| Homo rudolfensis | |
|---|---|
.png) | |
| Reconstruction of the KNM-ER 1470 skull | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Mammalia |
| Order: | Primates |
| Family: | Hominidae |
| Genus: | Homo (?) |
| Species: | †H. rudolfensis |
| Binomial name | |
| †Homo rudolfensis (Alekseyev, 1986) | |
| Synonyms | |
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Homo rudolfensis is an extinct species ofarchaic human from theEarly Pleistocene of East Africa about 2 million years ago (mya). BecauseH. rudolfensis coexisted with several otherhominins, it is debated what specimens can be confidently assigned to this species beyond thelectotype skull KNM-ER 1470 and other partial skull aspects. No bodily remains are definitively assigned toH. rudolfensis. Consequently, both itsgeneric classification and validity are debated without any wide consensus, with some recommending the species to actually belong to the genusAustralopithecus asA. rudolfensis orKenyanthropus asK. rudolfensis, or that it issynonymous with the contemporaneous and anatomically similarH. habilis.
H. rudolfensis is distinguished fromH. habilis by larger size, but it is also argued that this species actually consists of maleH. habilis specimens, assuming thatH. habilis wassexually dimorphic and males were much larger than females. Because no bodily remains are definitely identified, body size estimates are largely based on the stature ofH. habilis. Using this, maleH. rudolfensis may have averaged about 160 cm (5 ft 3 in) in height and 60 kg (130 lb) in weight, and females 150 cm (4 ft 11 in) and 51 kg (112 lb). KNM-ER 1470 had a brain volume of about 750 cc (46 cu in). Like other earlyHomo,H. rudolfensis had largecheek teeth and thickenamel.
EarlyHomo species exhibit marked brain growth compared toAustralopithecus 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.
The first fossils were discovered in 1972 alongLake Turkana (at the time called Lake Rudolf) in Kenya, and were detailed by Kenyan palaeoanthropologistRichard Leakey the following year. The specimens were: a large and nearly complete skull (KNM-ER 1470, thelectotype) discovered by Bernard Ngeneo, a local; a rightfemur (KNM-ER 1472) discovered by J. Harris; an upper femur (proximal) fragment (KNM-ER 1475) discovered by fossil collectorKamoya 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 genusHomo 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 thehorizon 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.[1] This challenged the major model of human evolution at the time whereAustralopithecus africanus gave rise toHomo about 2.5 mya, but ifHomo had already existed at this time, it would call for serious revisions.[2] However, the area was redated to about 2 mya in 1977 (the same time period asH. habilis andH. ergaster/H. erectus),[3] and more precisely to 2.1–1.95 mya in 2012.[4] They were first assigned to the specieshabilis in 1975 by anthropologistsColin Groves andVratislav Mazák. In 1978, in a joint paper with Leakey and English anthropologistAlan Walker, Walker suggested the remains belong inAustralopithecus (and that the skull was incorrectly reconstructed), but Leakey still believed they belonged toHomo, though they both agreed that the remains could belong tohabilis.[5]
Homo family tree showingH. rudolfensis andH. habilis at the base as offshoots of the human line:[6]
KNM-ER 1470 was much larger than the Olduvai remains, so the termsH. habilissensu lato ("in the broad sense") andH. habilissensu stricto ("in the strict sense") were used to include or exclude the larger morph, respectively.[7][8] In 1986, English palaeoanthropologist Bernard Wood first suggested these remains represent a differentHomo species, which coexisted withH. habilis andH. ergaster/H. erectus. CoexistingHomo species conflicted with the predominant model of human evolution at the time which was that modern humans evolved in a straight line directly fromH. ergaster/H. erectus which evolved directly fromH. habilis.[9] In 1986, the remains were placed into a new species,rudolfensis, by Russian anthropologistValery Alekseyev[10] (but he used the genusPithecanthropus, which was changed toHomo three years later by Groves).[11] In 1999, Kennedy argued that the name was invalid because Alekseyev had not assigned aholotype.[12] Pointing out that this is in fact not mandatory, Wood the same year nevertheless designated KNM-ER 1470 as the lectotype.[13] However, the validity of this species has also been debated on material grounds, with some arguing thatH. habilis was highlysexually dimorphic like modern non-human apes, with the larger skulls classified as "H. rudolfensis" actually representing maleH. habilis.[8][13] In 1999, Wood and biological anthropologist Mark Collard recommended movingrudolfensis andhabilis toAustralopithecus based on the similarity of dental adaptations. However, they conceded that dental anatomy is highly variable amonghominins and not always reliable when formulating family trees.[14]
