Homo rudolfensis

''Homo rudolfensis'' is an extinct species of archaic human from the Early Pleistocene of East Africa about 2 million years ago (mya). Because ''H. rudolfensis'' coexisted with several other hominins, it is debated what specimens can be confidently assigned to this species beyond the lectotype skull KNM-ER 1470 and other partial skull aspects. No bodily remains are definitively assigned to ''H. rudolfensis''. Consequently, both its generic classification and validity are debated without any wide consensus, with some recommending the species to actually belong to the genus ''Australopithecus'' as ''A. rudolfensis'' or ''Kenyanthropus'' as ''K. rudolfensis'', or that it is synonymous with the contemporaneous and anatomically similar ''H. habilis''.

''H. rudolfensis'' is distinguished from ''H. habilis'' by larger size, but it is also argued that this species actually consists of male ''H. habilis'' specimens, assuming that ''H. habilis'' was sexually dimorphic and males were much larger than females. Because no bodily remains are definitely identified, body size estimates are largely based on the stature of ''H. habilis''. Using this, male ''H. rudolfensis'' may have averaged about in height and in weight, and females and . KNM-ER 1470 had a brain volume of about . Like other early ''Homo'', ''H. rudolfensis'' had large cheek teeth and thick enamel.

Early ''Homo'' species exhibit marked brain growth compared to ''Australopithecus'' predecessors, which is typically explained as a change in diet with a calorie-rich food source, namely meat. Though not associated with tools, dental anatomy suggests some processing of plant or meat fiber before consumption, though the mouth could still effectively chew through mechanically challenging food, indicating tool use did not greatly affect diet.

Research history

The first fossils were discovered in 1972 along Lake Turkana (at the time called Lake Rudolf) in Kenya, and were detailed by Kenyan palaeoanthropologist Richard Leakey the following year. The specimens were: a large and nearly complete skull (KNM-ER 1470, the lectotype) discovered by Bernard Ngeneo, a local; a right femur (KNM-ER 1472) discovered by J. Harris; an upper femur (proximal) fragment (KNM-ER 1475) discovered by fossil collector Kamoya Kimeu; and a complete left femur (KNM-ER 1481) discovered by Harris. However, it is unclear if the femora belong to the same species as the skull. Leakey classified them under the genus ''Homo'' because he had reconstructed the skull fragments so that it had a large brain volume and a flat face, but did not assign them to a species. Because the horizon they were discovered in was, at the time, dated to 2.9–2.6 million years ago (mya), Leakey thought these specimens were a very early human ancestor. This challenged the major model of human evolution at the time where '' Australopithecus africanus'' gave rise to ''Homo'' about 2.5 mya, but if ''Homo'' had already existed at this time, it would call for serious revisions. However, the area was redated to about 2 mya in 1977 (the same time period as ''H. habilis'' and '' H. ergaster''/''H. erectus''), and more precisely to 2.1–1.95 mya in 2012. They were first assigned to the species ''habilis'' in 1975 by anthropologists Colin Groves and Vratislav Mazák. In 1978, in a joint paper with Leakey and English anthropologist Alan Walker, Walker suggested the remains belong in ''Australopithecus'' (and that the skull was incorrectly reconstructed), but Leakey still believed they belonged to ''Homo'', though they both agreed that the remains could belong to ''habilis''.

KNM-ER 1470 was much larger than the Olduvai remains, so the terms ''H. habilis sensu lato'' ("in the broad sense") and ''H. habilis sensu stricto'' ("in the strict sense") were used to include or exclude the larger morph, respectively. In 1986, English palaeoanthropologist Bernard Wood first suggested these remains represent a different ''Homo'' species, which coexisted with ''H. habilis'' and ''H. ergaster''/''H. erectus''. Coexisting ''Homo'' species conflicted with the predominant model of human evolution at the time which was that modern humans evolved in a straight line directly from ''H. ergaster''/''H. erectus'' which evolved directly from ''H. habilis''. In 1986, the remains were placed into a new species, ''rudolfensis'', by Russian anthropologist Valery Alekseyev (but he used the genus ''Pithecanthropus'', which was changed to ''Homo'' three years later by Groves). In 1999, Kennedy argued that the name was invalid because Alekseyev had not assigned a holotype. Pointing out that this is in fact not mandatory, Wood the same year nevertheless designated KNM-ER 1470 as the lectotype. However, the validity of this species has also been debated on material grounds, with some arguing that ''H. habilis'' was highly sexually dimorphic like modern non-human apes, with the larger skulls classified as "''H. rudolfensis''" actually representing male ''H. habilis''. In 1999, Wood and biological anthropologist Mark Collard recommended moving ''rudolfensis'' and ''habilis'' to ''Australopithecus'' based on the similarity of dental adaptations. However, they conceded that dental anatomy is highly variable among hominins and not always reliable when formulating family trees.

