40.2: Paleoanthropology

If you read about human evolution and the field of paleoanthropology, you will also encounter other primates you may consider old friends (e.g., chimpanzees) and a bunch of unusual primates who are extinct. You will also explore elements of the field of archaeology, the study of material culture. This last piece is pretty unique to research into hominid evolution, as there is no archaeological record for other organisms. The closest analog we get for archaeological materials in the rest of the living world is ichnology, the study of trace fossils. Archaeology is divided into two subfields. Historical archaeology explores human culture after the development of writing. Prehistorical archaeology explores everything else. Archaeology is typically a subfield of anthropology, the study of humans.

Paleoanthropology is a confluence of paleontology and anthropology that seeks to understand our human ancestry and associated culture. The field seeks a holistic exploration. This means that paleoanthropologists are interested in human biology, social structure, cultural development, material culture (archaeology), linguistic development, and more.

There have been many rather famous paleoanthropologists. This is because the field tends to engender a good deal of interest among people outside of the scientific community. Famous names include many members of the Leakey family. Louis and Mary Leakey and their son Richard Leakey who, with his partner Meave Leakey, have contributed much of what we know about our shared African ancestry. Their work in locations like Olduvai Gorge, Tanzania and nearby environs has provided important windows into how humans evolved, migrated, and are related to our primate cousins. Ian Tattersall, a curator at the American Museum of Natural History in New York City, has worked to understand more deeply our connection to current primate cousins but also to standardize terminology in the field. Still others, like Sonia Harmand, have dedicated their research to studying human cultural development, as exemplified in her work with stone tools.

Paleoanthropology is a dynamic and very young field. New insights into our human past are being published almost monthly. To keep up with these changes, refer to two particular locations included in the section on further reading at the end, the Smithsonian Institution’s human evolution program and the Institute of Human Origins at Arizona State University.

Hominid phylogeny terminology

As you dive into the study of human origins, you will encounter a bunch of confusing terms. We will begin by outlining some of these here.

Primate – Primates are a large group of social mammals that arose between 85 Ma and 55 Ma, probably from small tree-dwelling mammals. Shared characteristics of primates include relatively large brains, visual acuity, color vision, dexterous hands (prehensile with opposable thumbs for grasping), and large ranges of motion in their shoulder joints. Primates are a part of the order Primata, which contains two suborders, strepsirrhines and haplorhines. Humans are a member of this latter suborder.

Figure \(\PageIndex{1}\): Skeletons of members of the ape superfamily, Hominoidea. (Image: Public Domain; Wikimedia)

Hominidae – The hominids are the human taxonomic family. It is more colloquially referred to as the “great apes”. The term “hominid” is sometimes also used generally. Within this family, there are currently eight extant (living) species, contained within four genera. These species include us, Homo sapiens, Orangutans (Pongo abelii, Pongo tapanuliensis, Pongo pygmaeus), Gorillas (Gorilla gorilla, Gorilla beringei), and Chimpanzees (Pan troglodytes, Pan paniscus).

Homininae – The hominins are our subfamily and refers specifically to “African Apes”. This subfamily is divided into two “tribes” (the term is no connection to that which is used for indigenous human groups), Hominini and Gorillini.

Figure \(\PageIndex{2}\): Evolutionary tree of the family Hominoidea. The subfamily Homininae is highlighted and Gorillini and Hominini are tribes within the subfamily Homininae. Humans are more closely related to members of the genus Pan (Chimpanzees) than to gorillas. (Image: CC BY-SA 3.0; By EoD – Own work, Wikimedia)

Hominini – The hominini is a taxonomic tribe that includes our human ancestors and chimpanzees – two genera only. Sometimes chimps (Pan sp.) are separated out for their own tribe. Ultimately, the search for a common human ancestor between our close cousins chimpanzees extends to about 4-8 Ma (Patterson et al., 2006) but the split probably happened around 6 Ma. Once this split occurs, the phylogenetic tree is entirely focused on our ancestral lineage, all of whom are now extinct except for us. These genera include Homo and Australopithecus.

