By the end of this section, you will be able to:
- Compare and contrast some early hominin species.
- Identify some key adaptations and characteristics found in early hominins.
- Identify key adaptations and derived traits that emerged in changing environments.
Walking on Two Feet
The term hominin refers to all species considered to be in direct lineage to humans, which include the genera Homo, Australopithecus, Paranthropus, and Ardipithecus. Hominids refers to all modern and extinct great apes, which include humans, gorillas, chimpanzees, and orangutans and their ancestors. These terms have been understood to represent different things over the years, but the definitions provided here are the most current. While all hominins may differ in varying ways from one another, they all share one anatomical behavioral complex: bipedal locomotion.
Scientists can hypothesize about how a creature moved by analyzing several aspects of its morphology. Brachiators, animals that move by swinging from branch to branch, generally have long arms, while leapers, animals that propel their bodies through the force of their lower limbs, have relatively long legs. Arboreal primates have arms and legs of equal length. In bipedal locomotion, one leg is called the stance leg, and the other is called the step leg. While the stance leg is on the ground, the step leg is off the ground and striding forward. During normal walking, both feet are on the ground only about 25 percent of the time. As speed of locomotion increases, the percentage of time that both feet are on the ground decreases. As a result, for most of the time that bipedal organisms are moving, their body is balanced on only one of their legs (the stance leg). To ensure that bipedal organisms do not fall over while balanced on their stance leg, they have undergone many anatomical changes since the earliest hominin ancestors.
One of the most important anatomical changes that facilitate successful bipedalism is the angling of the femur (upper leg bone) inward at what is referred to as a valgus angle, which positions the knees and feet under the center of the pelvis. Bipedal hominins have also evolved spinal curves that make it possible for the hips to balance the weight of the upper body. The evolution of the arch in the foot as well as the realignment of the big toe so that it is parallel to the other toes is also instrumental in transmitting weight during the step phase of bipedal locomotion.
The most important evidence of early hominin bipedalism is provided by the work of English paleoanthropologist Mary Leakey. In the 1980s, Mary Leakey discovered a 75-foot trail of footprints made by three bipedal individuals who had crossed a thick bed of wet volcanic ash about 3.5 MYA. These footprints were found in East Africa at the site of Laetoli. Based on the date and the location, it is probable that these footprints were made by Australopithecus afarensis. Analysis of the Laetoli footprints indicates a modern striding gait.
The evolution of hominin bipedalism required complex anatomical reorganization. For natural selection to produce such a tremendous amount of change, the benefits of these changes must have been great. There have been dozens of hypotheses for these changes, ranging from freeing hands to carry tools, food, or offspring to increasing energy efficiency or thermoregulation (the ability to maintain the body’s temperature) by exposing more of the body’s surface. None of the hypotheses are testable, making it truly challenging to understand why humanity’s ancestors made such a huge behavioral shift. The next sections explore some of the key discoveries of early hominin fossils in which anthropologists see some of the earliest indications of the adaptation of bipedalism in the human story.
The first hominid fossils appear in the late Miocene, 10 to 5 MYA. Sometime between 7 MYA and 4 MYA, hominids moved out of the trees and began to adapt more fully to a ground-based living niche. Unfortunately, the fossil evidence from this time period is extremely sparse, but new finds continue to be discovered.
A complete cranium of Sahelanthropus tchadensis was found in 2002 by French paleoanthropologist Michel Brunet and his team in Chad in West Africa. Sahelanthropus is a fossil ape that lived approximately 7 MYA and is claimed by some researchers to be the last common ancestor of humans and chimpanzees. Genetic studies indicate that humans and chimpanzees diverged from one another sometime between 5 MYA and 7 MYA, so this species lived right at the time of the divergence. The cranial capacity is a mere 350 cubic centimeters (cc), which is equivalent to a chimpanzee; the modern human cranial capacity is approximately 1,400 cc. Sahelanthropus also has a very large brow ridge (the large bone above the eyes), and the location of the foramen magnum, the opening at the base of the skull where the spinal column enters the skull, suggests that its head was not held over its spine and thus it was not bipedal.
Orrorin tugenensis was found in Kenya in 2001 by geologist Martin Pickford of the Collège de France and paleontologist Brigitte Senut of France’s National Museum of Natural History. Orrorin tugenensis was dated to approximately 6 MYA. Orrorin was proposed to be a hominin due to anatomical traits that suggest bipedalism. For example, the femoral head (the big, rounded ball at the top of the leg bone that connects the leg to the hip) is much larger than in quadrupedal apes, suggesting the femur was being used to support the weight of the upper body. The muscles attached to the femur also suggest bipedal movement. Another feature that suggests that Orrorin is truly a hominin is the teeth, which exhibit thick dental enamel and small, square molars, much like modern humans.
