Australopithecus, Homo habilis and Homo Ergaster

The Place of Australopithecines● The situation of australopithecines as

the immediate antecedents of the genus Homo is well established

● Nonetheless, it must be recognised that there is significant diversity in the specimens and that their distributions and ages also differ

● A general distinction between gracile forms and robust forms has been made, with the former leading onward to the development of Homo habilis and Homo ergaster, whereas the latter were an evolutionary cul-de-sac that developed more similarities with the apes

● Some forms of robust australopithecines, commonly called Paranthropus, continued to flourish until as late as 1.2 million years ago in South Africa

Directions of Australopithecine Evolution

Distribution● The distribution of australopithecines and

Paranthropus overlaps largely with the region where Homo first emerged

● All the specimens have been found in a broad arc from East Africa to South Africa, with a curious outlier in southern Chad

● Although there were certainly local differences in the environment, we can characterise this region as one where savannah and open woodland prevailed

● This obviously was important for the subsistence opportunities afforded to these species and also for the adaptive advantages of being bipedal and exploiting arboreal environments

● It is therefore adaptation to these environments that probably triggered many of the changes in morphological characteristics

The Fossil Evidence

Principal Finds of Australopithecines and Their Characteristics

Australopithecus anamensis● The first fossilized specimen of the

species, though not recognised as such at the time, was a single arm bone found in Pliocene strata in the Kanapoi region of East Lake Turkana in 1965

● In 1995, Meave Leakey and her associates, noting salient differences between Australopithecus afarensis and the new finds, assigned them to a new species, A. anamensis, which derives its name from the Turkana word anam, meaning 'lake'

● It was determined that this species was independent and does not represent an intermediate species of any type

● In 2006, a new A. anamensis find was officially announced, extending the range of A. anamensis into north east Ethiopia

Relationship to Later Species● It is believed that Australopithecus

anamensis often climbed trees

● Tree climbing was one behavior retained by early hominins until the appearance of the first Homo species about 2.5 million years ago

● A. anamensis shares many traits with Australopithecus afarensis and is likely its direct predecessor

● A. anamensis is thought to have lived from 4.1 and 3.9 million years ago

● The older specimens were found between two layers of volcanic ash, dated to 4.17 and 4.12 million years, coincidentally when A. afarensis appears in the fossil record

● The A. anamensis find is dated to about 4.2 million years ago, the Ar. ramidus find to 4.4 million years ago, placing only 200,000 years between the two species

Australopithecus afarensis● The remains from A. afarensis are

relatively plentiful, and we are fortunate that perhaps the most complete early hominim skeleton is of this species, namely, the spectacular find of 'Lucy'

● This species lived between 3.8 and 2.9 million years ago and therefore immediately follows A. anamensis in the fossil record

● The distribution of this species is currently limited to East Africa, the most famous finds being made at Hadar, Omo, and Koobi for a

● It is widely maintained that this species led on to the Homo genus whereas other members of the Australopithecine family were evolutionary cul-de-sacs

Physiology● A. afarensis has reduced canines and

molars in comparison to those of the apes, although they are still relatively larger than in modern humans

● This species also has a relatively small cranial capacity (~380–430 cm³) and a prognathic face

● The small cranial capacity was something of a shock to many of the palaeoanthropologists, for they assumed that an increase in brain size was the first major adaptive change leading onward to the genus Homo and bipedalism

● There is considerable debate regarding the locomotor behaviour of A. afarensis

● Some believe that A. afarensis was almost exclusively bipedal, while others believe that the creatures were partly arboreal

Locomotion and Habitat

● The anatomy of the hands, feet and shoulder joints support an interpretation of A. afarensis being at least partly an arboreal dweller

● The curvature of the phalanges resembles that of modern-day apes, and is most likely reflective of their ability to efficiently grasp branches and climb

● The presence of a wrist-locking mechanism might suggest that they engaged in knuckle-walking

● The shoulder joint is also oriented more cranially than in modern humans

● Combined with the relatively long arms A. afarensis are thought to have had, this is thought by many to be reflective of a heightened ability to use the arm above the head in climbing behaviour

