Reassessment of 1950s fossil find provides early evidence for hominins in Central Africa

2.0 to 2.6-million-year-old tooth is from australopithecine or early human.

A reassessment of a fossil tooth from an old archaeological collection suggests that early hominins had extended their range to the western branch of Africa’s Great Rift Valley by no later than two million years ago. Since the late 1950s, large numbers of early hominin fossils have been found in the eastern branch of the Great Rift Valley, which is often described as the Cradle of Humanity. However, up until now, none have been found in the western branch.

Ishango 11 is an archaeological site in the Democratic Republic of Congo; it is located alongside the Semliki River, in the western branch of the Great Rift Valley. In the 1950s, the site was excavated by the Belgian geologist Jean de Heinzelin, who recovered numerous fossil human and animal remains, together with stone and bone artefacts. The assemblage dates mainly to the early part of the African Late Stone Age, from 25,000 to 19,000 years ago. It is housed in the Department of Anthropology and Prehistory at the Royal Belgian Institute of Science, Brussels.

However, the finds also included an upper left first molar that did not appear to be from such a recent period. Known as #Ish25, doubts were cast on its affinities to modern humans as long ago as 1958. A recent study has shown that #Ish25 probably originated from an earlier geological layer than the other fossils and artefacts. Animal remains associated with this layer suggest that it dates to between 2.6 and 2.0 million years ago. These dates make #Ish25 the earliest fossil hominin find from the western branch of the Great Rift Valley (though not the earliest from Central Africa, as much earlier hominins are known from Chad).

Various statistical analyses of the shape and size of #Ish25 suggest closer affinities to hominins from the Late Pliocene/Early Pleistocene than those from the Middle Pleistocene to Recent epochs. The exact hominin species to which the tooth belongs cannot be determined with certainty; Australopithecus africanus, Paranthropus robustus and early Homo are all possibilities.

The western Great Rift Valley underwent episodes of climate change 3.0, 2.6 and 1.8 million years ago; these led to the partial replacement of Congo flora and fauna with those typical of the East Africa; the latter are adapted to more open grassland conditions. The #Ish25 findings suggest that these conditions led to a dispersal of hominins into the region from either East Africa or South Africa.

The study also demonstrates how valuable knowledge can often be gained by applying modern techniques to old anthropological collections.

 Reference:

1. Crevecoeur, I. et al., First Early Hominin from Central Africa (Ishango, Democratic Republic of Congo). PLoS One 9 (1), e84652 (2014).

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Australopithecus sediba: a possible human ancestor

Australopithecus sediba is a possible human ancestor discovered in South Africa in 2010. The discovery was made at Malapa, a fossil-bearing cave located about 15 km (9.3 miles) NE of the well-known South African hominid-bearing sites of Sterkfontein and Swartkrans and about 45 km (28 miles) NNW of Johannesburg  (Berger, et al., 2010). It is situated within the Cradle of Humankind World Heritage Site. The recovery effort was led by Lee Berger, a paleoanthropologist at the University of the Witwatersrand, Johannesburg. The find was made when Matthew, Lee’s 9 year old son, discovered hominin collar bone embedded in a rock (Balter, 2010).

The find comprised two extremely well-preserved partial skeletons that were initially thought be somewhere between 1.78 and 1.95 million years old (Dirks, et al., 2010), later revised to 1.977 million years (Pickering, et al., 2011). These belonged to a juvenile male (MH1) aged 12 to 13 at time of his death and an adult female (MH2) (Berger, et al., 2010). They were found together buried in alluvial sediment, deep within the Malapa cave, part of an eroded cave system. Also found were the remains of wildcats, hyenas and a number of other mammals. On the ground above the cave are a number of ‘death traps’, or long vertical shafts. The smell of damp issuing from the shaft would have attracted animals. The pair – possibly mother and son – may have fallen to their deaths while searching for water. The sediments imply that subsequent high-volume water inflow, perhaps the result of a large storm, caused a debris flow. This carried the still partially articulated bodies deeper into the cave, to deposit them along a subterranean stream (Dirks, et al., 2010).

