6 Chapter 6: Early Hominins and Homo

Defining Hominins

The first apes on our lineage evolved during the Miocene, and one of the key differences between those apes and other Miocene species is that our ancestors were bipedal, or habitually walking upright on two feet. Hominin, then, means everyone on “our” side of the evolutionary lineage: humans and all of our extinct bipedal ancestors and relatives since our divergence from the last common ancestor (LCA) with chimpanzees. Hominins are apes defined by their reliance on bipedalism and also on their reduced canine size: unlike other primates and apes, our ancestors did not have big canines for social communication.

Historic interpretations of our evolution, prior to our finding of early hominin fossils, varied. Debates from the mid-1800s regarding hominin origins focused on two key issues:

1. Where did we evolve?
2. Which traits evolved first?

Within this conversation, naturalists and early paleoanthropologists (people who study human evolution) speculated as to which human traits came first. These included the evolution of a big brain (encephalization), the evolution of the strange way in which we move about on two legs (bipedalism), and the evolution of our strange flat faces and small teeth. We now know that bipedal locomotion is one of the first things that evolved in our lineage, with early relatives having small brains and more apelike dentition. In this chapter, we will tease out the details of what this looks like in terms of morphology (i.e. the study of the form or size and shape of things; in this case, skeletal parts).

We also know that early human evolution occurred in a very complicated fashion in Africa. We have multiple species and genera that are diverse in the extent to which they move like us and the diets on which they subsisted. Specimen finds have been made all along the Eastern African Rift System (EARS; in Ethiopia, Kenya, Tanzania, and Malawi), in limestone caves in South Africa, and in Chad. Dates of these early relatives range from around 7 million years ago (mya) to around 1 mya, overlapping temporally with members of our genus, Homo (Figure 1).

 

Yet there is still so much to understand. Modern debates now look at the relatedness of these species to us and to one another. Discussions regarding which of these species were able to make and use tools continue. Every discovery in the patchy hominin fossil record tells us more about our evolution. New scientific techniques provide us with insight into the diets, environments, and lifestyles of these ancient relatives that was not available to researchers even ten years ago.

A Note on Brain Size

It is worth noting that while brain size expansion is seen primarily in our genus, Homo, earlier hominin brain sizes were highly variable between and within taxa, from 300 cc (cranial capacity, cm³), estimated in Ardipithecus, to 550 cc, estimated in Paranthropus boisei. The lower estimates are well within the range of variation of non-human extant great apes, and body size variability also plays a role in the interpretation of whether brain size could be considered large or small for a particular species or specimen.

Increases in brain size do not necessarily correlate with an increase in intelligence in animals, especially if body size is not taken into consideration. However, the brain is an expensive tissue to build and maintain. Researchers therefore argue that the cost of maintenance must yield some evolutionary benefit. This is more easily understood in hominins where the stone tool record (an indication of behavior and intelligence) is well associated with the species.

Hominin Taxonomy

In an earlier chapter, you were introduced to ways of organizing living taxa. In the past, taxonomy was primarily based on morphology (i.e., the physical features of organisms). Today they are tied to known relationships based on molecular phylogeny or a combination of the two. This technique is complicated when applied to living taxa, but it becomes immensely more difficult when we seek to categorize ancestor-descendant relationships in long-extinct forms, where molecular information is no longer preserved. In many ways, we find ourselves falling back on morphological comparisons (often on teeth and partially fossilized skeletal material) in the absence of genetic material.

It is also worth considering the process of fossil discovery and publication. Some fossils are easily diagnostic to a species level and allow for easy and accurate interpretation. Some, however, are more controversial. This could be because they do not easily preserve or are incomplete, making it difficult to compare and place the fossil within a specific species. Researchers often need to make several important claims when announcing or publishing a find: a secure date (if possible), clear association with other finds, and an adequate comparison among multiple species (both extant and fossil). It is therefore not uncommon for the scientific community to know that an important find was made years before it is scientifically published.

Paleoenvironment and Hominin Evolution

There is so much more to paleoanthropology than digging up and grouping fossil hominins: the discipline seeks to explain and understand the evolution of our ancestors’ behavior and morphology. There is no doubt that one of the major drivers in hominin evolution is the environment. Large-scale changes in global and regional climate, as well as alterations to the environment, are all linked to hominin diversification, dispersal, and extinction (Maslin et al. 2014).

Environmental reconstructions often use modern analogs. Let us take, for instance, the hippopotamus. It is an animal that thrives in environments that have abundant water to keep its skin cool and moist. If the environment for some reason becomes drier, it is expected that hippopotamus populations will decline. If a drier environment becomes wetter, it is possible that hippopotamus populations may be attracted to the new environment and thrive. Such instances have occurred multiple times in the past, and the bones of some animals, like the hippopotamus, that are sensitive to these changes give us insights into these events.

Reconstructing a paleoenvironment relies on a range of techniques, which vary depending on whether research interests focus on local changes or more global environmental changes/reconstructions. For local environments (reconstructing those of a single site or region), looking at the faunal assemblages, collections of fossils of other animals found at a site, and comparing them to animals found in certain modern environments allow us to determine if the environments in the past mirror those seen today in the region. Changes in the faunal assemblages, as well as when they occur and how they occur, tell us about past environmental changes. Other techniques are also useful in this regard. Isotopes of these fauna, for instance, tell us about the relative diets of individual fauna (e.g., using carbon isotopes to differentiate between species eating more grassland-heavy diets and those consuming bushland/tree-heavy diets) and whether the environment of individual animals was wetter or drier than the present day (Kingston & Harrison 2007).

Global climatic changes in the distant past, which fluctuated between being colder and drier and warmer and wetter on average, would have global implications for environmental change (Figure 2). These can be studied by using marine core and terrestrial soil data and by comparing these lines of evidence across multiple localities/sites/regions. These techniques allow us to use chemistry, such as nitrogen and oxygen isotopes in shells and sediments, or pollen grains, which show directly the kinds of plants surviving in an environment at a specific time period. This means that there are multiple lines of evidence that allow us to visualize global trends over millions of years (although it should be noted that the direction and extent of these changes could differ by geographic region).

 

Both local and global climatic/environmental changes have been used to understand parameters affecting our evolution (DeHeinzelin et al. 1999; Kingston 2007). There are numerous hypotheses regarding how climate has driven and continues to drive human evolution. Environmental change acts as an important keystone in hypotheses regarding the onset of several important hominin traits that are seen in early hominins and which are discussed in this chapter. Namely, the environment has been interpreted as the driving force behind:

1. The evolution of bipedalism (terrestrial locomotion on two legs),
2. The changing and diversifying of early hominin diets, and
3. The diversification of multiple early hominin species.

