| Kenyanthropus | |
|---|---|
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| Cast of KNM-WT 40000 at theCantonal Museum of Geology,Lausanne | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Mammalia |
| Order: | Primates |
| Suborder: | Haplorhini |
| Family: | Hominidae |
| Tribe: | Hominini |
| Genus: | †Kenyanthropus Leakeyet al.,2001 |
| Type species | |
| †Kenyanthropus platyops Leakeyet al., 2001 | |
| Other species | |
| Synonyms | |
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Kenyanthropus ('man from Kenya'[2]) is a genus of extinct hominin identified from theLomekwi site byLake Turkana, Kenya, dated to 3.3 to 3.2 million years ago during theMiddle Pliocene. It contains one species,K. platyops, but may also include the two-million-year-oldHomo rudolfensis, orK. rudolfensis. Before its naming in 2001,Australopithecus afarensis was widely regarded as the only australopithecine to exist during the Middle Pliocene, butKenyanthropus evinces a greater diversity than once acknowledged.Kenyanthropus is most recognisable by an unusually flat face and small teeth for such an earlyhominin, with values on the extremes or beyond the range of variation for australopithecines in regard to these features. Multiple australopithecine species may have coexisted by foraging for different food items (niche partitioning), which may be the reason why these apes anatomically differ in features related to chewing.
The Lomekwi site also yielded the earlieststone toolindustry, the Lomekwian, characterised by the rudimentary production of simpleflakes by pounding acore against an anvil or with ahammerstone. It may have been manufactured byKenyanthropus, but it is unclear if multiple species were present at the site or not. The knappers were using volcanic rocks collected no more than 100 m (330 ft) from the site.Kenyanthropus seems to have lived on a lakeside orfloodplain environment featuring forests and grasslands.
In August 1998, field technician Blasto Onyango discovered ahominin partial leftmaxilla (upper jaw), specimen KNM-WT 38350, on the KenyanLomekwi dig site byLake Turkana, overseen by prominent paleoanthropologistsLouise andMeave Leakey. In August 1999 at the Lomekwi site, research assistant Justus Erus discovered an uncharacteristically flat-facedaustralopithecine skull, specimen KNM-WT 40000. The 1998–1999 field season subsequently uncovered 34 more craniodental hominin specimens, but the research team was unable to determine if these can be placed into the same species as the former two specimens (that is, if multiple species were present at the site).[1]
The specimens were recovered near the Nabetili tributary of the Lomekwi river in amudstone layer of theNachukui Formation. KNM-WT 40000 was recovered from the Kataboi Member, 8 m (26 ft) below the 3.4-million-year-old Tulu BorTuff, and 12 m (39 ft) above the 3.57-million-year-old Lokochot Tuff. Bylinear interpolation, KNM-WT 40000 is approximately 3.5 million years old, dating back to theMiddle Pliocene. Only three more specimens were recovered from the Kataboi Member at around the same level, the deepest KNM-WT 38341 probably sitting on 3.53-million-year-old sediments. KNM-WT 38350 was recovered from the Lomekwi Member 17 m (56 ft) above Tulu Bor, and is approximately 3.3 million years old. The other specimens from this member sit 16 to 24 m (52 to 79 ft) above Tulu Bor, roughly 3.3 million years old as well. The highest specimens—KNM-WT 38344, -55 and -56—may be around 3.2 million years old.[1]
In 2001, Meave Leakey and colleagues assigned the Lomekwi remains to a new genus and species,Kenyanthropus platyops, with KNM-WT 40000 theholotype, and KNM-WT 38350 aparatype. The genus name honours Kenya where Lomekwi and a slew of other major human-ancestor sites have been identified. The species name derives fromAncient Greekplatus "flat" andopsis "face" in reference to the unusually flat face for such an early hominin.[1]
The classification of early hominins with their widely varying anatomy has been a difficult subject matter. The 20th century generated an overabundance of hominin genera plunging the field into taxonomic turmoil, until German evolutionary biologistErnst Mayr, surveying a "bewildering diversity of names", decided to recognise only a single genus,Homo, containing a few species. Though other genera and species have since become popular, his more conservative view of hominin diversity has become the mainstay, and the acceptance of further genera is usually met with great resistance.[3] Since Mayr, hominins are classified intoAustralopithecus which gave rise toHomo (which includes modern humans) and the robustParanthropus (which is sometimes not recognised as its own genus), which by definition leavesAustralopithecuspolyphyletic (a non-natural group which does not comprise a common ancestor and all of its descendants). In addition toKenyanthropus, the 1990s saw the introduction ofA. bahrelghazali,Ardipithecus,Orrorin andSahelanthropus, which has complicated discussions of hominin diversity,[4] though the latter three have not been met with much resistance on account of their greater age (all predatingAustralopithecus).[3]
