Large and complex brains for our body size, small teeth, and upright walking are some of the characteristics that best define us as individually human.Additionally, our ability to manufacture a wide array of sophisticated tools, engage in complex symbolic and social behaviors, and communicate using spoken language has further distinguished us.
However, many of these characteristics, especially characteristics associated with culture, are not found in association with early hominins. As a result, researchers must reference the fossil record to help define which specific morphological characteristics have prevailed throughout our lineage. These characteristics can then be applied to any extinct or extant specimen to determine whether it is a Hominini or not. These sets of characteristics become extremely important when observing our earliest ancestors. With the earliest possible human ancestor dated at about 7 million years ago (MYA), and with the split between our chimp cousins and us estimated at the same time, applying these Hominini characteristics help researchers to determine who really was our earliest ancestor.
In 2001, French paleontologist Brigitte Senut and French geologist Martin Pickford discovered Orrorin tugenensis across the Tugen Hills region of central Kenya. Through the use of radioscopic, biochronology and paleomagnetic absolute dating techniques, O. tugenensis, given the name, millennial man, was dated at around 6 to 7 MYA, making it the second oldest possible hominin (Sawada et al. 2012 & Senut et al. 2001). Shalenthropus tchadensis is still the title holder for oldest possible hominin. With at least five individuals, O. tugenensis shows a variety of both hominin and chimp-like traits. When observing the craniofacial functional morphology of O. tugenensis, researchers see a relatively more robust mandible, smaller premolar complex, larger cheek teeth with thin dental enamel, and smaller canines compared to those seen in chimpanzees (Senut et. al 2001). While differences between the dental morphology of chimpanzees and O. tugenensis vary, O. tugenensis morphology still does not directly match up to later hominins. Rather, the dental morphology of O. tugenensis show gradual shifts that start to resemble later hominins such as Ardipithecus ramidus and Kenyanthropus platyopus.
When observing the postcranial remains of O. tugenensis, researchers note that the femoral neck is longer than those found in extant apes, suggesting bipedality. Additionally, scans made by computed tomography of the femur show deferential thickness (thicker inferiorly and thinner superiorly) in the cortical bone which is also seen in other hominin bipeds.
However, some of the muscle markings found on the proximal end of the femur could indicate either bipedality or arboreality. Curvature and lengthening of the manual phalanx are also present in O. tugenensis which similarly resembles the finger bones of our more arboreal cousins, the chimpanzees. Furthermore, the humerus also shows a straight lateral crest, where the brachioradialis muscle is able to insert itself (Senut et. al 2001 & Harcourt-Smith 2010). This same feature is seen in both modern chimpanzees and in Au. afarensis and has been linked to climbing adaptations. As a result, postcranially O. tugenensis shows evidence for both bipedalism and arboreal locomotion, creating debates as to how and why it would show adaptations for both.
We humans are unique among all living primates in the way we move around the earth. So exceedingly rare is our form of locomotion among mammals, that the striding bipedalism we engage in, is, in fact, only noted in one living mammal, us! It is for this reason that bipedalism has become a key indicator in determining whether or not a species is a hominin.
With such an unusual form of locomotion, certain specialized anatomical characteristics must be present in order for a body to maintain a committed striding bipedal pattern. In order to walk one foot over the other, the body must be in perfect alignment with its center of gravity. Additionally, since we move one foot at a time, our legs and feet must be fortified to maintain stability. As a result, our bodies have 7 key features which directly aid in balancing and stabilizing the body during bipedal striding. These features include a centrally placed foramen magnum, a sigmoid shaped spine, a shallow and broad shaped pelvis, medially angled femurs, an increased lateral lip on the patella, a robust ankle region (the talus & calcaneus bones) and a non-opposable big toe. The first three features facilitate the body’s ability to support its trunk vertically as well as efficiently transfer weight equally to the legs. The last three features allow for the knee and ankle joints to extend and lock to limit any hyperextension and increase shock absorption and stability (Harcourt-Smith W. 2010).
One of the central debates concerning hominin evolution is how certain characteristics like bipedalism arose. Many speculate that these features emerged due to changing environments, diet, climate and social dynamics. When observing bipedalism specifically, numerous suggestions have emerged as to what type of locomotion the last common ancestor between humans and chimps had and how different it was to the locomotion patterns we see in both lineages today. Suggestions for the origin of bipedalism include, for example, arboreal bipedalism (Thorp et al. 2007), gibbon-like suspension (Tuttle 1981), terrestrial quadrupedalism and swimming behavior (CITE).
In order to better address the origin of bipedalism, or any behavior for that matter and to help determine possible explanations about early hominins, chimpanzees are often used as a comparative model. This is due to their genetic closeness to humans and to the last common ancestor. However, while using comparative models may be incredibly insightful, researchers must remain vigilant since they may have developed their own suite of divergent traits since the spilt. Regardless, it is important to remember the complexities of reconstructing any form of behavior on extinct mammals. So, while some evidence points towards bipedality within O. tugenensis, there are still too many unsolved/uncertain variables making some researchers doubt the extent of bipedality within this species. As a result of this uncertainty, O. tugenensis, like Sahelanthropus, has been classified as a possible hominin. So, while not 100% hominin, O. tugenensis paved the way to the way we walk today!