Australopiths (4.2–1.2 mya)
- Early Australopith: Australopithecus anamensis and Australopithecus afarensis [Lucy]
- Later Australopiths : Paranthropus , A. africanus and Australopithecus sediba
The best-known, most widely distributed, and most diverse of the early African hominins are colloquially called australopiths. In fact, this diverse and very successful group of hominins is made up of two closely related genera, Australopithecus and Paranthropus. These hominins have an established time range of over 3 million years—stretching back as early as 4.2 mya and not becoming extinct until apparently close to 1 mya— making them the longest- enduring hominins yet documented. In addition, these hominins have been found in all the major geographical areas of Africa that have, to date, produced early hominin finds, namely, South Africa, central Africa (Chad), and East Africa. From all these areas combined, there appears to have been considerable complexity in terms of evolutionary diversity, with numerous species now recognized by most paleoanthropologists. There are two major subgroups of australopiths, an earlier one that is more anatomically primitive and a later one that is much more derived. These earlier australopiths, dated 4.2–3.0 mya, show several more primitive (ancestral) hominin characteristics than the later australopith group, whose members are more derived, some extremely so. These more derived hominins lived after 2.5 mya and are composed of two different genera, together represented by at least five different species. Given the 3-million-year time range as well as quite varied ecological niches, there are numerous intriguing adaptive differences between these varied australopith species. We’ll discuss the major adaptations of the various species in a moment. But first let’s emphasize the major features that all australopiths share:
- They are all clearly bipedal (although not necessarily identical to Homo in this regard).
- They all have relatively small brains (at least compared to Homo).
- They all have large teeth, particularly the back teeth, with thick to very thick enamel on the molars.
In short, then, all these australopith species are relatively small-brained, big toothed bipeds.
The earliest australopiths, dating to 4.2–3.0 mya, come from East Africa from a couple of sites in northern Kenya.
Among the fossil finds of these earliest australopiths so far discovered, a few postcranial pieces clearly indicate that locomotion was bipedal. Since these particular fossils have initially been interpreted as more primitive than all the later members of the genus Australopithecus, paleoanthropologists have provisionally assigned them to a separate species. This important fossil species is now called Australopithecus anamensis, and some researchers suggest that it is a potential ancestor for many later australopiths as well as perhaps early members of the genus Homo (White et al., 2006).
Australopithecus afarensis
Slightly later and much more complete remains of Australopithecus have come primarily from the sites of Hadar (in Ethiopia) and Laetoli (in Tanzania). Much of this material has been known for three decades, and the fossils have been very well studied; indeed, in certain instances, they are quite famous. For example, the Lucy skeleton was discovered at Hadar in 1974, and the Laetoli footprints were first found in 1978. These hominins are classified as members of the species Australopithecus afarensis. Literally thousands of footprints have been found at Laetoli, representing more than 20 different kinds of animals (Pliocene elephants, horses, pigs, giraffes, antelopes, hyenas, and an abundance of hares). Several hominin footprints have also been found, including a trail more than 75 feet long made by at least two—and perhaps three—individuals (Leakey and Hay, 1979; . Such discoveries of well-preserved hominin footprints are extremely important in furthering our understanding of human evolution.
For the first time, we can make definite statements regarding the locomotor pattern and stature of early hominins. Studies of these impression patterns clearly show that the mode of locomotion of these hominins was bipedal (Day and Wickens, 1980). Some researchers, however, have concluded that A. afarensis was not bipedal in quite the same way that modern humans are. From detailed comparisons with modern humans, estimates of stride length, cadence, and speed of walking have been ascertained, indicating that the Laetoli hominins moved in a slow-moving (“strolling”) fashion with a rather short stride. One extraordinary discovery at Hadar is the Lucy skeleton , found eroding out of a hillside by Don Johanson. This fossil is scientifically designated as Afar Locality (AL) 288-1, but is usually just called Lucy (after the Beatles song “Lucy in the Sky with Diamonds”). Representing almost 40 percent of a skeleton, this is one of the most complete individuals from anywhere in the world for the entire period before about 100,000 ya. Because the Laetoli area was covered periodically by ashfalls from nearby volcanic eruptions, accurate dating is possible and has provided dates of 3.7–3.5 mya. Dating from the Hadar region hasn’t proved as straightforward; however, more complete dating calibration using a variety of techniques has determined a range of 3.9–3.0 mya for the hominin discoveries from this area. Several hundred A. afarensis specimens, representing a minimum of 60 individuals (and perhaps as many as 100), have been removed from Laetoli and Hadar. At present, these materials represent the largest well-studied collection of early hominins and as such are among the most significant of the hominins.
