Pre-Australopiths (6.0+ – 4.4 mya)
The oldest and most surprising of these earliest hominins is represented by a cranium discovered at a central African site called Toros-Menalla in the modern nation of Chad (Brunet et al., 2002; . Provisional dating using faunal correlation (biostratigraphy) suggests a date of between 7 and 6 mya (Vignaud et al., 2002). Closer examination of the evidence used in obtaining this biostratigraphic date has led many paleoanthropologists to suggest that the later date (6 mya) is more likely.
The morphology of the fossil is unusual, with a combination of characteristics unlike that found in other early hominins. The braincase is small, estimated at no larger than a modern chimpanzee’s (preliminary estimate in the range of 320 to 380 cm3 ), but it is massively built, with huge browridges in front, a crest on top, and large muscle attachments in the rear. Yet, combined with these apelike features is a smallish vertical face containing front teeth very unlike an ape’s. In fact, the lower face, being more tucked in under the brain vault (and not protruding, as in most other early hominins), is more of a derived feature more commonly expressed in much later hominins, especially members of genus Homo .
What’s more, unlike the dentition seen in apes, the upper canine is reduced and is worn down from the tip (rather than shearing along its side against the first lower premolar). The lack of such a shearing canine/premolar arrangement (called a honing complex; ) is viewed by many researchers as an important derived characteristic of early hominins (White et al., 2009). Other experts are not entirely convinced and suggest that it could just as easily have evolved in both hominins and other hominoids due to homoplasy (Wood and Harrison, 2011).
In recognition of this unique combination of characteristics, paleoanthropologists have placed the Toros-Menalla remains into a new genus and species of hominin, Sahelanthropus chadensis (Sahel being the region of the southern Sahara in North Africa). These finds from Chad have forced an immediate and significant reassessment of early hominin evolution. Two cautionary comments, however, are in order. First, as we noted, the dating is only approximate, based, as it is, on biostratigraphic correlation with sites in Kenya (1,500 miles to the east). Second, and perhaps more serious, is the hominin status of the Chad fossil. Given the facial structure and dentition, it’s difficult to see how Sahelanthropus could be anything but a hominin. However, the position of its foramen magnum is intermediate between that of a quadrupedal ape and that of a bipedal hominin ; for this and other reasons, some researchers (Wolpoff et al., 2002) suggest that at this time, “ape” may be a better classification for Sahelanthropus. As we have previously said, the best-defining anatomical characteristics of hominins relate to bipedal locomotion. Unfortunately, no postcranial elements have been recovered from Chad—at least not yet. Consequently, we do not yet know the locomotor behavior of Sahelanthropus, and this raises even more fundamental questions: What if further finds show this form not to be bipedal? Should we still consider it a hominin? What, then, are the defining characteristics of our lineage? For all these reasons, several paleoanthropologists have recently grown more skeptical regarding the hominin status of all the pre-australopith finds, and Bernard Wood (2010) prefers to call them “possible hominins.”
Probably living at about the same time as Sahelanthropus, two other very early (possible) hominin genera have been found at sites in central Kenya in the Tugen Hills and from the Middle Awash area of northeastern Ethiopia. The earlier of these finds (dated by radiometric methods to around 6 mya) comes from the Tugen Hills and includes mostly dental remains, but also some quite complete lower limb bones. The fossils have been placed in a separate early hominin genus called Orrorin. The postcranial remains are especially important, since they seem to indicate bipedal locomotion (Pickford and Senut, 2001; Senut et al., 2001; Galik et al., 2004; Richmond and Jungers, 2008). As a result of these further analyses, Orrorin is the pre- australopith generally recognized as having the best evidence to establish it as a hominin (compared to less clear evidence for Sahelanthropus and Ardipithecus).
The last group of possible hominins dating to the late Miocene (that is, earlier than 5 mya) comes from the Middle Awash in the Afar Triangle of Ethiopia. Radiometric dating places the age of these fossils in the very late Miocene,5.8–5.2 mya. The fossil remains themselves are very fragmentary. Some of the dental remains resemble some later fossils from the Middle Awash (discussed shortly), and Yohannes HaileSelassie, the researcher who first found and described these earlier materials, has provisionally assigned them to the genus Ardipithecus (Haile-Selassie et al., 2004; see “At a Glance: Pre-Australopith Discoveries”). In addition, some postcranial elements have been preserved, most informatively a toe bone, a phalanx from the middle of the foot. From clues in this bone, Haile-Selassie concludes that this primate was a well-adapted biped (once again, the best-supporting evidence of hominin status).