In 2003, Australian anthropologist David Cameron concluded that the earlier australopithecine ''Kenyanthropus platyops'' was the ancestor of ''rudolfensis'', and reclassified it as ''K. rudolfensis''. He also believed that ''Kenyanthropus'' was more closely related to '' Paranthropus'' than ''Homo''. In 2008, a re-reconstruction of the skull concluded it was incorrectly restored originally, though agreed with the classification as ''H. rudolfensis''. In 2012, British palaeoanthropologist Meave Leakey described the juvenile partial face KNM-ER 62000 discovered in Koobi Fora, Kenya; noting it shares several similarities to KNM-ER 1470 and is smaller, she assigned it to ''H. rudolfensis'', and, because prepubescent male and female bones should be indistinguishable, differences between juvenile ''H. rudolfensis'' and adult ''H. habilis'' specimens support species distinction. She also concluded that the jawbone KNM-ER 1802, an important specimen often used in classifying other specimens as ''H. rudolfensis'', actually belongs to a different (possibly undescribed) species, but American palaeoanthropologist Tim D. White believes this to be premature because it is unclear how wide the range of variation is in early hominins. The 2013 discovery of the 1.8 Ma Georgian Dmanisi skulls which exhibit several similarities with early ''Homo'' have led to suggestions that all contemporary groups of early ''Homo'' in Africa, including ''H. habilis'' and ''H. rudolfensis'', are the same species and should be assigned to ''H. erectus''. There is still no wide consensus on how ''rudolfensis'' and ''habilis'' relate to ''H. ergaster'' and descendent species.

Beyond KNM-ER 1470, there is disagreement on which specimens actually belong in ''H. rudolfensis'' as it is difficult to assign with accuracy remains that do not preserve the face and jaw. No ''H. rudolfensis'' bodily elements have been definitively associated with a skull and thus to the species. Most proposed ''H. rudolfensis'' fossils come from Koobi Fora and date to 1.9–1.85 mya. Remains from the Shungura Formation, Ethiopia, and Uraha, Malawi, are dated as far back as 2.5–2.4 mya, which would make it the earliest identified species of ''Homo''. The latest potential specimen is KNM-ER 819 dating to 1.65–1.55 mya.

Nonetheless, ''H. rudolfensis'' and ''H. habilis'' generally are recognised members of the genus at the base of the family tree, with arguments for synonymisation or removal from the genus not widely adopted. Though it is now largely agreed upon that ''Homo'' evolved from ''Australopithecus'', the timing and placement of this split has been much debated, with many ''Australopithecus'' species having been proposed as the ancestor. The discovery of LD 350-1, the oldest ''Homo'' specimen, dating to 2.8 mya, in the Afar Region of Ethiopia may indicate that the genus evolved from '' A. afarensis'' around this time. The species LD 350-1 belongs to could be the ancestor of ''H. rudolfensis'' and ''H. habilis'', but this is unclear. Based on 2.1 million year old stone tools from Shangchen, China, possibly an ancestral species to ''H. rudolfensis'' and ''H. habilis'' dispersed across Asia.

Anatomy

Skull

In 1973, Mr. Leakey had reconstructed the skull KNM-ER 1470 with a flat face and a brain volume of . In 1983, American physical anthropologist Ralph Holloway revised the base of the skull and calculated a brain volume of . For comparison, ''H. habilis'' specimens average about , and ''H. ergaster'' . Anthropologist Timothy Bromage and colleagues revised the face again at a 5° incline (slightly prognathic) instead of completely flat, but pushed the nasal bone back directly beneath the frontal bones. He then said it was possible to predict brain size based on just the face and (disregarding the braincase) calculated , and chalked up the errors of Leakey's reconstruction to a lack of research of the biological principles of facial anatomy at the time as well as confirmation bias, as a flat-faced reconstruction of the skull aligned with the predominant model of human evolution at the time. This was refuted by American palaeoanthropologist John D. Hawks because the skull remained more or less unchanged except for the 5° rotation outwards. Bromage and colleagues returned in 2008 with a revised skull reconstruction and brain volume estimate of .

Fossils have generally been classified into ''H. rudolfensis'' due to large skull size, flatter and broader face, broader cheek teeth, more complex tooth crowns and roots, and thicker enamel compared to ''H. habilis''. Early ''Homo'' are characterised by larger teeth compared to later ''Homo''. The cheek teeth of KNM-ER 60000, a jawbone, in terms of size are on the lower end for early ''Homo'', except for the third molar which is within range. The molars increase in size towards the back of the mouth. The tooth rows of KNM-ER 1470, KNM-ER 60000, and KNM-ER 62000 are rectangular, whereas the tooth row of KNM-ER 1802 is U-shaped, which may indicate that these two morphs represent different species, or demonstrate the normal range of variation for ''H. rudolfensis'' jaws. In UR 501 from Uraha, Malawi—the oldest ''H. rudolfensis'' specimen dating to 2.5–2.3 mya—the tooth enamel thickness is the same as in other early ''Homo'', but the enamel on the molars is almost as thick as '' Paranthropus'' molars (which have some of the thickest enamel of any hominin). Such a wide variation in enamel thickness across the cheek teeth is not exhibited in KNM-ER 1802, which may indicate regional differences among ''H. rudolfensis'' populations.