Bipedalism● Many traits, however, suggest that A.

afarensis was bipedal

● The traits are so strongly developed that it suggests that bipedalism emerged well before A. afarensis appeared, which is being corroborated by the finds of Sahelanthropus tchadensis which have been dated to ca. 7 million years

● The pelvis is far more human-like than ape-like, whereas he iliac blades are short and wide, the sacrum is wide and positioned directly behind the hip joint, and there is clear evidence of a strong attachment for the knee extensors

● While the pelvis is not wholly human-like (being markedly wide with flared with laterally orientated iliac blades), it is suitable for regular and sustained bipedal locomotion

Australopithecus bahrelghazali● The species designation of

Australopithecus bahrelghazali is a new one, currently gaining more and more support among palaeoanthrologists

● Arguments against the case for it being its own species are based upon its resemblances to Australopithecus afarensis

● A. bahrelghazali was discovered by Michel Brunet in 1993, in the ancient riverbed of Bahr el Ghazal in Chad, located 2,500 kilometers west of the East African Rift Valley

● This is currently the first and only australopithecine specimen to be found in North-Central Africa, and is also the furthest west of any specimen found to date

● Australopithecus bahrelghazali dates to between 3.5 and 3.0 million years ago

Australopithecus africanus● This species flourished between 3

million and 2 million years ago and had a much broader geographic distribution than A. afarensis

● It is also well represented in the fossil record, affording numerous specimens that permit a much more accurate reconstruction of their anatomical structure than is often possible for other hominim species

● Some of the main finds have been made in South Africa, suggesting that Australopithecines began to expand outwith East Africa and were adaptively successful, colonising different environments

● Most notable are the finds of the Taung child, Sterkfontein, Gladysvale and Makapansgat

Anatomical Structure● A. africanus was similar in most respects

to A. afarensis, being bipedal and having arms slightly longer than the legs

● This species also had curved phalanges, like A. africanus, interpreted as either an adaptive advantage for climbing trees or a vestigial feature

● It also had a relatively small cranial capacity, but again comparable to that of A. afarensis

● Some ostensible 'primitive' features have suggested to some that this species was not a direct descendant of Homo, but rather was the first in a line that was leading to more robust forms of Australopithecine

● We therefore see Australopithecine emerging into two groups: one more gracile and one more robust

Paranthropus aethiopicus● This species was formerly known as A.

aethiopicus, but the robustness of its features and the divergence from the Australopithecines that were becoming more gracile led to some re-classifying this line as 'Paranthropus'

● The species inhabited East Africa from 2.7 – 2.0 million years ago and is best represented by the skull and bones found at West Turkana (KNM WT 17000), known as the 'black skull' due to the discolouration caused by high levels of manganese

● Lower jaw and teeth fragments have been uncovered from some specimens

● It is therefore possible to state that P. aethiopicus had a large sagittal crest and zygomatic arch adapted for heavy chewing, which resembled the mastication complex of gorillas

Robust Forms: A Divergent Line ?● Some are uncomfortable with the

argument that Australopithecines began to diverge into a more robust type that ultimately failed, whereas the more gracile forms evolved into the genus Homo

● It is argued that some of the forms of Paranthropus derived from separate forms of Australopithecine—namely, Paranthropus robustus evolving from A. africanus whereas the others from A. afarensis—and therefore invalidates the existence of Paranthropus as a separate genus

● Ultimately, the trajectory of evolution seems to suggest that the robust forms commonly assigned to Paranthropus diverged from the gracile forms and followed a different course of evolution that did not contribute to the emergence of Homo

Australopithecus afarensis

Australopithecus africanus Paranthropus aethiopicus

Paranthropus boisei

Paranthropus robustus

Australopithecus garhi

Homo habilis

?

