Australopithecus

The First of the Great South Africans
The first discovery of a bipedal ape was made by the Australian anthropologist Raymond Dart in 1924. Investigating a box of fossil-bearing rock from a limestone quarry in Taung in North West Province, South Africa, Dart discovered fossilised remains of a juvenile apelike creature. The remains comprised the face, part of the cranium, the complete lower jaw and a brain endocast formed when material within the skull hardened to rock. Dart concluded that the brain was humanlike and that the foramen magnum was placed centrally in the basicranium as with humans, rather than towards the rear as with apes. The canine teeth were small – again like humans. Dart described what became known as the Taung Child in February 1925 in the journal Nature (Dart, 1925), naming it Australopithecus africanus (Southern ape of Africa).

Dart’s claim was strongly criticised at the time, largely because the (fake) Eoanthropus dawsoni (Dawson’s dawn man) better known as Piltdown Man fitted the then prevalent view that brains had expanded before bipedal walking had evolved. The find was widely dismissed as a fossilised ape.

However in 1936 the Scottish palaeontologist Robert Broom, a long-time supporter of Dart, instigated new searches for early human fossils and soon discovered the braincase of an adult specimen in a set of limestone caves at Sterkfontein near Krugersdorp 25km northwest of Johannesburg. Further finds followed at nearby Kromdraai in 1938 but then the war intervened and the sites were closed down for the duration. Broom (who was in his seventies) spent the war years preparing a monograph on the finds which was published in 1946, after which the australopithecines were generally accepted as hominins.

Broom assigned his finds to two species, Plesiantropus transvaalensis (“Near-man from the Transavaal”) and Paranthropus robustus (“alongside-man”). After the war, now assisted by John T. Robinson, Broom resumed his excavations. In April 1947 the pair discovered the nearly complete adult skull of a female Plesianthropus (STS 5) which became popularly known as Mrs Ples. Another find from Sterkfontein, a fossilized pelvis, vertebral column and fragmentary rib and femur discovered by Broom in 1947 and known as STS 14, may well be part of the same individual as STS 5.
Plesianthropus transvaalensis is now regarded as the same species as Australopithecus africanus. Paranthropus was later “lumped” in with Australopithecus but the current trend for “splitting” has led some to resurrect it as a separates genus, incorporating the so-called robust australopithecines. The morphological differences between the robust australopithecines and the earlier australopithecines, colloquially referred to as gracile australopithecines, are far less than those between the earliest (Homo habilis) and modern humans (H. sapiens), which are not given separate genera. I will therefore follow Klein (1999), Conroy (1997), Lewin & Foley (1998, 2004), etc in not adopting this convention.

A. africanus is known from 2.8 to 2.3 mya; A. robustus is more recent, known from 1.8 to possibly as late as 1.0 mya. Both species were small-brained in comparison to human. Australopithecus africanus is believed to have had a cranial capacity of around 430-520 cc. There was a considerable degree of sexual dimorphism in both. Males typically measured 4ft 6in tall and weighed 40 kg, whereas females measured 3ft 8 in tall and weighed 30 kg. Australopithecus robustus had a cranial capacity of 500-545 cc. Males measured 4ft 4 in tall and weighed 40 kg. Females measured 3ft 6 in tall and weighed 32 kg.

In 2004, Mrs Ples made the Top 100 in SABC3’s television series Great South Africans, placing her in the company of Nelson Mandela, Steve Biko and Dr Christiaan Barnard.

Zinj
In the 1950s Louis and Mary Leakey began excavating at Olduvai Gorge, a steep-sided ravine in the Great Rift Valley located in the eastern Serengeti, Tanzania. For many years the Leakeys recovered only stone tools, but in 1959 they made the first of a series of discoveries that was to make them world famous and led to Olduvai Gorge becoming known as the Cradle of Mankind. On 17 July of that year, with Louis back at camp unwell, Mary discovered an almost complete hominin cranium. The find was initially classified as Zinjanthropus boisei – Zinj was the medieval Arab name for this region of East Africa. However it was later reclassified as a robust australopithecine and hence is now known as Australopithecus boisei (or Paranthropus boisei). The specific name boisei honoured the expedition sponsor Charles Boise.

