to them. They are best known from Europe. Hollywood has fashioned them into lumbering cave men, an early anatomist fashioned them into brutish louts, and anthropologists recently have fashioned them into a kinder, gentler creature. As I write this, I am reminded of this aphorism: We see the world not the way it is, but the way we are.

We do know that Neandertals were ruggedly built hunter-gatherers adapted to the European Ice Age. They had short stocky bodies dominated by enormous noses. They rarely lived past 45, had severe arthritis, worn teeth, and fractures like rodeo riders. They have left evidence of skilled stone tool technology called Mousterian tradition. They cared for their sick and elderly, had intentional burials possible with flowers, and possibly practiced cave bear cults. They suddenly disappeared about 35,000 years ago, replaced by modern humans like us.

Their brains were larger than ours, exceeding 1450 ml. The Neandertal skull was shaped differently than ours: it had a low crown and bulged on the side and back. A characteristic trait was the presence of an occipital 'bun' at the rear of the skull. Examine these features in the diagrams below.

Figure Sixteen from left to right Neandertal, modern H. sapiens and finally, Neandertal and modern human superimposed

Neandertals had a receding chin (or no chin), large cheek bones, prominent brow ridges connected across the nasal bridge, and a large nasal cavity (possibly an adaptation to the cold). The Neandertal face is extraordinarily robust. It may be constructed to counteract the considerable forces developed between the upper and lower teeth.

Neandertals have very large canines and incisors relative to their molars and premolars. Many individuals exhibit very heavy wear on the front teeth, possibly due to chewing hides or other materials. Neandertal face and jaw morphology may have been shaped by their use of the front teeth as tools.

The dentition as a whole is placed forward relative to the skull vault. Because of this anatomical change there is a gap, known to dentists as the retromolar space, to be seen in lateral view behind the last molar. This feature combined with proximal wear and physiological mesial drift would mean no third molar impaction problems.

Another frequent feature of Neandertal molars was taurodontism, meaning 'bull-like' teeth. It is occasionally seen in modern populations; in that setting it is often interpreted as a developmental disturbance.

Taurodontism is described as a molar with an elongated crown and apically placed furcation of the roots, resulting in an enlarged rectangular coronal pulpal chamber. What sort of functional adaptation they represent is not clear.

Figure Eighteen Taurodont Molar

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Anatomically modern Homo sapiens first appeared approximately 100,000 years ago. By 35,000 years ago, all fossil humans are anatomically modern in form. Remarkably, modern forms preceded Neandertals in many areas, and may have lived side-by-side as suggested by sites in Israel.

The features of the modern Homo sapiens is a description that you know well. The skull is high and well rounded. The forehead rises vertically above the eye orbits and does not slope as in Neandertal. The brow ridges are small. The face does not protrude very much (an orthognathic face) and a strong chin is evident. Modern humans have small faces tucked under enlarged brain cases.

The teeth progressively become reduced in size, a change that is concurrent with the reduction of the masticatory apparatus. This implies reduced chewing and probably a reduction in the use of teeth as tools. Canines become less sexually dimorphic and they no longer protrude past the occlusal plane.

With appearance of modern humans there is a burst of cultural expression: highly diverse tools in stone, bone and ivory; figurines, exquisite cave art, beads, and evidence of tailored clothing. Neural changes have made possibly rapidly spoken language; language makes possible meanings and beliefs which dramatically change culture. Language and symbolic thought are breakthroughs. Changes in culture allow humans to adapt to Ice Age conditions, permitting people to exploit many new areas of the Earth. Humans entered the New World 12,000 years ago when ocean levels are 330 feet lower than today. Over time, biology and culture interact to make modern humans a product of biocultural evolution.

Fully modern people now find themselves with a new arsenal of cultural possibilities and use them to adapt as the the last glacial period ends and the abundant megafauna die out. Humans settle down; they plant crops instead of foraging for their harvest. Their formerly hunted prey became their partners. From these events come metallurgy, urban living, and the of civilization to which we are heir today. From the simple act of planting seeds, humans become the most widespread and dominant species on earth fueled with the power of culture and symbolic thought.

..... CJ '99

Resources

Boaz, N and Almquist, A. Biological Anthropology. Upper Saddle River: Prentice Hall, 1997.

Boyd, R. and Silk, J. How Humans Evolved. New York: W. W. Norton and Company, 1997.

Campbell, B. and Loy, J. Humankind Emerging 7th ed. HarperCollins College Publishers, 1996.

Feder, K. and Park, M. Human Antiquity. Mountain View: Mayfield Publishing Company, 1997.

Larsen, C. et al Human Origins The Fossil Record, 3nd ed. Prospect Heights: Waveland Press, Inc., 1998.

Poirier, F. et al In Search of Ourselves, 5th ed. Englewood Cliffs: Prentice Hall, 1994.

Relethford, J. The Human Species 3rd ed. Mountain View: Mayfield Publishing Company, 1997.

Zihlman, A. The Human Evolution Coloring Book. New York: Barnes and Noble, 1982.