MH1 and MH2 were assigned to a new australopithecine species, Australopithecus sediba. The word ‘sediba’ means ‘fountain’ or ‘wellspring’ in the Sotho language. The more complete cranium of the juvenile MH1 has a capacity of 420cc, probably at least 95 percent of adult size. The remains share numerous similarities with Australopithecus africanus in the cranial vault, facial skeleton, lower jawbone and teeth, but there are also significant differences in the cranial, dental and postcranial anatomy. Homo-like features include smaller molars and premolars and less pronounced cheekbones. Certain features of the pelvis are similar to those seen in Homo erectus. The lower-to-upper limb bone proportions are also similar to those of later Homo, and unlike the more apelike proportions of Homo habilis. The anatomy of its hip, knees and ankles suggest that Australopithecus sediba was a habitual biped. Overall, it was claimed that Australopithecus sediba shares more derived features with early Homo than it does with other australopithecines. However, Berger was reluctant to place the new discovery within Homo, preferring to classify it as an australopithecine (Berger, et al., 2010)

The initial announcement of Australopithecus sediba attracted extensive news coverage, but not everybody was convinced by the claims made for it. Australian anthropologist Darren Curnoe was reported (MacKnight, 2010) as claiming that Australopithecus sediba is in the wrong place at the wrong time to be a human ancestor. He noted that Homo habilis emerged in East Africa well before the time of Australopithecus sediba. However, his argument does assume that Homo habilis is indeed an early human.  This may not be the case. It is also possible that at least some of Australopithecus sediba’s humanlike features could have evolved independently, and may not necessarily imply shared ancestry (Wood & Harrison, 2011).

Nevertheless, subsequent studies do support Berger’s initial claims. They suggest that aspects of the brain, dental morphology, pelvis, hand and foot of Australopithecus sediba could be interpreted as incipient humanlike features. A virtual endocast of the brain, obtained from synchrotron scanning, revealed an australopithecine-like size and pattern of convolutions. However, the orbitofrontal region showed possible development towards a humanlike frontal lobe. Possibly some neural reorganization of the brain preceded its later size increase in early humans (Carlson, et al., 2011).

The teeth of MH1 and MH2 are a mosaic of primitive and derived traits. Cladistic analysis of 22 dental traits suggest that Australopithecus sediba was a sister species of Australopithecus africanus(i.e. the two shared a common ancestor) and that the two were further evolved in the direction of Homo than were the australopithecines from East Africa (Irish, Guatelli-Steinberg, Legge, de Ruiter, & Berger, 2013). The lower jawbone morphology reduced dentition (especially canines and premolars) confirms that Australopithecus sediba was a distinct species to Australopithecus africanusand not merely a late-surviving form of that species (de Ruiter, et al., 2013).

The upper ribcage of Australopithecus sediba exhibits an apelike funnel shape, unlike the barrel shape associated with Homo. The funnel shape, as noted above, may be an adaptation to under-branch suspensory locomotion. The barrel shape may be associated with the increased chest volume and lung function necessary for endurance walking and running. The lower thorax, however, appears less flared than that of apes and more closely approximates the morphology found in humans (Schmid, et al., 2013). The spine is long and flexible, a form that has more in common with early Homo than with other australopithecines. Curvature of the lower spine is a hallmark of walking upright (Williams, Ostrofsky, Frater, Churchill, Schmid, & Berger, 2013).

The upper limbs were still predominantly apelike, suggesting the retention of substantial climbing and suspensory abilities (Churchill, et al., 2013). The hands show a mixture of australopithecine and human features. They retained adaptations for tree-climbing, but there was also a long thumb and shorter fingers. These suggest precision gripping of the type associated with tool manufacture and use (Kivell, Kibii, Churchill, Schmid, & Berger, 2011).

The pelvis and foot presented a mosaic of apelike and humanlike characteristics. These suggested adaptations to a more efficient (albeit not entirely human) form of bipedalism, at the expense of reduced arboreal efficiency (Kibii, et al., 2011; Zipfel, DeSilva, Kidd, Carlson, Churchill, & Berger, 2011). The bipedal mechanics differed from those reconstructed for other australopithecines, suggesting that there may have been several forms of hominin bipedalism at this time. The adaptations of Australopithecus sediba may have enabled it to both walk and climb reasonably well and thus survive in a dual arboreal/terrestrial world (DeSilva, et al., 2013).