The Pliocene epoch extended from 5 MYA to 1.8 MYA. Fossils from the Pliocene show evidence of the evolution of hominins that are clearly bipedal. They also show evidence of clear, albeit primitive, cultural behavior. Climatically, the Pliocene was colder than the preceding Miocene, which resulted in changing sea levels and an increase in ice at the poles, opening up some previously inaccessible areas. During this period, North and South America became connected through the Isthmus of Panama, and a land bridge across the Bering Strait appeared between Alaska and Siberia.
Ardipithecus ramidus was found in Ethiopia in 1992 by American paleoanthropologist Tim White and was dated to about 4.4 MYA. This is the first discovered hominin species to be dated to the Pliocene era. Based on the forward position of the foramen magnum, it can be concluded that Ardipithecus was bipedal. Also, the upper arm bones are very small, suggesting that the arms were not used to support weight during locomotion. Ardipithecus possesses numerous traits, such as thin dental enamel, evidence of a reduced canine, and an opposable big toe. As a result of the latter trait, many believe that Ardipithecus was bipedal on the ground and quadrupedal in the trees. This hypothesis is supported by the fact that the fossil bones were found in relatively heavily forested environments. The reduced canine is a derived trait appearing even earlier than A. ramidus and is not what we would typically see in African ape males who have large intimidating canines. Current hypotheses suggest that over time smaller canines became dominant when there became less need to show aggression along with a female preference for males with milder temperaments (Suwa, G., et al. 2021).
The Robust and Gracile Australopithecines
The next few sections will examine various australopithecine species that had diverse physical characteristics related to morphology of the teeth and skull. Based on these characteristics, paleoanthropologists classified these species into gracile and robust forms, as illustrated in Figure 4.33. Gracile species had a more pronounced projection of the jaw (prognathism), less flared cheeks with no sagittal crest, and smaller teeth and jaws. The sagittal crest in the robust australopithecines accommodated large temporalis jaw muscles for chewing tough plant materials.
Species considered to be the gracile include Australopithecus anamensis, A. afarensis, A. africanus, A. garhi, and A. sediba. The robust australopithecines (classified under the genus Paranthropus) include Paranthropus robustus, P. boisei, and P. aethiopicus. The gracile species emerged around 4 MYA and disappeared 2 MYA, while robust species continued to exist for another million years. The next sections will first take a look at some of the gracile forms of australopithecine, followed by the robust forms.
Australopithecus africanus was the first australopithecine discovered, in 1924, and was described by Australian anatomist and anthropologist Raymond Dart, who found the fossil in a box of fossils sent to him by lime quarry workers at a site called Taung in South Africa. The most notable specimen in the box was a skull from a child, which Dart had to chip away from the stone it was embedded in. It took Dart four years to separate the teeth. The skull is now known as the Taung skull or Taung child. Dart argued that the Taung child represents “an extinct race of apes intermediate between living anthropoids and man” (Wayman 2011). He noted that the skull was long and narrow, not rounded as in modern humans, and its brain averaged a mere 422 cc, equivalent to a chimpanzee. However, the Taung child did not possess brow ridges, had circular orbits, and had minimal prognathism as well as small canines and no diastema (space in the jaw for large canines to be positioned when the mouth closes). These latter traits are all analogous to modern humans. Most importantly, Dart noted that the forward position of the foramen magnum indicated that the skull was poised on top of the vertebral column, suggesting bipedalism and an upright posture.
In 1973, a good portion of a skeleton (about 40 percent) was found in the Afar region of Ethiopia by American paleoanthropologist Donald Johanson. He called the skeleton Lucy, after a Beatles song. It was dated to around 3.75–2.8 MYA and was determined to be a member of the species Australopithecus afarensis. Like all fossils recently discovered, Lucy was given an identification or accession number, KNM-AL-288. The KNM acronym stands for the Kenya National Museum, where the fossil is housed, and AL stands for the Afar locality where the fossil was found. Since then, more specimens of this species have been found in Kenya, Tanzania, and Ethiopia, all in East Africa.
Australopithecus afarensis is dated from 3.9 to 2.9 MYA with an endocranial capacity of around 400 cc, which is approximately the same as a common chimpanzee. There are two morphological features that provide evidence that A. afarenis moved more like a great ape than a human. First, it had arms that were substantially longer than modern humans’. Long arms are generally found in animals that hang from branches, suggesting that A. afarensis also exhibited this behavior. Also, A. afarensis possesses finger and toe bones that are long and curved, another characteristic of animals that hang from branches. However, there is one important morphological feature of A. afarensis that suggests that this species may have moved somewhat like modern humans. The shape of A. afarensis’s pelvis (hip bones) looks substantially more like a modern human’s than it does an ape’s. Instead of the hip bones being long and narrow, they are short and wide. Most paleoanthropologists believe that this change in pelvic shape indicates that A. afarensis moved like modern humans do, on two legs. While A. afarensis may have locomoted bipedally, the morphological differences between A. afarensis and modern humans suggest they did not move in exactly the same way. Current consensus is that A. afarensis was both tree dwelling and bipedal. Other anatomical evidence of bipedalism includes a more anterior position of the foramen magnum and the angle of the femoral head and neck.