Without question, A. afarensis is more primitive than any of the other later australopith fossils from South or East Africa (discussed shortly). By “primitive” we mean that A. afarensis is less evolved in any particular direction than are later-occurring hominin species. That is, A. afarensis shares more primitive features with late Miocene apes and with living great apes than do later hominins, who display more derived characteristics.
For example, the teeth of A. afarensis are quite primitive. The canines are often large, pointed teeth. Moreover, the lower first premolar provides a shearing surface for the upper canine (although it is not a full honing complex as seen in many monkeys and apes). Lastly, the tooth rows are parallel, even converging somewhat toward the back of the mouth. The cranial portions that are preserved also display several primitive hominoid characteristics, including a crest in the back as well as several primitive features of the cranial base.
Cranial capacity estimates for A. afarensis show a mixed pattern when compared with later hominins. A provisional estimate for the one partially complete cranium—apparently a large individual—gives a figure of 500 cm3 , but another, even more fragmentary cranium is apparently quite a bit smaller and has been estimated at about 375 cm3 (Holloway, 1983). Thus, for some individuals (males?), A. afarensis is well within the range of other australopith species, but others (females?) may have a significantly smaller cranial capacity. However, a detailed depiction of cranial size for A. afarensis is not possible at this time; this part of the skeleton is unfortunately too poorly represented. One thing is clear: A. afarensis had a small brain, probably averaging for the whole species not much over 420 cm3.
On the other hand, a large assortment of postcranial pieces representing almost all portions of the body of A. afarensis has been found. Initial impressions suggest that relative to lower limbs, the upper limbs are longer than in modern humans (also a primitive Miocene ape condition). (This statement does not mean that the arms of A. afarensis were longer than the legs.) In addition, the wrist, hand, and foot bones show several differences from modern humans (Susman et al., 1985). From such excellent postcranial evidence, stature can be confidently estimated: A. afarensis was a short hominin. From her partial skeleton, Lucy is estimated to be only 3 to 4 feet tall. However, Lucy—as demonstrated by her pelvis—was probably a female, and there is evidence of larger individuals as well. The most economical hypothesis explaining this variation is that A. afarensis was quite sexually dimorphic: The larger individuals are male, and the smaller ones, such as Lucy, are female. Estimates of male stature can be approximated from the larger footprints at Laetoli, inferring a height of not quite 5 feet. If we accept this interpretation, A. afarensis was a very sexually dimorphic form indeed. In fact, for overall body size, this species may have been as dimorphic as any living primate (that is, as much as gorillas, orangutans, or baboons).
Significant further discoveries of A. afarensis have come from Ethiopia in the last few years, including two further partial skeletons. The first of these is a mostly complete skeleton of an A. afarensis infant discovered at the Dikika locale in northeastern Ethiopia, very near the Hadar sites mentioned earlier.
What’s more, the infant comes from the same geological horizon as Hadar, with very similar dating: 3.3–3.2 mya (Alemseged et al., 2006). This find of a 3-year-old infant is remarkable because it’s the first example of a very well-preserved immature hominin prior to about 100,000 ya. From the infant’s extremely well-preserved teeth, scientists hypothesize that she was female. A comprehensive study of her developmental biology has already begun, and many more revelations are surely in store as the Dikika infant is more completely cleaned and studied.
For now, and accounting for her immature age, the skeletal pattern appears to be quite similar to what we’d expect in an A. afarensis adult. What’s more, the limb proportions, anatomy of the hands and feet, and shape of the scapula (shoulder blade) reveal a similar “mixed” pattern of locomotion. The foot and lower limb indicate that this infant would have been a terrestrial biped; yet, the shoulder and (curved) fingers suggest that she was also capable of climbing about quite ably in the trees.
The second recently discovered A. afarensis partial skeleton comes from the Woranso-Mille research area in the central Afar, only about 30 miles north of Hadar (Haile-Selassie et al., 2010). The dating places the find at close to 3.6 mya (almost 400,000 years earlier than Lucy). Moreover, the individual was considerably larger than Lucy and likely was male. Analysis of bones preserved in this new find reinforces what was previously known about A. afarensis as well as adding some further insights. The large degree of sexual dimorphism and well-adapted bipedal locomotion agree with prior evidence. What’s more, a portion of a shoulder joint (with a scapula; see Appendix A) confirms that suspensory locomotion was not a mode of arboreal locomotion; nevertheless, arboreal habitats could still have been effectively exploited.