Figure \(\PageIndex{3}\): A Model of the phylogeny of Hominini over the past 10 million years. A pink line for Gorillini splits off around 8 Ma, a purple line for Pan splits off around 6 Ma, and Homo and Australopithecines continue on to today, represented by a lone species, Homo sapiens. (Image: CC BY-SA 4.0; Dbachmann – Own work, Wikimedia)

Did I Get It? Who are Hominids?

Exercise \(\PageIndex{1}\)

Which of the following phylogenetic terms refers to a taxonomic tribe of hominids?

a. Hominidae

b. Hominini

c. Homininae

d. Hominid

Answer

b. Hominini

Hominid Skull Terminology

Below there will be a good deal of discussion that uses hominid skull development to discuss hominid evolutionary change. Certainly, there are other anatomical features to use, when there are more complete skeletal remains. Skulls and skull fragments are probably the most common single fossil, so they are used in this case study. Below is a summary of some of the major parts of hominid skull anatomy.

Figure \(\PageIndex{4}\): A summary of hominid skull features, comparing Human and Australopithecine skulls. (Image: Public Domain)

If we look in detail at parts of the skull, some of the major areas become important in the story of human development. The first of these is the mandible, or lower jaw. A key difference between apes and humans in this feature is sheer size. Apes eat a great deal more tough, fibrous material than humans do, the chewing of which requires a more robust lower jaw. Humans also developed an accentuated chin structure to help provide some additional strength to what is a much weaker area of our lower jaw.

Figure \(\PageIndex{5}\): Mandible, or lower jaw. (Image: Public Domain)

Figure \(\PageIndex{6}\): Dental pattern of primate jaws, including humans. (Image: Public Domain)

The arrangement of teeth in apes and hominids is very similar. The dental arrangement includes molars and premolars for chewing, incisors for cutting, and canine teeth for tearing. Teeth are also one way that apes, as opposed to humans, can display sexual dimorphism (differences between genders). Male apes tend to have much larger canine teeth than female apes. There is no real advantage to the larger canine except for sexual selection, with females giving preference over one male for this particular feature over another. While certainly humans do notice one another’s teeth, mates are not selected based upon the size of their canine teeth. One other major difference between ape and human teeth is enamel thickness, with human tooth enamel being much thicker.

Figure \(\PageIndex{7}\): The dental arcade is made up of the teeth in the upper jaw of a primate. One of the key features is the diastema, a gap that exists for the large canine teeth. (Image: Public Domain)

On the upper jaw, the dental arcade provides additional areas where we can look for physiological changes in hominids. The shape of the dental arcade is an obvious starting point, where humans have a much more V-shaped arrangement and apes a more U-shaped arrangement. This is largely due to the shortening and flattening of the face through evolutionary change.

The other large difference here is the diastema, or the gap between the premolars and incisors where the large canine teeth reside. Humans have no diastema while hominid ancestors do.

Three other skull features are related to chewing, or lower jaw motion. These are the zygomaticus arch, the sagittal crest, and the supraorbital brow ridge.

The sagittal crest was lost to hominids as the demands of chewing became less strenuous. Because these crests were a primary area of attachment for the temporalis muscles that control the lower jaw, they could be quite pronounced in very early hominids.

Figure \(\PageIndex{8}\): The sagittal crest on a primate skull indicates the presence of very strong jaw muscles. (Image: Public Domain)

Figure \(\PageIndex{9}\): Zygomaticus arch. (Image: Public Domain)

The zygomaticus arch is much larger in earlier primates than it is in humans. This is due to the need for much thicker and robust temporal muscles. These muscles attached to the lower jaw through and behind the zygomaticus arch and then attach to the side of the cranium. They are the muscles that move the lower jaw and, the larger they are, the more powerful the chewing ability of the organism.