Here, we will explore a few popular hypotheses.

Savannah Hypothesis (or Aridity Hypothesis)

This popular theory was first penned by Charles Darwin and supported by anthropologists like Raymond Dart (Darwin 1871; Dart 1925). It suggests that the expansion of the savannah (or less densely forested, drier environments) forced early hominins from an arboreal lifestyle to a terrestrial one where bipedalism was a more efficient form of locomotion (Figure 3). This hypothesis stems from the idea that the Last Common Ancestor (LCA) between us and chimpanzees was a knuckle-walking quadruped like chimpanzees and gorillas. However, this idea was supported by little fossil or paleoenvironmental evidence and was later refined as the Aridity Hypothesis. The Aridity Hypothesis states that the long-term aridification and, thereby, expansion of savannah biomes were drivers in diversification in early hominin evolution (deMenocal 2004). It advocates particularly for periods of accelerated aridification leading to early hominin speciation events.

While early bipedal hominins are often associated with wetter, more closed environments (i.e., not supporting the Savannah Hypothesis), both marine and terrestrial records seem to support general cooling, drying conditions, with isotopic records indicating an increase in grasslands (i.e., colder and wetter climatic conditions) between 8 mya and 6 mya across the African continent (Cerling et al. 2011). This can be contrasted with later climatic changes derived from aeolian dust records (sediments transported to the site of interest by wind), which demonstrate increases in seasonal rainfall between 3 mya and 2.6 mya, 1.8 mya and 1.6 mya, and 1.2 mya and 0.8 mya (deMenocal 2004).

Despite a relatively scarce early hominin record, it is clear that two important factors occur around the time period in which we see increasing aridity. The first factor is the diversification of taxa, where high morphological variation between specimens has led to the naming of multiple hominin genera and species. The second factor is the observation that the earliest hominin fossils appear to have traits associated with bipedalism and are dated to around the drying period (as based on isotopic records). Some have argued that it is more accurately a combination of bipedalism and arboreal locomotion, which will be discussed later. However, the local environments in which these early specimens are found (as based on the faunal assemblages) do not appear to have been dry.

Forest Hypothesis

Based on contrasting environmental evidence to the Savannah hypothesis, R. J. Rayner and colleagues (1993) hypothesized that forested environments, rather than savannahs, were a key influence on the development of bipedalism in hominins. Unlike the Savannah Hypothesis, one prediction from this hypothesis is that the last common ancestor (LCA) between chimpanzees and us used an arboreal form of bipedal locomotion (i.e., walking along branches using the arms for stability), similar to orangutans, and was not a knuckle-walker like contemporary chimpanzees.

Pollen evidence from the site of Makapansgat in South Africa indicated that around the time early hominins occupied the area, it was a closed, wooded environment. Similarly, the earliest evidence for bipedalism occurs in specimens (associated with taxa such as Orrorin and Ardipithecus spp. as well as Australopithecus anamensis) found in sites with evidence of closed habitats (Suwa et al. 2009). Furthermore, evidence of knuckle-walking in older hominin species is sorely lacking or highly contested.

This hypothesis can be considered in contrast to the Savannah Hypothesis, and it does appear to be evidence based. However, it is worth noting that preservation and resulting fossilization might be better in these kinds of environments, biasing this interpretation of the fossil record.

Variability Selection Hypothesis

This hypothesis was first articulated by paleoanthropologist Richard Potts (1998). It links the high amount of climatic variability over the last 7 million years to both behavioral and morphological changes. Unlike previous notions, this hypothesis states that hominin evolution does not respond to habitat-specific changes or to specific aridity or moisture trends. Instead, long-term environmental unpredictability over time and space influenced morphological and behavioral adaptations that would help hominins survive, regardless of environmental context. The Variability Selection Hypothesis states that hominin groups would experience varying degrees of natural selection due to continually changing environments and potential group isolation. This would allow certain groups to develop genetic combinations that would increase their ability to survive in shifting environments. These populations would then have a genetic advantage over others that were forced into habitat-specific adaptations.

The evidence for this theory is large climatic variability and higher survivability of generalist species versus specialists. However, this hypothesis accommodates for larger time-scales of extinction and survival events. In this way, the Variability Selection Hypothesis allows for a more flexible interpretation of the evolution of bipedalism in hominins, accommodating the discrepancies in evidence between the conflicting Savannah and Forest Hypotheses. In some ways, this hypothesis accommodates both environmental data and our interpretations of an evolution toward greater variability among species and the survivability of generalists.

Derived Adaptations: Bipedalism

The unique form of locomotion exhibited by modern humans, called obligate bipedalism, is important in distinguishing our species from the other extant great apes. The ability to walk habitually upright is one of the defining attributes of the hominin lineage. We also differ from other animals that walk bipedally (such as kangaroos) in that we do not have a tail to balance us as we move.

There are many current ideas regarding selective pressures that would lead to early hominins adapting upright posture and locomotion. Many of these selective pressures, as we have seen in the previous section, coincide with a shift in environmental conditions, supported by paleoenvironmental data. In general, however, it appears as though early hominins thrived in forested regions, similar to extant great apes, with dense tree coverage, which would indicate an arboreal lifestyle. As the environmental conditions changed and a savannah/grassland environment became more widespread, the tree cover would become less dense, scattered, and sparse and bipedalism therefore would become more important.

There are several proposed selective pressures for bipedalism:

1. Energy conservation: modern bipedal humans conserve more energy than extant chimpanzees, which are predominantly knuckle-walking quadrupeds when walking over land. While chimpanzees, for instance, are faster than humans terrestrially, they expend large amounts of energy being so. Adaptations to bipedalism include “stacking” the majority of the weight of the body over a small area around the center of gravity (i.e., the head is above the chest, which is above the pelvis, which is over the knees, which is above the feet). This reduces the amount of muscle needed to be engaged during locomotion to “pull us up” and allows us to travel longer distances expending far less energy.
2. Thermoregulation: less surface area (i.e., only the head and shoulders) is exposed to direct sunlight during the hottest parts of the day (i.e., midday). This means that the body is exposed to less heat and has less need to employ additional “cooling” mechanisms such as sweating, which additionally means less water loss.
3. Bipedalism freed up our ancestors’ hands such that they could more easily gather food and carry tools or infants. This further enabled the use of hands for more specialized adaptations associated with the manufacturing and use of tools.