From another million years or so later in the geological record in the Middle Awash region, a very large and significant assemblage of fossil hominins has been discovered at a site called Aramis. Radiometric dating firmly places these remains at about 4.4 mya. The site, represented by a 6-foot-thick bed of bones, has yielded more than 6,000 fossils. From this key site, excavations reveal both large and small vertebrates—birds and other reptiles and even very small mammals. Additionally, fossil wood and pollen samples have been recovered. All this information is important for understanding the environments in which these ancient hominins lived. Hominin fossil remains from Aramis include several individuals, the most noteworthy being a partial skeleton. At least 36 other hominins are represented by isolated teeth, cranial bones, and a few limb bones. All the bones were extremely fragile and fragmentary and required many years of incredibly painstaking effort to clean and reconstruct. Indeed, it took 15 years before the partial skeleton was in adequate condition to be intensively studied. But the wait was well worth it, and in 2009, Tim White and colleagues published their truly remarkable finds. By far, the most informative fossil is the partial skeleton. Even though it was found crushed and fragmented into hundreds of small pieces, the years of work and computer imaging have now allowed researchers to interpret this 4.4-million-year-old individual. The skeleton, nicknamed “Ardi,” has more than 50 percent of the skeleton represented; however, since it was found in such poor condition, any reconstruction must be seen as provisional and open to varying interpretations. Ardi has been sexed as female and contains several key portions, including a skull, a pelvis, and almost complete hands and feet (White et al., 2009;. Brain size, estimated between 300 and 350 cm3 , is quite small, being no larger than a chimpanzee’s. However, it is much like that seen in Sahelanthropus, and overall, the skulls of the two hominins also appear to be similar. The fact that remains of the postcranial skeleton are preserved is potentially crucial, because key body elements, such as the pelvis and the foot, are only very rarely discovered. This is the earliest hominin for which we have so many different parts of the body represented, and it permits researchers to hypothesize more confidently about body size and proportions and, perhaps most crucially of all, the mode of locomotion. Height is estimated at close to 4 feet, with a body weight of around 110 pounds. Compared with other early hominins, such a body size would be similar to that of a male and well above average for a female. The pelvis and foot are preserved well enough to allow good-quality computer reconstructions. According to Tim White and colleagues, both areas of the body show key anatomical changes indicating that Ardipithecus was a competent biped. For example, the ilium is short and broad and the foot has been modified to act as a prop for propulsion during walking.
However, Ardi also contains some big surprises. While the shape of the ilium seems to show bipedal abilities, other parts of the pelvis show more ancestral (“primitive”) hominoid characteristics. In fact, the paleoanthropologists who analyzed the skeleton concluded that Ardi likely walked quite adequately, but might well have had difficulty running (Lovejoy et al., 2009a, 2009b). The foot is also an odd mix of features, showing a big toe that is highly divergent and capable of considerable grasping. Some researchers are not convinced that Ardi was bipedal, and considering all her other primitive characteristics, some have questioned whether Ardipithecus was really a hominin at all (Sarmiento, 2010). The extreme degree of reconstruction that was required (for the skull and pelvis especially) adds further uncertainty to understanding this crucial discovery. One thing that everyone agrees on is that Ardi was an able climber who likely was well adapted to walking on all fours along the tops of branches. It seems clear that she spent a lot of time in the trees.
Accepting for the moment that Ardipithecus was a hominin, it was a very primitive one, displaying an array of characteristics quite distinct from all later members of our lineage. The new evidence that Ardi provides has not convinced all paleoanthropologists that Ardipithecus or any of the other very early pre-australopiths are hominins; indeed, Ardi’s very odd anatomy has caused doubts to increase. One thing is for sure: It would take a considerable adaptive shift in the next 200,000 years to produce the more derived hominins we’ll discuss in a moment. All of these considerations have not only intrigued professional anthropologists; they have also captured the imagination of the general public. When did the earliest member of our linage first appear? The search still goes on, and professional reputations are made and lost in this quest.
Another intriguing aspect of all these late Miocene/early Pliocene locales (that is, Toros-Menalla, Tugen Hills, early Middle Awash sites, and Aramis) relates to the ancient environments associated with these potentially earliest hominins. Rather than the more open grassland savanna habitats so characteristic of most later hominin sites, the environment at all these early locales is more heavily forested. Perhaps we are seeing at Aramis and these other ancient sites the very beginnings of hominin divergence, not long after the division from the African apes.