Chapter eleven: from hominoid to hominin

Chapter Review

The Setting

In the Miocene epoch the earth's temperature cooled, and in the African tropics—where apes had enjoyed an adaptive radiation—rainfall decreased and woodlands and grasslands expanded. In response to these ecological changes and over many selective steps, some primates came down from the trees, took up residence on the savanna and the sparser woodlands, and moved around bipedally (on two feet instead of all four). These are the first hominins, our earliest non-ape ancestors. Contemporary hominins (humans!) share a number of derived traits that make them unique from their hominoid and some hominin ancestors, such as bipedal locomotion, a more parabolic dental arcade (tooth arch) with smaller canines and thick enamel, a very large brain with regard to body size, slow juvenile development, and a dependence on culture and language. The early hominins, however, looked and probably behaved more like apes, certainly more than we do now! This summary includes an inventory and explanation of all the early hominins with which we are familiar, as well as some theories on the evolution of bipedalism, our unique human mode of locomotion.

The Earliest Hominins

The earliest fossil hominins—while not yet classified as australopithecines—date to 6 million years ago (mya) and show a varied mix of anatomical features, some reminiscent of chimpanzees and others of humans. The Ardipithecus ramidus specimen (5.8 to 5.2 mya), for example, discovered in 1992 by Tim White's team (University of California at Berkeley) in the Middle Awash basin in Ethiopia, has been variably reported as an ancestor to all hominins or the ancestor to hominins and modern chimpanzees. A. ramidus has a downward facing foramen magnum (the large hole in the bottom of your skull where your spinal cord passes), signifying a more straight vertebral column and possible bipedalism. Apes and other quadrupeds have their foramen magnum positioned more toward the back of their skulls. The teeth of A. ramidus are also more human-like, with a reduced canine, but their molar teeth are more ape-like, and all their teeth have thin enamel, like hominoids. More recent finds of material classified in the Ardipithecus genus have appeared more ape-like, thus garnering a new species name Ardipithecus kadabba.
Another early hominin species, Orrorin tugenensis, found in 2001 by Brigitte Senut and Martin Pickford’s French-Kenyan team in the highlands of Kenya, dates to 6 mya and is another hominin with a mix of chimpanzee and human traits. In this species though, the tooth enamel is thick, like that of humans, but the shape and size of their teeth are more chimpanzee-like. Additionally, a surprising find in the country of Chad (far away from places like Kenya and Tanzania, from where most early hominins have been excavated) has transfixed the paleoanthropological world. The nearly complete skull and associated fragments, now named Sahelanthropus tchadensis, has turned out to be the oldest hominin ever found, dating from about 7 to 6 million years ago. Nicknamed Toumaï (meaning “hope of life” in the local people’s language), this individual looks more like hominins from around 2 million years ago! It has a large browridge and a relatively flat face, characteristics that anthropologists previously thought had never existed in the earliest hominins. This find suggests that both the range and features of early hominins may be more diverse than anyone could imagine.

A Very Brief History

The meaning of the word Australopithecus is "southern ape," which harkens back to the name given to the original early hominin specimen. In 1924, Raymond Dart examined the strange immature skull of a small-brained creature extracted from the cave breccia of a South African mine. This was the first discovery of a recognized early hominin in Africa and was named Taung, after the site where it was found; the species designation Dart gave it was apt—Australopithecus africanus, or "southern ape from Africa." The acknowledgment of the Taung specimen as a member of the human lineage remained hotly contested for many years after the initial finding, primarily because of preconceived notions about which characteristics would eventually be understood to embody the human precursor. Also, many researchers of Dart's time were convinced that the human line started in Asia and not Africa, despite the paucity of evidence from both regions.

The Fossils

Australopithecus anamensis

The genus Australopithecus spans a portion of geologic time from approximately 4.2 mya to about 1.0 mya. The earliest form, a species called Australopithecus anamensis, was announced by Maeve Leakey from the Kenya National Museum in 1994. Dated from between 3.8 and 4.2 mya, these fossils exhibit characteristics of the postcranium suggesting that A. anamensis was bipedal, although dental and mandibular morphology (besides the presence of thick enamel) indicates a more ape-like physical disposition. These fossils have been found in the western Lake Turkana region in northern Kenya.

Australopithecus afarensis

The most famous specimen of Australopithecus—commonly known as Lucy—belongs to the species A. afarensis. Found at sites in East Africa (Laetoli and Hadar most notably), this species, which spans a time range between 4 and 3 mya, has provided a great deal of fossil information concerning the morphology of early hominins. Most of the characteristics are continuous, and their condition within A. afarensis lies somewhere between other hominins (relatively lower) and chimpanzees (relatively higher)

subnasal prognathism (degree of facial projection beyond the borders of the
braincase when viewed in profile)
canine size dimorphism
modest diastema
endocranial capacity approximately 404 cubic centimeters (cc), which is quite close to that of modern chimpanzees
compound temporal-nuchal crests
sagittal crest
V-shaped dental arcade

These traits are variably expressed across later australopithecine species.

Australopithecus africanus

A. africanus includes the previously mentioned Taung child. A. africanus is similar to A. afarensis in its postcranial anatomy (elements of the pelvis, ribs, and vertebrae). The cranium of A. africanus is somewhat different and more gracile, with a higher degree of cranial flexion and no crests (sagittal or compound temporal-nuchal), as observed in A. afarensis. These specimens have been found predominantly at the South African sites of Sterkfontein and Makapansgat, in addition to Taung, and date from 3 to 2.2 mya.