These selective pressures are not mutually exclusive, and bipedality could have evolved from a combination of these selective pressures, in ways that increased the chances of early hominin survival.

Skeletal Adaptations for Bipedalism

Humans, as the only obligate bipedal species among primates, have highly specialized adaptations to facilitate this kind of locomotion (Figure 4). Many of these adaptations occur within the soft tissue of the body (e.g., muscles and tendons). However, when analyzing the paleoanthropological record for evidence of the emergence of bipedalism, all that remains is the fossilized bone. Interpretations of locomotion are therefore often based on comparative analyses between fossil remains and the skeletons of extant primates with known locomotor behaviors.

The majority of these adaptations occur in the postcranium, the skeleton from below the head, and are outlined in the chart below. In general, these adaptations allow for greater stability and strength in the lower limb, by allowing for more shock absorption, for a larger surface area for muscle attachment, and for the “stacking” of the skeleton directly over the center of gravity to reduce energy needed to be kept upright. These adaptations often mean less flexibility in areas such as the knee and foot.

However, these adaptations come at a cost. Evolving from a non-obligate bipedal ancestor means that the adaptations we have are evolutionary compromises. For instance, the valgus knee (angle at the knee) is an essential adaptation to balance the body weight above the ankle during bipedal locomotion. However, the strain and shock absorption at an angled knee eventually takes its toll, with runners often experiencing joint pain. Similarly, the long neck of the femur absorbs stress and accommodates for a larger pelvis, but it is a weak point, resulting in painful damage to the hip joint being commonplace among the elderly, especially in cases where the bone additionally weakens through osteoporosis. Finally, the S-shaped curve in our spine allows us to stand upright, relative to the more curved C-shaped spine of an LCA. Yet the weaknesses in the curves lead to pinching of nerves and back pain. Since many of these problems primarily are only seen in old age, they can potentially be seen as an evolutionary compromise.

Region	Feature	Obligate Biped (H. sapiens)	Non-obligate Biped
Cranium	Position of the foramen magnum	Positioned inferiorly (immediately under the cranium) so that the head rests on top of the vertebral column for balance and support (head is perpendicular to the ground	Posteriorly positioned (to the back of the cranium). Head is positioned parallel to the ground
Postcranium	Body proportions	Shorter upper limb (not used for locomotion)	Non-human apes: Longer upper limbs (used for locomotion: brachiation or knuckle-walking)
Postcranium	Spinal curvature	S-curve due to pressure exerted on the spine from bipedalism (lumbar lordosis)	C-curve
Postcranium	Vertebrae	Robust lumbar (lower-back) vertebrae (for shock absorbance and weight bearing). Lower back is more flexible than that of apes as the hips and trunk swivel when walking (weight transmission).	Gracile lumbar vertebrae compared to those of modern humans
Postcranium	Pelvis	Shorter, broader bowl-shaped pelvis (for support); very robust. Broad sacrum with large sacroiliac joint surfaces	Longer, flatter, elongated ilia, more narrow and gracile, narrower sacrum, relatively smaller sacroiliac joint surface
Postcranium	Lower limb	In general, longer, more robust lower limbs and more stable, larger joints Large femoral head and longer neck (absorbs more stress and increases the mechanical advantage). Valgus angle of knee, positions knee over the ankle and keeps the center of gravity balanced over stance leg during stride cycle (shock absorbance). Distal tibia (lower leg) of humans has a large medial malleolus for stability.	In general, smaller, more gracile limbs with more flexible joints Femoral neck is smaller in comparison to modern humans and has a shorter neck. The legs bow outward, there is no valgus angle of the knee (no “knock-knees”). The distal tibia in chimpanzees is trapezoid (wider anteriorly) for climbing and allows more flexibility.
Postcranium	Foot	Rigid, robust foot, without a midtarsal break. Non-opposable and large, robust big toe (for push off while walking) and large heel for shock absorbance.	Flexible foot, midtarsal break present (which allows primates to lift their heels independently from their feet), opposable big toe for grasping.

Despite relatively few postcranial fragments, the fossil record in early hominins indicates a complex pattern of emergence of bipedalism. Key features, such as a more anteriorly placed foramen magnum, are argued to be seen even in the earliest discovered hominins, indicating an upright posture (Dart 1925). Some early species appear to have a mix of primitive (arboreal) and derived (bipedal) traits, which indicates a mixed locomotion and a more mosaic evolution of the trait. Some early hominins appear to, for instance, have bowl-shaped pelvises (hip bones) and angled femurs suitable for bipedalism but also have retained an opposable big toe or curved fingers and longer arms (for arboreal locomotion). These mixed morphologies may indicate that earlier hominins were not fully obligate bipeds and thus thrived in mosaic environments.

It is also worth noting that, while not directly related to bipedalism per se, other postcranial adaptations are evident in the hominin fossil record from some of the earlier hominins. For instance, the hand and finger morphologies of many of the earliest hominins indicate adaptations consistent with arboreality. These include longer hands, more curved metacarpals and phalanges, and a shorter, relatively weaker thumb. This allows for gripping onto curved surfaces during locomotion. The earliest hominins appear to have mixed morphologies for both bipedalism and arborealism. However, among Australopiths, there are indications for greater reliance on bipedalism as the primary form of locomotion. Similarly, adaptations consistent with tool manufacture (shorter fingers and a longer, more robust thumb, in contrast to the features associated with arborealism) have been argued to appear before the genus Homo.

Derived Adaptations: Early Hominin Dentition

The Importance of Teeth

Teeth are abundant in the fossil record, primarily because they are already highly mineralized as they are forming, far more so than even bone. Because of this, teeth preserve readily. And, because they preserve readily, they are well-studied and better understood than many skeletal elements. Even in the sparse hominin and primate fossil record, teeth are, in some cases, all we have.

Teeth also reveal a lot about the individual from whom they came. We can tell what they evolved to eat, which other species they may be more closely related to, and even, to some extent, the level of sexual dimorphism, or general variability, within a given species. This is powerful information that can be contained in a single tooth. With a little more observation, the wear patterns on a tooth can tell us about the diet of the individual in the weeks leading up to its death. Furthermore, the way in which a tooth is formed, and the timing of formation, can reveal information about changes in diet, or even mobility, over infancy and childhood, using isotopic analyses. When it comes to our earliest hominin relatives, this information is vital for understanding how they lived.