Figure \(\PageIndex{10}\): Supraorbital brow ridge. (Image: Public Domain)

The supraorbital brow ridge is one of the last skull features lost in hominid evolution, with humans no longer displaying it at all. This structure provided strength to the facial portion of the skull. As the powerful temporal muscles placed a good deal of strain on the face as they pulled on the cranium while food was being masticated. This ridge kept the eye sockets from collapsing under this stress.

Other features provide support for changes in locomotion, specifically the evolution from walking on all fours to mostly on the hind legs to full bipedalism, or walking upright on two feet. Today, humans are bipedal. Skull features that accompany this progressive change in stance include the nuchal area and the foramen magnum.

Figure \(\PageIndex{11}\): Nuchal area, where the neck muscles attach to the back of the skull. (Image: Public Domain)

The nuchal (pronounced “new-kul”) area is located on the base toward the rear of the skull. This is the area where neck muscles attach to the skull. Our primate ancestors have a much larger nuchal area. This is because much larger muscles are required to keep the individual looking forward when the spine is situated so far back in the skull.

Figure \(\PageIndex{12}\): Foramen magnum, where the spinal cord enters the brain. (Image: Public Domain)

The foramen magnum is the hole in the base of the skull where the spinal cord enters. Depending upon the primate species, the foramen magnum is located at a fulcrum, or balance point, and so its position is highly indicative of overall locomotion. In humans, it is in a more forward position while in apes, who walk mostly on all fours, it is situated further toward the rear of the skull.

The final and perhaps most peculiar change in the hominid skull is the cranial capacity. Until the genus Homo sp. evolves, hominid skull capacity does not change in any substantial way. However, it certainly increases quite a bit in our most recent ancestors. In Homo habilis, which will be discussed below, the cranial volume is a mere 600 cm\(^3\). In modern humans, by contrast, it is 1400 cm\(^3\). Some Neanderthals even exceeded this, with cranial volumes approaching 1700 cm\(^3\).

Figure \(\PageIndex{13}\): Changes in cranial capacity (volume) are significant through the course of hominid evolution. (Image: Public Domain)

The greater cranial volume evolved to fit a growing brain. One might be tempted to attribute this larger brain simply to greater intelligence, but this is a matter of great debate. While there does seem to be a correlation between the two variables, it is not necessarily causal, as intelligence (however you define that term) can potentially be attributed to other characteristics of the brain. Further, larger cranial capacities are also directly associated with larger body size, and adaptations to colder climates (which certainly was necessary in Neanderthals and modern humans). There is also some degree of sexual dimorphism among hominids. On average, males tend to have slightly larger cranial capacities than females of various hominid species. This is not an issue of greater intelligence, but rather cranial capacity being directly linked to larger average body size.

This is not to say that intelligence does not play a role in cranial capacity. It is more likely that brain physiology controls that. For decades, it was assumed that brain growth in humans was linked to a kind of reinforcing feedback loop between intelligence, technological development, and hunting meat. There may be some truth to this and, in the context of a discussion of Homo habilis below, the connection between meat and brain growth will be explored more specifically. Overall, as hominids evolved from a strictly vegetarian diet to scavenging bone marrow to directly hunting game, cranial capacity and brain growth followed. These were accentuated by technological developments that made scavenging and hunting more efficient. Simple wooden tools gave way to knapped stone scrapers. These gave way to elaborate arrow and spear points. Eventually – humans learned to smelt metals. Copper was followed by bronze and iron, each with their own “age,” and now there’s silicon. Are these all the result of greater cranial capacity and brain size? Perhaps some of it is, but certainly there are many other variables to consider.

Below is an excellent video showcasing hominid skull features by Dr. Daniel Lieberman, Professor of Human Evolutionary Biology at Harvard University.

Did I Get It? Hominid Skull Physiology

Exercise \(\PageIndex{1}\)

Early hominids had powerful jaw muscles. Which skull feature protected the face from collapse under this stress and was also one of the last to go away in anatomically modern humans?

a. Mandible

b. Super Orbital Brow Ridge

c. Sagittal Crest

d. Zygomaticus Arch

Answer

b. Super Orbital Brow Ridge