Australopithecus garhi

Found at and around a site called Bouri, in Ethiopia, A. garhi was named relatively recently. The general morphology of this hominin is similar to the other australopithecines, though A. garhi has a sagittal crest and the canines are generally larger. The discovery of this species prompts researchers to ask new questions about the lineage leading to Homo sapiens sapiens, since it is dated to 2.5 mya, which is approximately the right time for it to be the ancestor of the first Homo species.

Australopithecus habilis/rudolfensis

The first specimens of a different type of hominin emerged approximately 2.4 mya in East Africa. These hominins exhibited a different suite of characteristics, including the following:

Higher cranial capacity
Reduction in tooth size (particularly molars and premolars)
Reduction in masticatory apparatus
Parabolic dental arcade
Thinner tooth enamel than other australopithecine species
Less prognathic facial profile

These specimens have been traditionally attributed to Homo habilis, although recent investigators have alleged that the specimens should be assigned to the genus Australopithecus—a more updated scheme we use here—because they are more similar to the australopithecines than they are to any other fossils that are classed in the genus Homo. Additionally, there are two distinct types of these fossils, a larger and more robust species named A. rudolfensis and a smaller species named A. habilis. Furthermore, around this time stone tools begin to appear in the archaeological record, a technology known as the Oldowan tool industry, which is discussed in greater detail in Chapter 12.

The Genus Paranthropus

Paranthropus robustus

P. robustus fossils differ markedly from the more gracile hominin forms described above. Their crania exhibit remarkably well-fortified structures and buttresses to counteract increased forces in mastication (chewing). Some of these characteristics include a large sagittal crest as well as flaring zygomatic arches (cheekbones), both of which provide additional leverage and anchor points for the muscles of mastication. In addition, the teeth of P. robustus are very large, particularly compared to the anterior (front) dentition.

Paranthropus aethiopicus

P. aethiopicus, discovered by Alan Walker (Pennsylvania State University), stirred quite a controversy over the phylogenetic arrangement of the hominin lineage. Until this discovery, most phylogenies had the australopithecines evolving down one exclusive lineage, while Homo was evolving separately down another (although both shared a common ancestor, presumably A. afarensis). A major portion of this puzzle included a tight relationship between the robust australopithecines—classed as Paranthropus in our text—an arrangement that required rethinking when the P. aethiopicus specimen KNM-WT 17000 (commonly called the Black Skull) was found. This specimen bore a number of similarities with A. afarensis, which suggest its placement as an intermediate form between A. afarensis and P. boisei, thus the previous arrangement with the South African P. robustus became questionable. Now, though, the paranthropines are often classed as distinct from the australopithecines because of their anatomical derived traits and adaptive differences.

Paranthropus boisei

P. boisei, the East African robust form, exhibits a hyperrobust morphology, with exaggerated features similar in kind to those of P. aethiopicus, but far greater in degree. The massive dentition is coupled with a sagittal crest; increased dishing of the face also characterizes these forms. A reduction in postnasal facial projection probably came as a result of increased demands to combat masticatory forces. Whereas earlier interpretations of their dental specialization suggested dietary shifts (for example, consuming a tougher, lower-quality diet), recent analyses of dental microwear suggest that P. boisei incorporated a diverse range of foods in its diet.

The Genus Kenyanthropus

K. platyops is a relatively new fossil find, dated between 3.5 and 3.2 mya; it was uncovered on the west side of Lake Turkana in Kenya by Maeve Leakey's team in 1999. The combined anatomical features of this hominin are unique when compared with both australopithecines and paranthropines, including its comparably small molars and a broad face.

The Evolution of Bipedality

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Figure 11.1 Chimpanzee in a bipedal posture reaching for fruit.
Image Credit: Robert Boyd.

The principal feature binding all hominins together into a unique group is their choice of locomotion. All hominins exhibit features of their anatomy that indicate habitual (if not totally efficient) bipedalism. A great many features provide direct or subtle evidence for this behavior; some of the main features of the skeletal system include the following:

Shape and proportioning of the pelvic girdle
Positioning of the head and neck of the femur
Position of the foramen magnum, the hole on the underside of the cranium, which transmits the brainstem and spinal cord

Furthermore, reconfiguration of muscular attachments, especially the hip abductors, is required to maintain proper balance while moving in a bipedal gait. These characteristics, when observed in fossilized skeletal elements, argue for an organism that used a highly specialized mode of locomotion shared with all human beings today (Table 11.1). A striking find at the site of Laetoli in East Africa confirms the efficient bipedality of at least one fossil hominin. Mary Leakey and her coworkers found the fossilized footprints of three hominins preserved in volcanic ash. Researchers have shown that the patterns of these footprints are virtually indistinguishable from those made by modern humans. Yet, we cannot pin the footprints on a single hominin species; two known species (and perhaps some as yet unknown!), Australopithecus afarensis and Kenyanthropus platyops, lived in this region of East Africa at the time the footprints were made.

Table 11.1 Theories on the Origin of Bipedality

Early Hominin Fossil Resources Online

To keep up on current thought, check out the Recent Developments in Paleoanthropology page of the Fossil Hominids FAQ at the archives of talk.origins. The Prominent Hominid Fossils page includes a handy list with nicknames, museum specimen numbers, descriptions, and links.

http://talkorigins.org/faqs/homs/recent.html http://www.talkorigins.org/faqs/homs/specimen.html

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