The purpose of comparing different hominin species is to better understand the functional morphology as it applies to dentition. In this, we mean that the morphology of the teeth or masticatory system (which includes jaws) can reveal something about the way in which they were used and, therefore, the kinds of foods these hominins ate. When comparing the features of hominin groups, it is worth considering modern analogs to make more appropriate assumptions about diet. In this way, hominin dentition is often compared with that of chimpanzees, gorillas (our two closest relatives), and/or modern humans.

The most divergent group, however, is humans. Humans around the world have incredibly varied diets. Among hunter-gatherers, it can vary from a honey- and plant-rich diet, as seen in the Hadza in Tanzania, to a diet almost entirely reliant on animal fat and protein, as seen in Inuits in polar regions of the world. We are therefore considered generalists, more general than the largely frugivorous chimpanzee or the [pb_glossary id=”1684″]folivorous[/pb_glossary] gorilla.

One way in which all humans are similar is our reliance on the processing of our food. We cut up and tear meat with tools using our hands, instead of using our front teeth (incisors and canines). We smash and grind up hard seeds, instead of crushing them with our hind teeth (molars). This means that, unlike our ape relatives, we can rely more on developing tools to navigate our complex and varied diets. Evolutionarily, partially in response to our increased reliance on our hands and brain, our teeth have reduced in size and our faces are flatter. Similarly, a reduction in teeth and a more generalist dental morphology could also indicate an increase in softer and more variable foods, such as the inclusion of more meat. These trends begin early on in our evolution.

General Dental Trends in Early Hominins

Early on in human evolution, we see a reduction in canine size. This implies strongly that, over evolutionary time, the need for display and dominance among males has reduced, as has our sexual dimorphism. This implies a less sexually dimorphic social structure in the earlier hominins relative to modern-day chimpanzees and gorillas.

Earliest Hominins: Sahelanthropus

We see evidence for bipedalism in some of the earliest fossil hominins, dated from within genetic estimates of our divergence from chimpanzees. These hominins, however, also indicate evidence for arboreal locomotion.

The earliest dated hominin find (between 6 mya and 7 mya, based on radiometric dating of volcanic tufts) has been argued to come from Chad and is named Sahelanthropus tchadensis (Figure 5; Brunet et al. 2002). The find has a small cranial capacity (360 cc) and has canines smaller than those in extant great apes, yet still larger and pointier than those in humans. A short cranial base and a foramen magnum, the hole through which the spinal cord enters the cranium, that is more humanlike in positioning have been argued to indicate upright walking. However, the inclusion of Sahelanthropus in the hominin family has been debated by researchers, since the evidence for bipedalism was initially based on cranial evidence alone.

 

Earliest Hominins: The Genus Ardipithecus

Another genus, Ardipithecus, is argued to be represented by at least two species: Ardipithecus ramidus and Ardipithecus kadabba.

Ardipithecus ramidus (“ramidu” means root in the Afar language) is currently the best known of the earliest hominins (Figure 6). Dated to 4.4 mya, Ar. ramidus was found in Ethiopia, in the Middle Awash region and in Gona. Unlike Sahelanthropus, this species has a large sample size, with over 110 specimens from the site of Aramis alone. This species was announced in 1994 by American palaeoanthropologist Tim White, based on a partial female skeleton nicknamed “Ardi” (ARA-VP-6/500; White et al. 1994). Ardi demonstrates a mosaic of ancestral and derived characteristics in the postcranial skeleton. For instance, she had an opposable big toe (hallux), similar to chimpanzees (i.e., ancestral), which could have aided in climbing trees effectively. However, the pelvis and hip show that she could walk upright (a derived trait), supporting her hominin status. A small brain (300 cc to 350 cc), midfacial projection, and slight prognathism show retained ancestral cranial features, but the cheek bones are less flared and robust than in later hominins.

Ardipithecus kadabba (the species name means “oldest ancestor” in the Afar language) is known from localities on the western margin of the Middle Awash region, the same locality where Ar. ramidus has been found.

The Genus Australopithecus

The Australopithecines are a diverse group of hominins comprising various species, though they can all be described as small-bodied, bipedal apes. Australopithecus is the given group or genus name. It stems from the Latin word Australo, meaning “southern,” and the Greek word pithecus, meaning “ape.” Within this section, we will outline these differing species’ geological and temporal distributions across Africa, unique derived and/or shared traits, and importance in the fossil record.

Between 3 mya and 1 mya, there seems to be differences in dietary strategy between species of hominins designated as Australopithecines, which is evident from the peculiar size of the molars in one of the groups. This pattern of larger posterior dentition (even relative to the incisors and canines), thick enamel, and cranial evidence for large chewing muscles is far more pronounced in a group known as the robust australopithecines, as opposed to their earlier contemporaries or predecessors, the gracile australopithecines, and certainly larger than those seen in early Homo, which emerges during this time. This pattern of incredibly large posterior dentition (and very small anterior dentition) has led people to refer to robust australopithecines as megadont hominins (Figure 7).

This section has been categorized into “gracile” and “robust” Australopithecines, highlighting the morphological differences between the two groups (which many researchers have designated as separate genera: Australopithecus and Paranthropus, respectively) and then focusing on the individual species. It is worth noting, however, that not all researchers accept these clades as biologically or genetically distinct, with some researchers insisting that the relative gracile and robust features found in these species are due to parallel evolutionary events, toward similar dietary niches.

Despite this genus’ ancestral traits and small cranial capacity, all members show evidence of bipedal locomotion. It is generally accepted that Australopithecus species display varying degrees of arborealism and bipedality—these individuals were walking on the ground on two legs but were probably still comfortable with climbing trees.

Eastern African Gracile Australopithecines

This section below describes individual species from across Eastern Africa. These species have coined the term “gracile australopithecines” because of the less exaggerated, smaller, and less robust features seen in them.

The earliest known Australopithecine, Australopithecus anamensis (after “Anam,” meaning “lake” from the Turkana region in Kenya) is dated to 4.2 mya to 3.8 mya. (Leakey et al. 1995; Patterson and Howells 1967).  The species is currently found from sites in the Turkana region of Kenya and Middle Awash of Ethiopia. A small brain size (370 cc), relatively large canines, projecting cheekbones, and primitive earholes show more ancestral features as compared to those of more recent Australopithecines. The most important element discovered associated with this species that indicates bipedalism is a fragment of a tibia (shinbone), which demonstrates features associated with weight transfer during bipedal walking. Ancestral traits in the upper limb (such as the humerus) indicate some retained arboreal locomotion. Almost 100 specimens, representing over 20 individuals, have been found to date.

