The unique form of locomotion exhibited by modern humans, called obligate bipedalism, is important in distinguishing our species from the extant (living) great apes. The ability to walk habitually upright is thus considered 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.
The origin of bipedalism in hominins has been debated in paleoanthropology, but at present there are two main ideas:
that early hominins descended from trees, and so we were a product of an arboreal last common ancestor (LCA) or
that our LCA was a terrestrial quadrupedal knuckle-walking species, more similar to extant chimpanzees.
Most research supports the theory of an arboreal LCA (i.e., idea 1) based on skeletal morphology of early hominin genera that demonstrate adaptations for climbing but not for knuckle-walking. This would mean that both humans and chimpanzees can be considered “derived” in terms of locomotion since chimpanzees would have independently evolved knuckle-walking.
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:
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.
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.
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 9.5). 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. These adaptations occur throughout the skeleton and are summarized in Table 9.1.
Figure \(\PageIndex{1}\): Compared to gorillas (right) and other apes, humans (left) have highly specialized adaptations to facilitate bipedal locomotion.
The majority of these adaptations occur in the postcranium (the skeleton from below the head) and are outlined in Figure 9.6. 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 hip replacements 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.
Table 9.2.1: Skeletal comparisons between modern humans (obligate bipeds) and non-obligate bipeds (e.g., chimpanzees).
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)
Nonhuman apes: Longer upper limbs (used for locomotion)
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 hallux (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 (long bones in the hand and fingers), 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.
Earliest Hominins: Sahelanthropus and Orrorin
We see evidence for bipedalism in some of the earliest fossil hominins, dated from within our 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 9.7; Brunet et al. 1995). The initial discovery was made in 2001 by Ahounta Djimdoumalbaye and announced in Nature in 2002 by a team led by French paleontologist Michel Brunet. 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. This implies strongly that, over evolutionary time, the need for display and dominance among males has reduced, as has our sexual dimorphism. 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 is based on cranial evidence alone. Researchers have suggested that in order to conclude if it is a truly bipedal species, we need to find postcranial remains such as a pelvis or a leg bone, which would support the idea of upright walking. An unpublished femur (thigh bone) thought to belong to Sahelanthropus was discovered in 2001 and could potentially shed light on this topic once it is fully studied. However, the extent to which this femur is hominin-like is currently unknown.
Figure \(\PageIndex{2}\): Sahelanthropus tchadensis exhibits a set of a set of derived features, including a long, low cranium; a small, ape-sized braincase; and relatively reduced prognathism.
Orrorin tugenensis (Orrorin meaning “original man”; dated to between 6 mya and 5.7 mya) was discovered near Tugen Hills in Kenya in 2000. Smaller cheek teeth (molars and premolars) than those in even more recent hominins (i.e., derived), thick enamel, and reduced, but apelike, canines characterize this species. This is the first species that clearly indicates adaptations for bipedal locomotion, with fragmentary leg, arm, and finger bones having been found but few cranial remains. One of the most important elements discovered was a proximal femur, BAR 1002’00. The femur is the thigh bone, and the proximal part is that which articulates with the pelvis—it is very important when studying posture and locomotion. This femur indicates that Ororrin was bipedal, and recent studies suggest that it walked in a similar way to later Pliocene hominins. Some have argued that features of the finger bones suggest potential tool-making capabilities, although many researchers argue that these features are also consistent with climbing.
Earliest Hominins: The Genus Ardipithecus
Another genus, Ardipithecus, is argued to be represented by at least two species: Ardipithecus ramidus and Ar. kadabba.
Ardipithecus ramidus (“ramid” means root in the Afar language) is currently the best known of the earliest hominins (Figure 9.8). Unlike Sahelanthropus and Orrorin, this species has a large sample size of over 110 specimens from Aramis alone. Dated to 4.4 mya, Ar. ramidus was found in Ethiopia (in the Middle Awash region and in Gona). 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 postcrania. For instance, she had an opposable big toe (hallux), 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. A small brain (300 cc to 350 cc), midfacial projection, and slight prognathism show retained primitive cranial features, but the cheek bones are less flared and robust than in later hominins.
Figure \(\PageIndex{3}\): Researchers believe that Ardipithecus ramidus was able to walk upright, although not as efficiently as later humans. It possessed the musculature required for tree climbing, and while moving quadrupedally, it likely placed weight on the palms of the hands rather than on the knuckles.
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. Specimens include mandibular fragments and isolated teeth as well as a few postcranial elements from the Asa Koma (5.5 mya to 5.77 mya) and Kuseralee (5.2 mya) Members (well-dated and understood- but temporally separate- volcanic layers in East Africa). This species was discovered in 1997 by paleoanthropologist Dr. Yohannes Haile-Selassie. Originally these specimens were referred to as a subspecies of Ar. ramidus. In 2002, six teeth were discovered at Asa Koma and the dental-wear patterns confirmed that this was a distinct species, named Ar. kadabba, in 2004. One of the postcranial remains recovered included a 5.2 million-year-old toe bone that demonstrated features that are associated with toeing off (pushing off the ground with the big toe leaving last) during walking, a characteristic unique to bipedal walkers. However, the toe bone was found in the Kuseralee Member, and therefore some doubt has been cast by researchers about its association with the teeth from the Asa Koma Member.
Bipedal Trends in Early Hominins
Trends toward bipedalism are seen in our earliest hominin finds. However, many specimens also indicate retained capabilities for climbing. Trends include a larger, more robust hallux; a more compact foot, with an arch; a robust, long femur, angled at the knee; a robust tibia; a bowl-shaped pelvis; and a more anterior foramen magnum. While the level of bipedality in Salehanthropus tchadenisis is debated since there are few fossils and no postcranial evidence, Orrorin tugenensis and Ardipithecus show clear indications of some of these bipedal trends. However, some retained primitive traits, such as an opposable hallux in Ardipithecus, indicate some retention in climbing ability.
Figure \(\PageIndex{1}\): Compared to gorillas (right) and other apes, humans (left) have highly specialized adaptations to facilitate bipedal locomotion.
Figure \(\PageIndex{2}\): Sahelanthropus tchadensis exhibits a set of a set of derived features, including a long, low cranium; a small, ape-sized braincase; and relatively reduced prognathism.
Figure \(\PageIndex{3}\): Researchers believe that Ardipithecus ramidus was able to walk upright, although not as efficiently as later humans. It possessed the musculature required for tree climbing, and while moving quadrupedally, it likely placed weight on the palms of the hands rather than on the knuckles.