“Zinj” or “Dear Boy” as the specimen was affectionately known is dated to 1.75 mya. A. boisei is now also known from East Turkana, Kenya (KNM-ER 406 - male cranium; KNM-ER 732 – female cranium) and from Ethiopia. The species is known from 2.3 to 1.2 mya. It is again quite small brained at 500-545 cc. Males measured 4ft 6 and weighed 45 to 80 kg, again much larger than the females at 4ft tall and 36 kg.

Lucy
Although the discovery of Australopithecus boisei proved that australopithecines had not been confined to South Africa, Mrs Ples and her relatives remained unchallenged as the oldest known bipedal apes, but in 1974 the baton passed to what is undoubtedly the most celebrated fossil hominin ever discovered.

In the 1970s diggings in the Afar Depression began to reveal evidence of an australopithecine species that considerably predated Australopithecus africanus. The first specimen, a fossilized knee-joint known as AL 129-1, was discovered in November 1973 by a young American PhD student, Donald Johanson, at the Middle Awash site along the Awash River. Its humanlike oblique femoral shaft indicated that it had belonged to a biped. A year later an expedition led by Johanson and French anthropologist Yves Coppens recovered a 40% complete skeleton designated AL 288-1, some 2.5 km from the site of AL 129-1. Nicknamed Lucy after the Beatles song Lucy in the Sky with Diamonds (which was played on a cassette-recorder at the campsite at a party held to mark the find), AL 288-1 became world famous overnight. The new species was named Australopithecus afarensis.

Shortly after the discovery of Lucy, Johanson’s team found a 3.2 million year old fossil bed containing 333 separate fragments. The site was accordingly dubbed Locality 333. The remains were associated with a group of 13 A. afarensis including males, females and infants. It is believed that this group – which became known as the First Family - were drowned by a flash flood as there is no evidence that they were attacked by predators. If they were indeed all members of a single social group, this suggests A. afarensis lived in relatively large groups of mixed sexes and ages.

In 1978 Mary Leakey discovered a set of hominin footprints preserved for 3.7 million years in volcanic ash at Laetoli in Tanzania, 45 km south of Olduvai Gorge. Generally attributed to Australopithecus afarensis, the footprints were made by three bipedal individuals, all walking in the same direction. The individuals had human-like arched feet, lacked the mobile big toes of apes and appear to have been moving at a leisurely stroll.

Australopithecus afarensis is known from 3.9 to 2.8 mya. Its cranial capacity was 380-485 cc. Males measured 5ft tall and weighed 45 kg, females measured 3ft 3in and weighed 30 kg; thus again the species was highly sexually dimorphic.

The face was prognathic – that is to say the lower jaw jutted forward. The cranium was long and low, with a nuchal crest at the back to which were attached powerful neck muscles, needed to balance the head because of the prognathic lower face. The foramen magnum was centrally-placed, confirming upright posture. Males had a sagittal crest, implying strong, ape-like jaw muscles.

The canines and incisors, though large, were reduced in comparison to modern ape; the molars were thick-enamelled and large.

The human-like pelvis had a short, broad, backwardly-extended iliac blade, which centres the trunk over the hip joints, reducing fatigue during upright bipedal walking. However it had relatively ape-like limb proportions: very short thighs, powerful arms with forearms long in proportion to upper arms, similar to chimpanzees. The ribcage was probably cone-shaped as opposed to barrel-shaped in humans. This implies that it was an adept climber and not yet a wholly-committed biped, though it was undoubtedly a bipedal walker on the ground.

The Australopithecine family grows
In the decade and a half that followed the discovery of Lucy, the australopithecine family continued to grow.

In the mid 1980s, a new robust type, Australopithecus aethiopicus, was identified. As the name implies, the first specimen (Omo 18) came to light in Ethiopia. But this find, made in 1967, comprised a partial mandible fragment and the species wasn’t recognised until 1985 when Alan Walker and Richard Leakey discovered a skull at West Turkana, Kenya. KNM WT 17000, known as the Black Skull due to manganese colouration, which has a cranial capacity of 410 cc. There is insufficient material to estimate its size. It lived 2.7 to 2.5 mya and may be ancestral to A. robustus and A. boisei, but this is uncertain. It is not even universally accepted that Omo 18 and the Black Skull belong to the same species.