Australopithecus afarensis is one of the oldest and most well-known australopithecine species and consists of a large number of fossil remains. Au. afarensis (which means “from the Afar region”) is dated to between 2.9 mya and 3.9 mya and is found in sites all along the EARS system, in Tanzania, Kenya, and Ethiopia. The most famous individual stemming from this species is a partial female skeleton discovered in Hadar in Ethiopia, which was later nicknamed “Lucy,” after the Beatles’ song “Lucy in the Sky with Diamonds,” which was played in celebration of the find (Johanson et al. 1978). This skeleton was found in 1974 by Donald Johanson and dates to approximately 3.2 mya (Figures 9 and 10). In addition, in 2002 a juvenile of the species was found by Zeresenay Alemseged and given the name “Selam” (meaning “peace,” DIK 1-1), though it is popularly known as “Lucy’s Child” or as the “Dikika Child”(Alemseged et al. 2006). Similarly, the Laetoli Footprints were made by Australopithecus afarensis have drawn much attention.

 

Skeletal evidence indicates that this species was bipedal, primarily through examining the pelvis and lower limb, which demonstrate a humanlike femoral neck, a valgus knee, and bowl-shaped hip. More evidence of bipedalism is found not in the skeleton but in the footprints of this species. Au. afarensis is associated with the Laetoli Footprints, a 24-meter trackway of hominin fossil footprints preserved in volcanic ash discovered by Mary Leakey in Tanzania and dated to 3.5 mya to 3 mya. This set of prints is thought to have been produced by three bipedal individuals as there are no knuckle imprints, no opposable big toes, and a clear arch is present. The infants of this species are thought to have been more arboreal than the adults, as was discovered through analyses of the foot bones of the Dikika Child dated to 3.32 mya (Alemseged et al. 2006).

Although not found in direct association with stone tools, potential evidence for cut marks on bones, found at Dikika, and dated to 3.39 mya indicates a potential temporal/geographic overlap between meat eating, tool use, and this species. However, this evidence is fiercely debated. Others have associated the cut marks with the earliest tool finds from Lomekwi, Kenya, temporally (3.3 mya) and in close geographic proximity to this species.

South African Gracile Australopithecines

Since the discovery of the Taung Child in 1924, there have been numerous Australopithecine discoveries from the region in South Africa known as “The Cradle of Humankind,” recently given UNESCO World Heritage Site status as “The Fossil Hominid Sites of South Africa.” The limestone caves found in the Cradle allow for the excellent preservation of fossils. Past animals navigating the landscape and falling into cave openings, or caves used as dens by carnivores, led to the accumulation of deposits over millions of years.

While these sites have historically been difficult to date, with mixed assemblages accumulated over large time periods, advances in techniques such as uranium-series dating have allowed for greater accuracy. Historically, the excellent faunal record from East Africa has traditionally been used to compare sites based on relative dating. In this, the knowledge of environmental/faunal changes and extinction events allows us to know which hominin finds are relatively younger or older than others.

The discovery of the Taung Child shifted the focus of palaeoanthropological research from Europe to Africa, although acceptance of this shift was slow. The species with which it is assigned, Australopithecus africanus (name meaning “Southern Ape of Africa”), is currently dated to between 3.3 mya and 2.1 mya (Pickering and Kramers 2010), with discoveries from Sterkfontein, Taung, Makapansgat, and Gladysvale in South Africa (Figure 12). A relatively large brain (400 cc to 500 cc), small canines without an associated diastema, and more rounded cranium and smaller teeth than Au. afarensis indicate some derived traits. Similarly, the postcranial remains (in particular, the pelvis) indicate bipedalism. However, the sloping face and curved phalanges, indicative of retained arboreal locomotor abilities, show some primitive features. 

Another famous Au. africanus skull (the skull of “Mrs. Ples”) was previously attributed to Plesianthropus transvaalensis, meaning “near human from the Transvaal,” the old name for Gauteng Province, South Africa (Broom 1947. The name was shortened by contemporary journalists to “Ples” (Figure 13). Due to the prevailing mores of the time, the assumed female found herself married, at least in name, and has become widely known as “Mrs. Ples.” It was later reassigned to Au. africanus and is now argued to be a young male rather than an adult female cranium (Thackeray et al. 2002).

 

Paranthropus “Robust” Australopithecines

In the robust australopithecines, the specialized nature of the teeth and masticatory system, such as flaring zygomatic arches (cheekbone) to accommodate the large temporalis (chewing) muscle, indicated a shift in diet in these taxa. Some argued that the diet of the robust australopithecines was so specific that any change in environment would have accelerated their extinction. The generalist nature of the teeth of the gracile australopithecines, and certainly early Homo, would have made these hominins more capable of surviving through and adapting to to environmental change. However, some have suggested that the features of the robust australopithecines might have developed more in response to effectively eating fallback foods in hard times rather than indicating a lack of adaptability.

Paranthropus is usually referred to by scholars as the “robust” australopithecine, because of its defining distinct morphological features. Features that are closer to those of the assumed ancestral type are referred to as P. aethiopicus, and those that have become derived are referred to as both P. boisei and P. robustus. These features include a large, broad, dish-shaped face and zygomatic arches that are forward facing, including a large mandible with extremely large posterior dentition. These three species have been grouped together by a majority of scholars as a genus as they share more derived features and are more closely related to each other than the other australopithecines. As a collective, this genus spans 2.7 mya to 1.0 mya, although the dates of the individual species differ.

In terms of diet, the tougher, chewing diets of the robust australopithecines are supported by the extreme morphology of their face and cranium. The earliest of the Paranthropus species, Paranthropus aethiopicus, is dated to between 2.7 mya and 2.3 mya and is currently found in Tanzania, Kenya, and Ethiopia. It is well known because of the “Black Skull” (KNM–WT 17000), so called because of the mineral manganese that stained it black during fossilization (Figure 16). As with all robust Australopithecines, P. aethiopicus has the shared derived traits of large, flat premolars and molars; large, flaring zygomatic arches for accommodating large chewing muscles (the temporalis muscle); a sagittal crest for increased muscle attachment of the chewing muscles to the skull; and a robust mandible and supraorbital torus (brow ridge).