References:
Balter, M. (2010, April 9). Candidate Human Ancestor From South Africa Sparks Praise and Debate. Science, 328, 154-155.
Berger, L., de Ruiter, D., Churchill, S., Schmid, P., Carlson, K., Dirks, P., et al. (2010, April 9). Australopithecus sediba: A New Species of Homo-Like Australopith from South Africa. Science, 328, 195-204.
Carlson, K., Stout, D., Jashashvili, T., de Ruiter, D., Tafforeau, P., Carlson, K., et al. (2011, September 9). The Endocast of MH1, Australopithecus sediba. Science, 333, 1402-1407.
Churchill, S., Holliday, T., Carlson, K., Jashashvili, T., Macias, M., Mathews, S., et al. (2013, April 12). The Upper Limb of Australopithecus sediba. Science, 340.
de Ruiter, D., DeWitt, T., Carlson, K., Brophy, J., Schroeder, L., Ackermann, R., et al. (2013, April 12). Mandibular Remains Support Taxonomic Validity of Australopithecus sediba. Science, 340.
DeSilva, J., Holt, K., Churchill, S., Carlson, K., Walker, C., Zipfel, B., et al. (2013). The Lower Limb and Mechanics of Walking in Australopithecus sediba. Science, 340.
Dirks, P., Kibii, J., Kuhn, B., Steininger, C., Churchill, S., Kramers, J., et al. (2010, April 9). Geological Setting and Age of Australopithecus sediba from Southern Africa. Science, 328, 205-208.
Irish, J., Guatelli-Steinberg, D., Legge, S., de Ruiter, D., & Berger, L. (2013, April 12). Dental Morphology and the Phylogenetic “Place” of Australopithecus sediba. Science(340).
Kibii, J., Churchill, S., Schmid, P., Carlson, K., Reed, M., de Ruiter, D., et al. (2011, September 9). A Partial Pelvis of Australopithecus sediba. Science, 333, 1407-1411.
Kivell, T., Kibii, J., Churchill, S., Schmid, P., & Berger, L. (2011, September 9). Australopithecus sediba Hand Demonstrates Mosaic Evolution of Locomotor and Manipulative Abilities. Science, 333, 1411-1417.
MacKnight, H. (2010, April 8). Experts reject new human species theory. Retrieved September 12, 2012, from Independent: http://www.independent.co.uk/news/science/experts-reject-new-human-species-theory-1939512.html
Pickering, R., Dirks, P., Jinnah, Z., de Ruiter, D., Churchil, S., Herries, A., et al. (2011, September 9). Australopithecus sediba at 1.977 Ma and Implications for the Origins of the Genus Homo. Science, 333, 1421-1423.
Schmid, P., Churchill, S., Nalla, S., Weissen, E., Carlson, K., de Ruiter, D., et al. (2013). Mosaic Morphology in the Thorax of Australopithecus sediba. Science, 340.
Williams, S., Ostrofsky, K., Frater, N., Churchill, S., Schmid, P., & Berger, L. (2013, April 12). The Vertebral Column of Australopithecus sediba. Science, 340.
Wood, B., & Harrison, T. (2011, February 17). The evolutionary context of the first hominins. Nature, 470, 347-352.
Zipfel, B., DeSilva, J., Kidd, R., Carlson, K., Churchill, S., & Berger, L. (2011, September 9). The Foot and Ankle of Australopithecus sediba. Science, 333, 1417-1420.

Homo habilis

Introduction:
Homo habilis (“handy man”) is an early human species that lived between 2.33 to 1.44 million years ago and is quite possibly the earliest member of genus Homo, though acceptance of it being an ancestor to modern humans or indeed of it being a human species at all is not universal. The species was first described by Louis Leakey in 1964.

Fossil Record:
Homo habilis is known from fossils recovered from Olduvai Gorge, Tanzania; East Turkana, Kenya; Sterkfontein, South Africa; and Hadar in the Awash Valley of Afar Depression, Ethiopia.

The first specimen to be discovered, now known as OH7, comprising a partial cranium and mandible, was discovered by Jonathon and Mary Leakey at Olduvai Gorge on 4 November 1960 and has been dated to 1.75 million years old. Other findings from Olduvai Gorge include OH 24, a cranium discovered by Peter Nzube in 1968 and believed to be 1.8 million years old; OH 13 (partial cranium, mandible and maxilla, discovered by N. Mbuika in 1963, 1.66 million years old); OH 8 (foot, probably from the same individual as OH 7, discovered 1960) and OH 62 (partial skeleton, including upper and lower limbs, believed to be female, discovered by Donald Johanson and Tim White in 1986, 2.0 million years old).

KNM-ER 1805 and KNM-ER 1813 were both discovered at Koobi Fora, East Turkana. KNM-ER 1805 is a partial cranium, maxilla, and mandible, dated to 1.74 million years old. The finder and the date of discovery are unknown. KNM-ER 1813 is a cranium dated to 1.8 million years old discovered by K. Kimeu in 1973.

KNM-ER 42703, also from Koobi Fora, is a right maxilla with an estimated geological age of 1.44 million years. It was discovered by John E. Kaatho in 2000. This is the youngest specimen assigned to Homo erectus, and is notable for being younger than KNM-ER 42700: a small, well preserved calvaria with an estimated geological age of 1.55 million years that has been assigned to (African) Homo erectus.

The Sterkfontein specimens include the partial cranium STW-53 discovered by A.R. Hughes in 1976 and dated 1.5 – 2.0 million years old.