What makes A. afarensis a hominin?
The answer is revealed by its manner of locomotion. From the abundant limb bones recovered from Hadar and other locales, as well as those beautiful footprints from Laetoli, we know unequivocally that A. afarensis walked bipedally when on the ground. (At present, we do not have nearly such good evidence concerning locomotion for any of the earlier hominin finds.) Whether Lucy and her contemporaries still spent considerable time in the trees and just how efficiently they walked have become topics of some controversy. Most researchers, however, agree that A. afarensis was an efficient habitual biped while on the ground. These hominins were also clearly obligate bipeds, which would have hampered their climbing abilities but would not necessarily have precluded arboreal behavior altogether.
Australopithecus afarensis is a crucial hominin group. Since it comes after the earliest, poorly known group of pre- australopith hominins, but prior to all later australopiths as well as Homo, it is an evolutionary bridge, connecting together much of what we assume are the major patterns of early hominin evolution. The fact that there are many well-preserved fossils and that they have been so well studied also adds to the paleoanthropological significance of A. afarensis. The consensus among most experts over the last several years has been that A. afarensis is a potentially strong candidate as the ancestor of all later hominins. Some ongoing analysis has recently challenged this hypothesis (Rak et al., 2007), but at least for the moment, this new interpretation has not been widely accepted. Still, it reminds us that science is an intellectual pursuit that constantly reevaluates older views and seeks to provide more systematic explanations about the world around us. When it comes to understanding human evolution, we should always be aware that things might change. So stay tuned.
Later More Derived Australopiths (3.0–1.2 mya)
Following 3.0 mya, hominins became more diverse in Africa. As they adapted to varied niches, australopiths became considerably more derived. In other words, they show physical changes making them quite distinct from their immediate ancestors. In fact, there were at least three separate lineages of hominins living (in some cases side by side) between 2.0 and 1.2 mya. One of these is a later form of Australopithecus; another is represented by the highly derived three species that belong to the genus Paranthropus; and the last consists of early members of the genus Homo.
The most derived australopiths are the various members of Paranthropus. While all australopiths are big-toothed, Paranthropus has the biggest teeth of all, especially as seen in its huge premolars and molars. Along with these massive back teeth, these hominins show a variety of other specializations related to powerful chewing. For example, they all have large, deep lower jaws and large attachments for muscles associated with chewing. In fact, these chewing muscles are so prominent that major anatomical alterations evolved in the architecture of their face and skull vault.
In particular, the Paranthropus face is flatter than that of any other australopith; the broad cheekbones (to which the masseter muscle attaches) flare out; and a ridge develops on top of the skull (this is called a sagittal crest, and it’s where the temporal muscle attaches). All these morphological features suggest that Paranthropus was adapted for a diet emphasizing rough vegetable foods. However, this does not mean that these very big- toothed hominins did not also eat a variety of other foods, perhaps including some meat. In fact, sophisticated recent chemical analyses of Paranthropus teeth suggest that their diet may have been quite varied (Sponheimer et al., 2006).
The first member of the Paranthropus evolutionary group (clade) comes from a site in northern Kenya on the west side of Lake Turkana. This key find is that of a nearly complete skull, called the “Black Skull” (owing to chemical staining during fossilization), and it dates to approximately 2.5 mya . This skull, with a cranial capacity of only 410 cm3 , is among the smallest for any hominin known, and it has other primitive traits reminiscent of A. afarensis. But here’s what makes the Black Skull so fascinating: Mixed into this array of distinctively primitive traits are a host of derived ones that link it to other, later Paranthropus species (including a broad face, a very large palate, and a large area for the back teeth). This mosaic of features seems to place this individual between the earlier A. afarensis and the later Paranthropus species. Because of its unique position in hominin evolution, the Black Skull (and the population it represents) has been placed in a new species, Paranthropus aethiopicus.
Around 2 mya, different varieties of even more derived members of the Paranthropus lineage were on the scene in East Africa. As well documented by finds dated after 2 mya from Olduvai and East Turkana, Paranthropus continues to have relatively small cranial capacities (ranging from 510 to 530 cm3 ) and very large, broad faces with massive back teeth and lower jaws. The larger (probably male) individuals also show that characteristic raised ridge (sagittal crest) along the midline of the cranium. Females are not as large or as robust as the males, indicating a fair degree of sexual dimorphism. In any case, the East African Paranthropus individuals are all extremely robust in terms of their teeth and jaws—although in overall body size they are much like other australopiths. Since these somewhat later East African Paranthropus fossils are so robust, they are usually placed in their own separate species, Paranthropus boisei. Paranthropus fossils have also been found at several sites in South Africa.