 

 First attributed as Zinjanthropus boisei (with the first discovery going by the nickname “Zinj” or sometimes “Nutcracker Man”), Paranthropus boisei was discovered in 1959 by Mary Leakey (see Figures 17 and 18; Leakey 1959). This “robust” australopith species is distributed across countries in East Africa at sites in Kenya (Koobi Fora, West Turkana, and Chesowanja), Malawi (Malema-Chiwondo), Tanzania (Olduvai Gorge and Peninj), and Ethiopia (Omo River Basin and Konso). The specimens of this species have been found by researchers to date to roughly 2.4 mya to 1.4 mya. Due to the nature of its exaggerated, larger, and more robust features, P. boisei has been termed hyper-robust, that is, even more heavily built than other robust forms, with very large, flat posterior dentition. Despite the cranial features of P. boisei indicating a tough diet of tubers, nuts, and seeds, isotopes indicate a diet high in C4 foods (e.g., grasses, such as sedges). This differs from what is seen in P. robustus. Another famous specimen from this species is the Peninj mandible from Tanzania, found in 1964 by Kimoya Kimeu.

Paranthropus robustus was the first taxon to be discovered within the genus in Kromdraai B by a school boy named Gert Terblanche, and subsequent fossil discoveries were made by researcher Robert Broom in 1938 (Figure 19; Broom 1938). Paranthropus robustus dates approximately to 2.0 mya to 1 mya and is the only taxon from the genus to be discovered in South Africa. It has been found in sites all over the Cradle of Humankind, such as Kromdraai B, Swartkrans, Gondolin, Drimolen, and Coopers Cave. P. robustus features are neither as “hyper-robust” as P. boisei nor as primitive as P. aethiopicus; instead, they have been described as being less derived, more general features that are shared with both East African species.

Early Tool Use and Technology

The Early Stone Age (ESA) marks the beginning of recognizable technology as made by our human ancestors. Stone-tool (or lithic) technology is defined by the fracturing of rocks and the manufacture of tools through a process called knapping. The Stone Age lasted for more than 3 million years and is broken up into chronological periods called the Early (ESA), Middle (MSA), and Later Stone Ages (LSA). Each period is further broken up in different techno-complexes, as explained below. The ESA spanned the largest technological time period of human innovation from over 3 million years ago to around 300,000 years ago and is associated almost entirely with hominin species prior to modern Homo sapiens. 

Currently, the oldest known stone tools, which form the techno-complex the Lomekwian, date to 3.3 mya (Harmand et al. 2015). They were found at a site called Lomekwi 3 in Kenya. This techno-complex is the most recently defined and pushed back the oldest known date for lithic technology. There is only one known site thus far and, due to the age of the site, it is associated with species prior to Homo.  The pieces are very chunky and do not display the same fracture patterns as seen in later techno-complexes. Lomekwian knappers likely aimed to get a sharp-edged piece on a flake, which would have been functional.

Stone tool use, however, is not only understood through the direct discovery of the tools. Cut marks on fossilized animal bones may illuminate the functionality of stone tools. In one study in 2010, researchers argued that cut marks on a pair of animal bones from Dikika in Ethiopia, dated to 3.4 mya, were from stone tools. The discoverers suggested that they be more securely associated, temporally, with Au. afarensis. However, others have noted that these marks are consistent with teeth marks from carnivores.

The Oldowan techno-complex is far more established in the scientific literature (Leakey 1971). It is called the Oldowan because it was originally discovered in Olduvai Gorge, Tanzania, but the oldest assemblage is from Gona in Ethiopia, dated to 2.6 mya (Semaw 2000). The techno-complex is defined as a core and flake industry. Like the Lomekwian, there was an aim to get sharp-edged flakes, but this was achieved through a different production method. Hominins were able to actively hold or manipulate the core being knapped, which they could directly hit using a hammerstone.

Hominin	Sahelanthropus tchadensis
Dates	7 mya to 6 mya
Region(s)	Chad
Famous discoveries	The initial discovery, made in 2001.
Brain size	360 cc average
Dentition	Smaller than in extant great apes, larger and pointier than in humans. Canines worn at the tips.
Cranial features	A short cranial base and a foramen magnum (hole in which the spinal cord enters the cranium) that is more humanlike in positioning, has been argued to indicate upright walking.
Postcranial features	Currently little published postcranial material.
Culture	N/A
Other	The extent to which this hominin was bipedal is currently heavily debated. If so, it would indicate an arboreal bipedal ancestor of hominins, not a knuckle-walker like chimpanzees.

Hominin	Orrorin tugenensis
Dates	6 mya to 5.7 mya
Region(s)	Tugen Hills (Kenya)
Famous discoveries	Original discovery in 2000.
Brain size	N/A
Dentition	Smaller cheek teeth (molars and premolars) than even more recent hominins (i.e., derived), thick enamel, and reduced, but apelike, canines.
Cranial features	Not many found
Postcranial features	Fragmentary leg, arm, and finger bones have been found. Indicates bipedal locomotion.
Culture	Potential toolmaking capability based on hand morphology, but nothing found directly.
Other	This is the earliest species that clearly indicates adaptations for bipedal locomotion.

Hominin	Ardipithecus ramidus
Dates	4.4 mya
Region(s)	Middle Awash region and Gona (Ethiopia)
Famous discoveries	A partial female skeleton nicknamed “Ardi” (ARA-VP-6/500).
Brain size	300 cc to 350 cc
Dentition	Little differences between the canines of males and females (small sexual dimorphism).
Cranial features	Midfacial projection, slightly prognathic. Cheekbones less flared and robust than in later hominins.
Postcranial features	Ardi demonstrates a mosaic of ancestral and derived characteristics in the postcrania. For instance, an opposable big toe similar to chimpanzees (i.e., “primitive” or more ancestral), which could have aided in climbing trees effectively. However, the pelvis and hip show that she could walk upright (i.e., it is derived), supporting her hominin status.
Culture	None directly associated
Other	Over 110 specimens from Aramis

Hominin	Ardipithecus kadabba
Dates	5.2 mya to 5.8 mya
Region(s)	Middle Awash (Ethiopia)
Famous discoveries	This species discovery in 1997 by Yohannes Haile-Selassie.
Brain size	N/A
Dentition	Larger hind dentition than in modern chimpanzees. Thick enamel and larger canines than in later hominins.
Cranial features	N/A
Postcranial features	A large hallux (big toe) bone indicates a bipedal “push off.”
Culture	N/A
Other	Faunal evidence indicates a mixed grassland/woodland environment.