Also found in Ethiopia, Australopithecus garhi is dated to approximately 2.5 MYA. Its cranial capacity is slightly greater than A. afarensis, at 450 cc. Australopithecus garhi has incisors that are larger than those of any of the known australopithecines or Homo. The function of the large incisors is not yet known. The most exciting aspect of A. garhi is that it provides evidence of the earliest use of stone tools by a hominin. Specifically, A. garhi fossils were found with fossil bones of ruminants, such as antelopes, that displayed numerous cut marks. Cut marks are made on bones by the process of removing meat from the bones with stone or metal tools. Based on this finding, biological anthropologists have hypothesized that A. garhi used some type of stone tool for butchering.
In 2008, the clavicle bone of Australopithecus sediba was discovered by Matthew Berger, the nine-year-old son of American paleontologist Lee Berger, in Malapa, South Africa. Further excavation in a cave feature uncovered two partial skeletons, one of an adult female and the other a younger juvenile. A. sediba is considered an important species because it appears in the fossil record around the time of the first emergence of the genus Homo around 2 mya. The classification of A. sediba was initially difficult to determine, due to its complex overlapping features, which include humanlike spine, pelvis, hands, and teeth and a chimpanzee-like foot. This combination of traits suggests both tree climbing and bipedal adaptations. After studying the characteristics collectively, anthropologists classified A. sediba as a species of Australopithecus. It is considered a direct ancestor of Homo erectus and Homo ergaster, which are discussed in Chapter 5, The Genus Homo and the Emergence of Us . It is believed that A. sediba could be a descendent of A. africanus, which suggests the species may be a dead end within the lineage to humans. Its classification and relationship with the genus Homo will likely remain highly debated.
Thirteen years after Raymond Dart’s discovery, South African paleontologist and medical doctor Robert Broom discovered Paranthropus robustus at a site called Kromdraai in South Africa. The most obvious difference between Dart’s and Broom’s respective fossils, A. africanus and P. robustus, is that the morphology of Broom’s fossil is much larger. Its features include a sagittal crest and a flared zygomatic arch for the attachment of a large temporalis muscle for chewing a diet reliant on hard nuts and seeds. This interpretation was further supported by scanning electron microscopy (SEM), which was used to evaluate the markings etched into the teeth. As the teeth increased in size the incisors and canines shrank, giving Paranthropus a flatter face with less projection of the jaw. There are some who argue that depending on the environment and locale, some Paranthropus may have been omnivores, with varied diets similar to those of H. ergaster. (Lee-Thorp, Thackeray, and van der Merwe 2000).
Following in Broom’s footsteps, other scientists began searching for fossils in East Africa. Beginning in 1931, Kenyan and British paleoanthropologist Louis Leakey and his wife, Mary Leakey, worked in what is known as the Eastern Rift Valley, which is a 1,200-mile trough extending through Ethiopia, Kenya, and Tanzania. They searched for almost 30 years before they found their first hominin fossil, Paranthropus boisei (OH-5)—originally classified as Zinjanthropus boisei—in 1959. It is often referred to as the hyper-robust hominin because of its mohawk of bone on the top of the skull. Other features include a low or absent forehead, a flat face, large jaws, and large attachment sites over the entire skull for chewing muscles.
We have little knowledge about Paranthropus aethiopicus (shown in Figure 4.37), which has been dated to about 2.5 MYA and is referred to as the “black skull.” It is believed that this species falls somewhere between the robust and gracile australopithecines, having characteristics of both. The species was discovered in Ethiopia in 1967 by a French expedition team headed by Camille Arambourg and Yves Coppens.
Landmarks and Questions
While the fossils discovered up to this point have provided a small window into the story of humanity’s past, they have also simultaneously raised numerous questions. Questions related to phylogenetic relationships and points of divergence are challenges for paleoanthropologists, who have only fragmentary fossil evidence to build hypotheses around. Nevertheless, the discoveries that have been made represent important landmarks in anthropologists’ understanding, providing clues that will lead to the next steps in the human journey.
Conduct a pedestrian survey to try to locate fossils near where you live (trilobites in New York, ammonites in Texas, shark teeth near riverbeds, arrowheads). Think about where you would most likely find a fossil and why. Try to extract one without destroying the environment around it, which provides important context. Try to figure out what kind of fossil it is by doing some Internet research. Why do you think that this fossil was preserved? What information would make the search for fossils easier?