Paranthropus boisei● This species is more recent that some

forms in the genus Homo, being dated to as recently as 1.75 million years

● The first specimens, found at Olduvai Gorge, were classified as Zinjanthropus boisei, but the discovery of more Australopithecines and Paranthropus finds led to a reclassification

● The brain volume is quite small, about 500 and 550 cm³, not much larger in comparison to Australopithecus afarensis and Australopithecus africanus

● It had a skull highly specialized for heavy chewing and several traits seen in modern day gorillas, with a sagittal crest and heavy dentition

● P. boisei inhabited savannah woodland environments and the carbon isotope ratios of P. boisei suggest that it had a diet dominated by C4 vegetation

Paranthropus robustus● This species lived in South Africa

between 2.0 and 1.2 million years ago

● P. robustus had large sagittal crests, jaws, jaw muscles, and post-canine teeth that were adapted to serve in the dry environment that they inhabited

● The teeth of these primates were larger and thicker than any gracile australopithecine found

● On the skull, a bony ridge is located above from the front to back indicating where the jaw muscles joined

● P. robustus males may have stood only 1.2m (4 feet) tall and weighed 54 kg (120 lb) while females stood just under 1 meter (3 feet 2 inches) tall and weighed only 40 kg (90 lb)

Australopithecus garhi

The Earliest Tool Maker and Progenitor of the Genus Homo

Importance of the Species● A species known as Australopithecus

garhi was found in Ethiopia in 1996

● This species dates between 2.6 million and 2.5 million years; a broader temporal determination for its persistence cannot be made at present because of the paucity of fossils

● It is particularly significant, though, because it is the earliest hominim species that can unequivocally be demonstrated to have made tools

● Most of the finds have been concentrated in Ethiopia, at a series of sites near Gona but also at Bouri

● Although clearly primitive and simple tools, they show affinities with the industry at Olduvai Gorge associated with Homo habilis

Dentition and Cranial Capacity● The traits of A. garhi fossils such as

BOU-VP-12/130 are somewhat distinctive from traits typically seen in Australopithecus afarensis and Australopithecus africanus

● An example of the distinction can be seen when comparing the Hadar maxilla (A. afarensis) to the Bouri specimen of A. gahri

● The cranial capacity of A. garhi measures 450cc, the same size as other australopithecines

● Studies made on the premolars and molar teeth have a few similarities with those of Paranthropus boisei since they are larger than any other gracile form of australopithecine

● It has been suggested that if A. garhi is ancestral to Homo (ie. Homo habilis) the maxillary morphology would have undergone a rapid evolutionary change in roughly 200,000 and 300,000 years

Gona, Ethiopia● As there is so little skeletal evidence, it

must be expected that the observations of differences between A. garhi and other hominims will be refined and expanded

● The main point of discussion concerning A. garhi is its status as the first hominim to produce tools

● Excavations at Gona in Ethiopia have resulted in the discovery of an handful of sites affording tools that have been dated between 2.6 million and 1.5 million years

● The earliest tools are associated with A. garhi, but it is not certain which hominim was responsible for the manufacture of the later tool assemblages

● Most of these are based on small cores and therefore conform to the 'chopper industry' seen at Olduvai Gorge and elsewhere in the earliest phases of the Lower Palaeolithic in Africa

Dating the Earliest Tools● We have little occasion to doubt the

early dates for these tools, nor for concern over the accuracy of the dates associated with the remains of A. garhi, for the stratigraphic succession affords several horizons of tephra that permit good dating opportunities

● The volcanic material also provided the raw material for the production of the tools—namely, small pebbles of trachyte

● It is possible to follow some of the tephra horizons elsewhere in the region, and numerous dating assays have confirmed the chronological succession

● In short, the tools here are certainly at least 500,000 years earlier than those at Olduvai Gorge

The Industrial Tradition● The tools were fashioned from small

pebbles of trachyte, being struck once or twice, and used as 'choppers'

● We nonetheless also see the use of small flakes detached from the pebbles

● Although relatively primitive, they differ little from the tools made at Olduvai Gorge by Homo habilis

● The raw material was also deliberately selected, as there are other rocks available in the vicinity

● This implies that a conscious decision was made to choose those rocks with the best flaking properties

● They were probably used to process vegetable food