As we discussed earlier, the geological context in South Africa usually does not allow as precise chronometric dating as is possible in East Africa. Based on less precise dating methods, Paranthropus in South Africa existed about 2.0–1.2 mya. Paranthropus in South Africa is very similar to its close cousin in East Africa, but it’s not quite as dentally robust. As a result, paleoanthropologists prefer to regard South African Paranthropus as a distinct species—one called Paranthropus robustus.
What became of Paranthropus?
After 1 mya, these hominins seem to vanish without descendants. Nevertheless, we should be careful not to think of them as “failures.” After all, they lasted for 1.5 million years, during which time they expanded over a considerable area of sub-Saharan Africa. Moreover, while their extreme dental/chewing adaptations may seem peculiar to us, it was a fascinating “evolutionary experiment” in hominin evolution. And it was an innovation that worked for a long time. Still, these big-toothed cousins of ours did eventually die out. It remains to us, the descendants of another hominin lineage, to find their fossils, study them, and ponder what these creatures were like. From no site dating after 3 mya in East Africa have fossil finds of the genus Australopithecus been found. As you know, their close Paranthropus kin were doing quite well during this time. Whether Australopithecus actually did become extinct in East Africa following 3 mya or whether we just haven’t yet found their fossils is impossible to say. South Africa, however, is another story. A very wellknown Australopithecus species has been found at four sites in southernmost Africa, in a couple of cases in limestone caves very close to where Paranthropus fossils have also been found. In fact, the very first early hominin discovery from Africa (indeed, from anywhere) came from the Taung site and was discovered back in 1924. The story of the discovery of the beautifully preserved child’s skull from Taung is a fascinating tale . When first published in 1925 by a young anatomist named Raymond Dart, most experts were unimpressed. They thought Africa to be an unlikely place for the origins of hominins. These skeptics, who had been long focused on European and Asian hominin finds, were initially unprepared to acknowledge Africa’s central place in human evolution. Only years later, following many more African discoveries from other sites, did professional opinion shift. With this admittedly slow scientific awareness came the eventual consensus that Taung (which Dart classified as Australopithecus africanus) was indeed an ancient member of the hominin family tree.
Like other australopiths, the “Taung baby” and other A. africanus individuals were small- brained, with an adult cranial capacity of about 440 cm3 . They were also big-toothed, although not as extremely so as Paranthropus. Moreover, from very well-preserved postcranial remains from Sterkfontein, we know that they also were well adapted bipeds. The ongoing excavation of a remarkably complete skeleton at Sterkfontein should tell us about A. africanus’ locomotion, body size and proportions, and much more . The precise dating of A. africanus, as with most other South African hominins, has been disputed. Over the last several years, it’s been assumed that this species existed as far back as 3.3 mya. However, the most recent analysis suggests that A. africanus lived approximately between 3 and 2 mya (Walker et al., 2006; Wood, 2010;.
New Connections: A Transitional Australopith?
All the evidence for the earliest appearance of our genus, Homo, has come from East Africa. So it’s no surprise that most researchers have assumed that Homo probably first evolved in this region of Africa. However, new and remarkably well-preserved fossil discoveries from South Africa may challenge this view.
In 2008, paleoanthropologists discovered two partial skeletons at the Malapa Cave, located just a few miles from Sterkfontein and Swartkrans. Actually, the first find was made by the lead researcher’s 9-year-old son, Matthew. His father (Lee Berger, from the University of Witwatersrand) and colleagues have been further investigating inside the cave, where several skeletons may be buried, and they announced and described their finds in 2010 (Berger et al., 2010).
Using paleomagnetic dating as well as more precise radiometric techniques than have been used before in South Africa (Dirks et al., 2010; Pickering et al., 2011), the fossils are dated to just a little less than 2 mya and show a fascinating mix of australopith characteristics along with a few features more suggestive of Homo. Because of this unique anatomical combination, these fossils have been assigned to a new species, Australopithecus sediba (sediba means “wellspring” or “fountain” in the local language). Australopith-like characteristics seen in A. sediba include a small brain (estimated at 420 cm3 ) , long arms with curved fingers, and several primitive traits in the feet. In these respects A. sediba most resembles its potential immediate South African predecessor, A. africanus.