From Hadar comes the maxilla AL-666-1, discovered by William Kimball and dated to 2.33 million years old, the oldest specimen with affinities to Homo habilis.

Description:
Homo habilis had a cranial capacity of between 509-675cc (Cameron & Groves, 2004), considerably larger than its putative ancestors, the australopithecines (375-500cc); and that of present-day chimpanzees (400cc), but much less than that of modern humans. It had a prognathic face (jutting jaw), moderate brow-ridges; no saggital keeling (a raised area along the centre of the skullcap); reduced dentition relative to australopithecines; and proportionately long arms and short legs relative to modern humans. It had an ape-like conical ribcage and possibly retained the ape-like ability of arboreal locomotion.

There was a degree of sexual dimorphism in the species. OH 62 which, as noted above, is believed to have been female, was about 1m tall and probably weighed 30kg. Males were probably around 1.3m tall and weighed around 36kg.

Technology:
Homo habilis is generally associated with the Oldowan industry (or Mode I), named for Olduvai Gorge by Louis and Mary Leakey because they were first recognised there. The Oldowan is the earliest stone tool industry and examples dating to around 2.5 million years old are known from Gona, Bouri and Hadar in Ethiopia; from Lokalelei, West Turkana, Kenya dating to 2.4 million years old; from Koobi Fora dating to 1.9 million years old and from Olduvai Gorge dating to 1.8 million years old.

The Oldowan is associated with a number of hominin species and is therefore a toll-making tradition rather than a particular culture. In addition to Homo habilis, the tradition may be associated with Parantropus (“robust australopithecines”) and the “late” gracile species Australopithecus garhi. The latter, discovered in 1997, was found in association with large animal bones showing cut-marks from stone tools. Although no actual tools were directly associated with the fossils, it seems likely that A. garhi made and used stone tools. The remains have been dated to 2.5 million years old. In addition, the Oldowan survived Homo habilis and is associated with a number of later human species.

The Oldowan is characterised by very simple stone tools. The main types are choppers made from cobbles or angular blocks of stone; hammer-stones, which are unmodified chunks that show signs of having been used as hammers in tool manufacture; scrapers made from both cores and retouched flakes detached from cores. Although there is a considerable variety and other types such as discoids and polyhedrons are recognised, much of the variation can be explained in terms of differences in the nature of the raw material available. The overall strategy was likely a least-effort strategy to produce either flakes with sharp cutting edges or cores for chopping. It seems unlikely that the Oldowan toolmakers mastered the sophisticated stone reduction strategies seen in later tool-making traditions.

Evolutionary history of Homo habilis:
The accepted view of human evolution has become considerably more complicated over the last twenty years and it is now recognised that the traditional view of progressive evolution from australopithecine to H. habilis to H. erectus and finally to H. sapiens is at best an oversimplification.

The picture for Homo habilis was complicated in the 1980s by the discovery that fossils previously assigned to Homo habilis actually belonged to two species – Homo habilis and Homo rudolfensis, although it does now seem likely that the latter is an australopithecine-type upright ape, off the line of human evolution.

Homo habilis is believed to have evolved from an australopithecine species, but there is no consensus as to which of those currently known, if any. The traditional assumption that Homo ergaster (i.e. African Homo erectus) evolved from Homo habilis has recently been challenged by the discovery that the two species were sympatric (co-existing) in the Lake Turkana basin in Kenya for almost half a million years, implying that they must have occupied different niches and leading some to believe that the two species diverged from a common ancestor 2.3 million years ago rather than one evolving from the other.

On the other hand Homo habilis appears in the fossil record some 300,000 years before Homo ergaster. Unless earlier H. ergaster remains come to light, it seems likely that a proto-ergaster population split away from an earlier population of H. habilis and only later came into contact with a population of the ancestral species.

For now, then, it is still widely accepted that Homo habilis is ancestral to all later human species, including Homo sapiens.

References:

Cameron D & Groves C (2004): Bones, Stones and Molecules: “Out of Africa” and Human Origins, Elsevier Academic Press.

Leakey LSB, Tobias PV & Napier JR (1964): A New Species of Genus Homo from Olduvai Gorge, Nature No. 4927, 4 April 1964.

Scarre C (2005) (Ed): “The human past”, Thames & Hudson.

F. Spoor, M. G. Leakey, P. N. Gathogo, F. H. Brown, S. C. Anton, I. McDougall, C. Kiarie, F. K. Manthi & L. N. Leakey (2007): Implications of new early Homo fossils from Ileret, east of Lake Turkana, Kenya, Nature Vol 448 9 August 2007.

© Christopher Seddon 2009

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