In 2003, Australian anthropologist David Cameron concluded that the earlieraustralopithecineKenyanthropus platyops was the ancestor ofrudolfensis, and reclassified it asK. rudolfensis. He also believed thatKenyanthropus was more closely related toParanthropus thanHomo.[15] In 2008, a re-reconstruction of the skull concluded it was incorrectly restored originally, though agreed with the classification asH. rudolfensis.[16] In 2012, British palaeoanthropologistMeave Leakey described the juvenile partial face KNM-ER 62000 discovered inKoobi Fora, Kenya; noting it shares several similarities to KNM-ER 1470 and is smaller, she assigned it toH. rudolfensis, and, because prepubescent male and female bones should be indistinguishable, differences between juvenileH. rudolfensis and adultH. habilis specimens support species distinction. She also concluded that the jawbone KNM-ER 1802, an important specimen often used in classifying other specimens asH. rudolfensis, actually belongs to a different (possibly undescribed) species,[17] but American palaeoanthropologistTim D. White believes this to be premature because it is unclear how wide the range of variation is in early hominins.[7] The 2013 discovery of the 1.8 Ma GeorgianDmanisi skulls which exhibit several similarities with earlyHomo have led to suggestions that all contemporary groups of earlyHomo in Africa, includingH. habilis andH. rudolfensis, are the same species and should be assigned toH. erectus.[18][19] There is still no wide consensus on howrudolfensis andhabilis relate toH. ergaster and descendent species.[20]
Beyond KNM-ER 1470, there is disagreement on which specimens actually belong inH. rudolfensis as it is difficult to assign with accuracy remains that do not preserve the face and jaw.[7][8] NoH. rudolfensis bodily elements have been definitively associated with a skull and thus to the species.[21] Most proposedH. rudolfensis fossils come from Koobi Fora and date to 1.9–1.85 mya. Remains from theShungura Formation, Ethiopia, and Uraha, Malawi, are dated as far back as 2.5–2.4 mya, which would make it the earliest identified species ofHomo. The latest potential specimen is KNM-ER 819 dating to 1.65–1.55 mya.[21]: 210
Nonetheless,H. rudolfensis andH. 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.[22] Though it is now largely agreed upon thatHomo evolved fromAustralopithecus, the timing and placement of this split has been much debated, with manyAustralopithecus species having been proposed as the ancestor. The discovery ofLD 350-1, the oldestHomo specimen, dating to 2.8 mya, in theAfar Region of Ethiopia may indicate that the genus evolved fromA. afarensis around this time. The species LD 350-1 belongs to could be the ancestor ofH. rudolfensis andH. habilis, but this is unclear.[23] Based on 2.1 million year old stone tools fromShangchen, China, possibly an ancestral species toH. rudolfensis andH. habilis dispersed across Asia.[24]
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_and_Homo_habilis_(KNM-ER_1813_cast)_at_G%25C3%25B6teborgs_Naturhistoriska_Museum_8595.jpg)
In 1973, Richard Leakey had reconstructed the skull KNM-ER 1470 with a flat face and a brain volume of 800 cc (49 cu in).[1] In 1983, American physical anthropologistRalph Holloway revised the base of the skull and calculated a brain volume of 752–753 cc (45.9–46.0 cu in).[25] For comparison,H. habilis specimens average about 600 cc (37 cu in), andH. ergaster 850 cc (52 cu in).[26] Anthropologist Timothy Bromage and colleagues revised the face again at a 5° incline (slightlyprognathic) instead of completely flat, but pushed thenasal bone back directly beneath thefrontal bones. He then said it was possible to predict brain size based on just the face and (disregarding the braincase) calculated 526 cc (32.1 cu in), 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 asconfirmation 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 palaeoanthropologistJohn D. Hawks because the skull remained more or less unchanged except for the 5° rotation outwards.[27] Bromage and colleagues returned in 2008 with a revised skull reconstruction and brain volume estimate of 700 cc (43 cu in).[16]