But some other aspects of A. sediba more resemble Homo. Among these characteristics are short fingers and possible indications of brain reorganization. All this is very new and quite complex. Indeed, initial paleoanthropological interpretations are highly varied (Balter, 2010; Gibbons, 2011). It will take some time for experts to figure it out.
Remember, too, that there are more fossils still in the cave. The initial consensus among paleoanthropologists is that A. sediba is quite different from other australopiths and shows a surprising and unique mix of primitive and derived characteristics. How it fits in with the origins of Homo remains to be determined. Certainly, more detailed studies of the A. sediba fossils, including further comparisons with other early hominins, will help further our understanding. For the moment, most paleoanthropologists still think the best evidence for the origins of our genus comes from East Africa.
General Description – IGNOU
Raymond Dart, a young Professor of Anatomy at Johannesburg on 7th Feb, 1925 announced the discovery of an early Pliocene- Pleistocene hominid with a small brain case. The general reaction of the world was one of the disbelief at that
time. The type specimen, an infant skull (about six years old), was discovered in the end of year 1924 by workers in a lime stone quarry at Taung (Ta- ung = place of lion), in what is now Botswana, South Africa. Dart named his find as Australopithecus africanus (Southern African ape) and mentioned the many ape like feature of the skull including the small size of the brain ( 380- 500 c.c) and pointed to its pongid affinities; albeit he also emphasised the fact that there were a number of features of the skull and the dentition which were man-like. He placed this new genus and species in a family intermediate between hominids and pongids i.e. Homo-simiidae. It took Dart 73 days to work the skull out of lime stone matrix and a total of four years to separate the lower jaw from the rest of skull and it was in this year i.e. 1929 that his judgment was strikingly vindicated
and an unquestionably hominid dentition was revealed. The teeth of the juvenile are very large, but morphologically similar to those of later hominids. No more hominids were discovered at Taung and unfortunately now this fossil site has been destroyed by quarry operations.
The gracile type and the robust forms of Australopithecus can be differentiated as follows. The gracile form is small and light (20-40 kg), with an estimated stature of 145 cm (4 feet and 9 inches). Its estimated cranial capacity is 442 cm3 . The facial skeleton is small, but the jaws are large in proportion to the rest of the skull. The disk shaped profile is obvious. Gracile forms mostly occur between 3 and 2 millions of years before present. On the other hand, the robust form is larger and more heavily built (35-55 kg). Its estimated stature is 153 cm (5 feet) and the average cranial capacity is 530 cm3 . The heavier jaw of robust form is associated with very large molar and premolars but relatively small incisors and canines. The massiveness of the jaw musculature is seen in the frequent development of a sagittal crest. The above variability can be explained on the basis of generic or species or subspecies or geographic variation or sexual dimorphism or dietary differences. The robust forms range between 2.1 and 1.3 million years before present
Phylogenetic status – IGNOU
Australopithecines were morphologically diverse group; the diversity is indicated by the various taxonomic schemes devised to accommodate their numbers.
Different researchers have different ideas, most of the ‘lumpers’ recognise only one genus called Australopithecus to accommodate at least four distinct species at present i.e. Australopithecus africanus in South Africa, Australopithecus robustus in East Africa, Australopithecus afarensis in Hadar (Ethiopia) and Laeotoli (Tanzania) and Australopithecus ramidus in Afar ( Ethiopia) region.
The overall pattern of early human evolution in Africa appears to have taken the following course. First was the early Australopithecus ramidus, the apparent ancestral stock which appeared about 5 to 6 million years before present; later came another smaller but better developed form, Australopithecus afarensis which appeared around 3.75 million years ago; after that came the gracile and robust forms i.e Australopithecus africanus and Australopithecus robustus which appeared about 2 to 1 million years before present, respectively.
Some investigators argue that the important differences in dental proportions between Australopithecus africanus and Australopithecus robustus, the gracile and robust forms, respectively, do not correspond with allometric trends noted among modern primates. According to them these differences may be morphologically significant adaptations, perhaps indicating dietary modifications or may be even social behaviour.
The genus Homo appeared to be and was apparently the descendant of one of the Australopithecus forms, but, it is still not clear what led to the evolution of the genus Homo. The phylogentic position of Australopithecus is quite clear that it is a hominid and that is why it is included under the subfamily Homininae and the family Hominidae to which the present day humans also belong.