Hominin	Australopithecus anamensis
Dates	4.2 mya to 3.8 mya
Region(s)	Turkana region (Kenya), Middle Awash (Ethiopia)
Famous discoveries	A 2019 find from Ethiopia, named MRD.
Brain size	370 cc
Dentition	Relatively large canines compared with more recent Australopithecines.
Cranial features	Projecting cheekbones and primitive earholes.
Postcranial features	Lower limb bones (tibia and femur) indicate bipedality; arboreal features in upper limb bones (humerus) found.
Culture	N/A
Other	Almost 100 specimens, representing over 20 individuals, have been found to date.

 

Hominin	Australopithecus afarensis
Dates	2.9 mya to 3.9 mya
Region(s)	Afar Region, Omo, Maka, Fejej, and Belohdelie (Ethiopia); Laetoli (Tanzania); Koobi Fora (Kenya).
Famous discoveries	Lucy, Selam (Dikika Child), Laetoli Footprints.
Brain size	380 cc to 430 cc
Dentition	Reduced canines and molars relative to great apes, but larger than in modern humans.
Cranial features	Prognathic face, facial features indicate relatively strong chewing musculature (compared with Homo), but less extreme than in Paranthropus.
Postcranial features	Clear evidence for bipedalism from lower limb postcranial bones. Laetoli Footprints indicate humanlike walking. Dikika Child bones indicate retained primitive arboreal traits in the postcrania.
Culture	None directly; but close in age and proximity to controversial cut marks at Dikika and early tools in Lomekwi.
Other	Au. afarensis is one of the oldest and most well-known australopithecine species and consists of a large number of fossil remains.

Hominin	Australopithecus bahrelghazali
Dates	3.6 mya
Region(s)	Chad
Famous discoveries	“Abel,” the holotype.
Brain size	N/A
Dentition	N/A
Cranial features	N/A
Postcranial features	N/A
Culture	N/A
Other	Arguably within range of variation of Au. afarensis

Hominin	Australopithecus deyiremada
Dates	3.5 mya to 3.3 mya
Region(s)	Woranso-Mille (Afar region, Ethiopia)
Famous discoveries	First fossil mandible bones were discovered in 2011 in the Afar region of Ethiopia by Yohannes Haile-Selassie.
Brain size	N/A
Dentition	Smaller teeth with thicker enamel than seen in Au. afarensis, with a potentially hardier diet.
Cranial features	Larger mandible and more projecting cheekbones than in Au. afarensis.
Postcranial features	N/A
Culture	N/A
Other	Contested species designation; arguably a member of Au. afarensis.

 

Hominin	Australopithecus garhi
Dates	2.5 mya
Region(s)	Middle Awash (Ethiopia)
Famous discoveries	N/A
Brain size	450 cc
Dentition	Larger hind dentition than seen in other gracile Australopithecines.
Cranial features	N/A
Postcranial features	A femur of a fragmentary partial skeleton, argued to belong to Au. garhi, indicates this species may be longer-limbed than Au. afarensis, although still able to move arboreally.
Culture	Crude/primitive stone tools resembling Oldowan (described later) have been found in association with Au. garhi.
Other	This species is not well documented or understood and is based on only a few fossil specimens.

Hominin	Australopithecus africanus
Dates	3.3 mya to 2.1 mya
Region(s)	Sterkfontein, Taung, Makapansgat, Gladysvale (South Africa)
Famous discoveries	Taung Child, “Mrs. Ples,” Little Foot (?).
Brain size	400 cc to 500 cc
Dentition	Smaller teeth (derived) relative to Au. afarensis. Small canines with no diastema.
Cranial features	A rounder skull compared with Au. afarensis in East Africa. A sloping face (primitive).
Postcranial features	Similar postcranial evidence for bipedal locomotion (derived pelvis) with retained arboreal locomotion (e.g., curved phalanges—fingers), as seen in Au. afarensis.
Culture	None with direct evidence.
Other	A 2015 study noted that the trabecular bone morphology of the hand was consistent with forceful tool manufacture and use, suggesting potential early tool abilities.

Hominin	Australopithecus sediba
Dates	1.97 mya
Region(s)	Malapa Fossil Site (South Africa)
Famous discoveries	Karabo (MH1)
Brain size	420 cc to 450 cc
Dentition	Small dentition with Australopithecine cusp-spacing.
Cranial features	Small brain size (Australopithecus-like), but gracile mandible (Homo-like).
Postcranial features	Scientists have interpreted this mixture of traits (such as a robust ankle but evidence for an arch in the foot) as a transitional phase between a body previously adapted to arborealism (tree climbing, particularly in evidence from the bones of the wrist) to one that adapted to bipedal ground walking.
Culture	None of direct association, but some have argued that a modern hand morphology (shorter fingers and a longer thumb) means that adaptations to tool manufacture and use may be present in this species.
Other	It was first discovered through a clavicle bone in 2008 by nine-year-old Matthew Berger, son of paleoanthropologist Lee Berger.

Hominin	Australopithecus prometheus
Dates	3.7 mya (debated)
Region(s)	Sterkfontein (South Africa)
Famous discoveries	“Little Foot” (StW 573)
Brain size	408 cc (Little Foot estimate)
Dentition	Heavy anterior dental wear patterns, relatively large anterior dentition and smaller hind dentition, similar to Au. afarensis.
Cranial features	Relatively larger brain size, robust zygomatic arch, and a flatter midface.
Postcranial features	The initial discovery of four ankle bnes indicated bipedality.
Culture	N/A
Other	Highly debated new species designation.

Hominin	Paranthropus aethiopicus
Dates	2.7 mya to 2.3 mya
Region(s)	West Turkana (Kenya), Laetoli (Tanzania), Omo River Basin (Ethiopia)
Famous discoveries	The ‘Black Skull” (KNM–WT 17000)
Brain Size	410 cc
Dentition	P. aethiopicus has the shared derived traits of large flat premolars and molars, although few teeth have been found.
Cranial features	Large flaring zygomatic arches for accommodating large chewing muscles (the temporalis muscle), a sagittal crest for increased muscle attachment of the chewing muscles to the skull, and a robust mandible and supraorbital torus (brow ridge).
Postcranial features	A proximal tibia indicates bipedality, and similar size to Au. afarensis.
Culture	N/A
Other	The “Black Skull” is so called because of the mineral manganese that stained it black during fossilization.