Build

Body size estimates of ''H. rudolfensis'' and ''H. habilis'' typically conclude a small size comparable to australopithecines. These largely depend on the ''H. habilis'' partial skeleton OH 62 estimated at in height and in weight. ''H. rudolfensis'' is thought to be bigger than ''H. habilis'', but it is unclear how big this species was as no bodily elements have been definitively associated with a skull. Based on just the KNM-ER 1470 skull, male ''H. rudolfensis'' were estimated to have been in height and in weight, and females and .

For specimens that might be ''H. rudolfensis'': the femur KNM-ER 1472 which may also be ''H. habilis'' or ''H. ergaster'' was estimated at and , the humerus KNM-ER 1473 and , the partial leg KNM-ER 1481 which may also be ''H. ergaster'' and , the pelvis KNM-ER 3228 which may also be ''H. ergaster'' and , and the femur KNM-ER 3728 which may be ''H. habilis'' or ''P. boisei'' and . It is generally assumed that pre-''H. ergaster'' hominins, including ''H. rudolfensis'' and ''H. habilis'', exhibited sexual dimorphism with males markedly bigger than females. However, relative female body mass is unknown in either species.

Early hominins, including ''H. rudolfensis'', are thought to have had thick body hair coverage like modern non-human apes because they appear to have inhabited cooler regions and are thought to have had a less active lifestyle than (presumed hairless) post-''ergaster'' species, and so probably required thick body hair to stay warm. The juvenile specimen KNM-ER 62000, a partial face, has the same age landmarks as a 13 to 14 year old modern human, but more likely died at around 8 years of age due to the presumed faster growth rate among early hominins based on dental development rate.

Culture

It is typically thought that the diets of early ''Homo'' had a greater proportion of meat than ''Australopithecus'', and that this led to brain growth. The main hypotheses regarding this are: meat is energy- and nutrient-rich and put evolutionary pressure on developing enhanced cognitive skills to facilitate strategic scavenging and monopolise fresh carcasses, or meat allowed the large and calorie-expensive ape gut to decrease in size allowing this energy to be diverted to brain growth. Alternatively, it is also suggested that early ''Homo'', in a drying climate with scarcer food options, relied primarily on underground storage organs (such as tubers) and food sharing, which facilitated social bonding among both male and female group members. However, unlike what is presumed for ''H. ergaster'' and later ''Homo'', short-statured early ''Homo'' were likely incapable of endurance running and hunting, and the long and ''Australopithecus''-like forearm of ''H. habilis'' could indicate early ''Homo'' were still arboreal to a degree. Also, organised hunting and gathering is thought to have emerged in ''H. ergaster''. Nonetheless, the proposed food-gathering models to explain large brain growth necessitate increased daily travel distance. Large incisor size in ''H. rudolfensis'' and ''H. habilis'' compared to ''Australopithecus'' predecessors implies these two species relied on incisors more. The large, ''Australopithecus''-like molars could indicate more mechanically challenging food compared to later ''Homo''. The bodies of the mandibles of ''H. rudolfensis'' and other early ''Homo'' are thicker than those of modern humans and all living apes, more comparable to ''Australopithecus''. The mandibular body resists torsion from the bite force or chewing, meaning their jaws could produce unusually powerful stresses while eating.

''H. rudolfensis'' is not associated with any tools. However, the greater molar cusp relief in ''H. rudolfensis'' and ''H. habilis'' compared to ''Australopithecus'' suggests the former two used tools to fracture tough foods (such as pliable plant parts or meat), otherwise the cusps would have been more worn down. Nonetheless, the jaw adaptations for processing mechanically challenging food indicates technological advancement did not greatly affect their diet. Large concentrations of stone tools are known from Koobi Fora. Because these aggregations are coincident with the emergence of ''H. ergaster'', it is probable ''H. ergaster'' manufactured them, though it is not possible to definitively attribute the tools to a species because ''H. rudolfensis'', ''H. habilis'', and ''P. boisei'' are also well-known from the area. The oldest specimen of ''Homo'', LD 350-1, is associated with the Oldowan stone tool industry, meaning this tradition had been in use by the genus since near its emergence.

Early ''H. rudolfensis'' and ''Paranthropus'' have exceptionally thick molars for hominins, and the emergence of these two coincides with a cooling and aridity trend in Africa about 2.5 mya. This could mean they evolved due to climate change. Nonetheless, in East Africa, tropical forests and woodlands still persisted through periods of drought. ''H. rudolfensis'' coexisted with ''H. habilis'', ''H. ergaster'', and ''P. boisei''.

See also

References

External links

Archaeology Info



Homo rudolfensis
– The Smithsonian Institution's Human Origins Program

Human Timeline (Interactive)
– Smithsonian, National Museum of Natural History (August 2016).

{{DEFAULTSORT:Homo Rudolfensis
1972 archaeological discoveries
Early species of Homo
Hominini
Fossil taxa described in 1978
Lower Paleolithic
Pleistocene primates
Prehistoric Kenya