Fossils have generally been classified intoH. rudolfensis due to large skull size, flatter and broader face, broadercheek teeth, more complex toothcrowns androots, and thickerenamel compared toH. habilis.[28] EarlyHomo are characterised by larger teeth compared to laterHomo. The cheek teeth of KNM-ER 60000, a jawbone, in terms of size are on the lower end for earlyHomo, except for the thirdmolar 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,[17] or demonstrate the normal range of variation forH. rudolfensis jaws.[7] In UR 501 from Uraha, Malawi—the oldestH. rudolfensis specimen dating to 2.5–2.3 mya—thetooth enamel thickness is the same as in other earlyHomo, but the enamel on the molars is almost as thick asParanthropus 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 amongH. rudolfensis populations.[28][29]
Body size estimates ofH. rudolfensis andH. habilis typically conclude a small size comparable to australopithecines. These largely depend on theH. habilis partial skeleton OH 62 estimated at 100–120 cm (3 ft 3 in – 3 ft 11 in) in height and 20–37 kg (44–82 lb) in weight.H. rudolfensis is thought to be bigger thanH. habilis, but it is unclear how big this species was as no bodily elements have been definitively associated with a skull.[30] Based on just the KNM-ER 1470 skull, maleH. rudolfensis were estimated to have been 160 cm (5 ft 3 in) in height and 60 kg (130 lb) in weight, and females 150 cm (4 ft 11 in) and 51 kg (112 lb).[26]
For specimens that might beH. rudolfensis: thefemur KNM-ER 1472 which may also beH. habilis orH. ergaster was estimated at 155.9 cm (5 ft 1 in) and 41.8 kg (92 lb), thehumerus KNM-ER 1473 162.9 cm (5 ft 4 in) and 47.1 kg (104 lb), the partial leg KNM-ER 1481 which may also beH. ergaster 156.7 cm (5 ft 2 in) and 41.8 kg (92 lb), thepelvis KNM-ER 3228 which may also beH. ergaster 165.8 cm (5 ft 5 in) and 47.2 kg (104 lb), and the femur KNM-ER 3728 which may beH. habilis orP. boisei 153.3 cm (5 ft) and 40.3 kg (89 lb).[30] It is generally assumed that pre-H. ergaster hominins, includingH. rudolfensis andH. habilis, exhibitedsexual dimorphism with males markedly bigger than females. However, relative female body mass is unknown in either species.[21]
Early hominins, includingH. 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.[31] 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.[17]
It is typically thought that the diets of earlyHomo had a greater proportion of meat thanAustralopithecus, 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 earlyHomo, in a drying climate with scarcer food options, relied primarily on undergroundstorage organs (such astubers) and food sharing, which facilitated social bonding among both male and female group members. However, unlike what is presumed forH. ergaster and laterHomo, short-statured earlyHomo were likely incapable ofendurance running and hunting, and the long andAustralopithecus-like forearm ofH. habilis could indicate earlyHomo were stillarboreal to a degree. Also, organisedhunting and gathering is thought to have emerged inH. ergaster. Nonetheless, the proposed food-gathering models to explain large brain growth necessitate increased daily travel distance.[32] Largeincisor size inH. rudolfensis andH. habilis compared toAustralopithecus predecessors implies these two species relied on incisors more. The large,Australopithecus-like molars could indicate more mechanically challenging food compared to laterHomo. Thebodies of the mandibles ofH. rudolfensis and other earlyHomo are thicker than those of modern humans and all living apes, more comparable toAustralopithecus. The mandibular body resists torsion from thebite force or chewing, meaning their jaws could produce unusually powerful stresses while eating.[21]
H. rudolfensis is not associated with any tools. However, the greater molarcusprelief inH. rudolfensis andH. habilis compared toAustralopithecus 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 ofH. ergaster, it is probableH. ergaster manufactured them, though it is not possible to definitively attribute the tools to a species becauseH. rudolfensis,H. habilis, andP. boisei are also well known from the area.[21] The oldest specimen ofHomo,LD 350-1, is associated with theOldowan stone toolindustry, meaning this tradition had been in use by the genus since near its emergence.[33]
EarlyH. rudolfensis andParanthropus 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.[28]H. rudolfensis coexisted withH. habilis,H. ergaster, andP. boisei.[21]
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