Hominin	Paranthropus boisei
Dates	2.4 mya to 1.4 mya
Region(s)	Koobi Fora, West Turkana, and Chesowanja (Kenya), Malema-Chiwondo (Malawi), Olduvai Gorge and Peninj (Tanzania), and Omo River basin and Konso (Ethiopia)
Famous discoveries	“Zinj,” or sometimes “Nutcracker Man” (OH5), in 1959 by Mary Leakey. The Peninj mandible from Tanzania, found in 1964 by Kimoya Kimeu.
Brain size	500 cc to 550 cc
Dentition	Very large, flat posterior dentition (largest of all hominins currently known). Much smaller anterior dentition. Very thick dental enamel.
Cranial features	Indications of very large chewing muscles (e.g., flaring zygomatic arches and a large sagittal crest).
Postcranial features	Evidence for high variability and sexual dimorphism, with estimates of males at 1.37 meters tall and females at 1.24 meters.
Culture	Richard Leakey and Bernard Wood have both suggested that P. boisei could have made and used stone tools. Tools dated to 2.5 mya in Ethiopia have been argued to possibly belong to this species.
Other	Despite the cranial features of P. boisei indicating a tough diet of tubers, nuts, and seeds, isotopes indicate a diet high in C4 foods (e.g., grasses, such as sedges). This differs from what is seen in P. robustus.

Hominin	Paranthropus robustus
Dates	2.3 mya to 1 mya
Region(s)	Kromdraai B, Swartkrans, Gondolin, Drimolen, and Coopers Cave (South Africa)
Famous discoveries	SK48 (original skull)
Brain size	410 cc to 530 cc
Dentition	Large posterior teeth with thick enamel, consistent with other Robust Australopithecines. Enamel hypoplasia is also common in this species, possibly because of instability in the development of large, thick enameled dentition.
Cranial features	P. robustus features are neither as “hyper-robust” as P. boisei or as primitive as P. aethiopicus, but have been described as less derived more general features that are shared with both East African species, e.g., the sagittal crest and zygomatic flaring.
Postcranial features	Reconstructions indicate sexual dimorphism.
Culture	N/A
Other	Several of these fossils are fragmentary in nature, distorted, and not well preserved, because they have been recovered from quarry breccia using explosives.

Hominin	Kenyanthopus platyops
Dates	3.5 mya to 3.2 mya
Region(s)	Lake Turkana (Kenya)
Famous discoveries	KNM–WT 40000
Brain size	Difficult to determine, but appears within the range of Australopithecus afarensis.
Dentition	Small molars/dentition (Homo-like characteristic)
Cranial features	Flatter (i.e., orthognathic) face
Postcranial features	N/A
Culture	Some have associated the earliest tool finds from Lomekwi, Kenya, temporally (3.3 mya) and in close geographic proximity to this species/specimen.
Other	Taxonomic placing of this species is quite divided. The discoverers have argued that this species is ancestral to Homo, in particular to Homo ruldolfensis.

Hominin	Homo habilis
Dates	2.5 million years ago to 1.7 million years ago
Region(s)	East and South Africa
Famous Discoveries	Olduvai Gorge, Tanzania; Koobi Fora, Kenya; Sterkfontein, South Africa
Brain Size	650 cc average (range from 510 cc to 775 cc)
Dentition	Smaller teeth with thinner enamel compared to Australopithecus; parabolic dental arcade shape
Cranial Features	Rounder cranium and less facial prognathism than Australopithecus
Postcranial Features	Small stature; similar body plan to Australopithecus
Culture	Oldowan tools

Hominin	Homo erectus
Dates	1.8 million years ago to about 200,000 years ago
Region(s)	East and South Africa; West Eurasia; China and Southeast Asia
Famous Discoveries	Lake Turkana, Olorgesailie, Kenya; Zhoukoudian, China; Dmanisi, Republic of Georgia
Brain Size	Average 900 cc; range between 650 cc and 1,100 cc
Dentition	Smaller teeth than Homo habilis
Cranial Features	Long, low skull with robust features including thick cranial vault bones and large brow ridge, sagittal keel, and occipital torus
Postcranial Features	Larger body size compared to Homo habilis; body proportions (longer legs and shorter arms) similar to Homo sapiens
Culture	Acheulean tools (in Africa); evidence of increased hunting and meat-eating; use of fire; migration out of Africa

For Further Exploration

The Smithsonian website hosts descriptions of fossil species, an interactive timeline and much more! It is a highly recommended website. http://humanorigins.si.edu/evidence

The Maropeng Museum website hosts a wealth of information regarding South African Fossil Bearing sites in the Cradle of Humankind. https://www.maropeng.co.za/content/page/human-evolution

A collation of 3-D files for visualizing (or even 3-D printing) for homes, schools, and universities: https://www.hetmp.com/

A wealth of information from the Australian Museum website, including species descriptions, family trees, and explanations of bipedalism and diet: https://australianmuseum.net.au/learn/science/human-evolution/

 

Terms You Should Know

Acheulean

Arboreal

Aridity Hypothesis

Bipedalism

Breccia

Closed habitat

Context

Cores

Early Stone Age (ESA)

East African Rift System (EARS)

Encephalization

Fallback foods

Faunal assemblages

Flakes

Foramen magnum

Fossil

Gracile Australopithecines

Holotype

Hominin

Knapping

Large Cutting Tools (LCT)

Molecular Phylogeny

Obligate bipedalism

Oldowan

Orthognathic

Paleoanthropologists

Paleoenvironment

Pleistocene

Postcranium

Prognathic

Relative dating

Robust Australopithecines

Taxa

Techno-complex

Thermoregulation

 

Study Questions

1. What is the difference between a “derived” versus a “primitive” trait? Give an example of both, seen in Au. afarensis.
2. How does past environment relate to the physical traits developed by hominins? To the diversity of hominin species?
3. Which anatomical features for bipedalism do we see in early hominins? Are these primarily obligate bipeds? Explain.
4. List the hominin species argued to be associated with stone tool technologies. Are you convinced of these associations? Why/why not?
5. List the key anatomical characteristics that are generally agreed to define the genus Homo.
6. Name some specific behaviors associated with Homo erectus in the areas of tool use, subsistence practices, migration patterns, and other cultural innovations.

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