The next discovery was Australopithecus bahrelghazali, discovered in 1993, known only from at Bahr el Ghazal valley near Koro Toro, Chad (KT-12H1). The find consists of a fragmentary upper third premolar and the anterior portion of a mandible retaining one incisor, the sockets for the remaining three, both canines and all four premolars. An age of 3.4 to 3 million years makes it contemporary with A. afarensis and its status as a separate species is disputed. However its location in Chad, 2,500 km from contemporary australopithecine sites in the Rift Valley suggests north-central Africa may also be important in human origins.

Further back: Ardipithecus ramidus and Australopithecus anamensis
Up to this point, Lucy continued to hold the title for oldest known hominin, and by implication, the closest to the last common ancestor between humans and living apes, but from the 1990s onwards, the date was successively pushed ever further back.
Australopithecus anamensis (“lake” in Turkana language) is another find dating back to the Sixties but not recognised until later. The first specimen, a single arm bone, was recovered 1965 at Lake Turkana, but it was not proposed as a new species until 1995 following discoveries in 1987 at Lake Turkana by Allan Morton and 1993 by Meave Leakey and Alan White. Leakey proposed the new species after noting differences between the new finds and A. afarensis.

Australopithecus anamensis was unquestionably bipedal, as shown by the form of its tibia, including the near right angle between the proximal shaft and the proximal articular surface, the large size of the lateral proximal condyle, and the human-like buttressing of the proximal and distal shaft. These features suggest human-like transfer of weight from one leg to another when walking. Like A. afarensis, A anamensis had powerful arms that would have aided tree climbing. Body weight is estimated to have been between 47 to 55 kg. Based on canine root size, it may have been more sexually dimorphic than A. afarensis.

Australopithecus anamensis lived from 4.2 to 3.9 mya and is the oldest known australopithecine, but round about the time its discovery was announced, an earlier, considerably more ape-like hominin, now known as Ardipithecus ramidus, came to light.

In September 1994 a research team headed by Dr Timothy White discovered the hominin fragments including skull, mandible, teeth and arm bones—from the Afar Depression in the Middle Awash river valley of Ethiopia. Eventually 45 percent of the total skeleton was recovered. Dated to 4.4 mya, the new species was originally classed as an australopithecine, A. ramidus (“ramid” means root in the native Afar language), but it has subsequently been assigned a new genus, Ardipithecus (“Ardi” means ground or floor” in Afar). More Ardipithecus ramidus finds were made in 2005 in As Duma, northern Ethiopia. The finds comprised 9 individuals who lived from between 4.5 to 4.3 mya.

Ardipithecus ramidus was about the size of a chimpanzee and had chimp-like dentition, including thin enamel, strongly-muscled arms which could be an aid to climbing. It is linked to later hominins by incisor-like canines and by forward position of foramen magnum, implying bipedalism. Leg bones show it was bipedal, but less so than its less ape-like successors. It clearly represented an earlier grade of organization than Australopithecus.

Ardipithecus ramidus seems to have lived in forest, scuppering theories about a savannah origin for bipedalism. Fossils are found with typical forest fauna. This is supported by implied dietary adaptations – thin molar enamel and small molar teeth, suggesting a diet of leaves, soft fruit and soft vegetables.

The discovery of a second Ardipithecus species, Ardipithecus kadabba (originally classed as a subspecies, A. ramidus kadabba) pushed the hominin lineage back still further. These later samples, also found at Middle Awash, represented five individuals and were older than the 1994 findings. Ardipithecus kadabba lived from 5.8 to 5.2 mya, not far from the 7 to 5 mya date for the human/chimp split obtained from recent molecular studies.

A fairly straightforward evolutionary relationship seemed to be indicated at this point, with Ardipithecus being ancestral to Australopithecus. Meanwhile, the discovery in 1999 of another “late” gracile australopithecine, A. garhi (“surprise” in Afar) seemed to fill in another gap between A. africanus and Homo habilis, the first human species.

However, the recent discoveries of Orrorin turgensis (Tugen Hills, Kenya, Pickford & Senut, 2000) and Sahelanthropus tchadensis (19 July 2001, TM 266 “Toumai” [Hope of Life]) have thrown the whole issue back into the melting pot.

© Christopher Seddon 2008