At the timeKenyanthropus was discovered,Australopithecus afarensis was the only recognised australopithecine to have existed between 4 and 3 million years ago, aside from its probable ancestorA. anamensis, makingA. afarensis the likely progenitor of all other australopithecines as theydiversified in the late Pliocene and into thePleistocene. Leakey and colleagues consideredKenyanthropus to be evidence of a greater diversity of Pliocene australopithecines than previously acknowledged.[1] In 2015, Ethiopian palaeoanthropologistYohannes Haile-Selassie and colleagues erected a new species,A. deyiremeda, which lived in the same time and region asKenyanthropus andA. afarensis.[5]
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Meave Leakey and colleagues drew attention to namely the flat face and smallcheek teeth, in addition to several other traits, to distinguish the genus from earlierArdipithecus, contemporary and laterAustralopithecus, and laterParanthropus.Kenyanthropus lacks any of the derived traits seen inHomo. They concededKenyanthropus could be subsumed intoAustralopithecus if the widest definition of the latter is used, but this conservative approach to hominin diversity leavesAustralopithecus agrade taxon, a non-natural grouping of similar-looking species whereby it effectively encompasses all hominins not classifiable intoArdipithecus orHomo regardless of how they may be related to each other.[1] Leakey and colleagues further drew parallels with KNM-WT 40000 and the two-million-year-old KNM-ER 1470 assigned toHomo rudolfensis, attributing differences inbraincase andnasal anatomy to archaicness. They suggestedH. rudolfensis may be better classified asK. rudolfensis.[1]
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In 2003, American palaeoanthropologistTim D. White was concerned that KNM-WT 40000 was far too distorted to obtain any accurate metrics for classification purposes, especially because the skull was splintered into over 1,100 pieces often measuring less than 1 cm (0.39 in) across. Because such damage is rarely even seen, he argues that it cannot be reliably reconstructed. Because the skulls of modern ape species vary widely, he suggested further fossil discoveries in the region may prove the Lomekwi hominins to be a local variant ofA. afarensis rather than a distinct genus or species.[6] In response, anthropologist Fred Spoor and Meave and Louise Leakey produced much more detailed digitaltopographical scans of the KNM-WT 40000 maxilla in 2010, permitting the comparison of many more anatomical landmarks on the maxillae of all other early hominins, modern humans,chimpanzees andgorillas, in order to more accurately correct the distortion. The new reconstruction more convincingly verifies the distinctness ofKenyanthropus.[7]
In 2003, Spanish writerCamilo José Cela Conde and evolutionary biologistFrancisco J. Ayala proposed resurrecting the genus "Praeanthropus" to house all australopithecines which are notArdipithecus,Paranthropus, orA. africanus, though they opted tosynonymiseKenyanthropus withHomo as "H. platyops".[4] Their recommendations have been largely rejected.[3]
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KNM-WT 40000 has been heavily distorted during the fossilisation process, the braincase shifted downwards and backwards, the nasal region to the right, and the mouth and cheek region forward. It is unclear if the specimen represents a male or a female.[1]
Kenyanthropus has a relatively flat face, including subnasally, between the nose and the mouth (the nasoalveolar clivus). The clivus inclines at 45° (there is relaxed sub-nasalprognathism), steeper than almost all other australopithecine specimens (on the upper end of variation forParanthropus), more comparable toH. rudolfensis andH. habilis. This is the earliest example of a flat face in the hominin fossil record. UnlikeA. afarensis,Kenyanthropus lacks the anterior pillars, bony columns running down from the nasal aperture (nose hole). It is also one of the longest early hominin clivi discovered at 32 mm (1.3 in). The nasal aperture (nose hole) is narrow compared to that ofAustralopithecus andParanthropus. The cheekbones are tall and steep, and theanterior surface (where the cheeks juts out the most) is positioned above the premolars, more frequently seen inParanthropus than other hominins. The zygomaticoalveolar crest (stretching between the cheek and the teeth) is low and curved. Overall, the face resemblesH. rudolfensis, though has longer nasal bones, a narrower nasal aperture, a shorter postcanine (themolars andpremolars) tooth row, and a less steeply inclined (less flat, more prognathic) midfacial region. Much laterParanthropus are also characterised by relatively flat faces, but this is generally considered to be an adaptation to maximise bite force through enormous teeth, whichKenyanthropus enigmatically does not have.[1]
Among all the specimens, only the M2 (2nd upper left molar) and thetooth sockets of the left side of the mouth of KNM-WT 40000 are preserved well enough to measure and study. With dimensions of 11.4 mm2 × 12.4 mm2 (0.0177 sq in × 0.0192 sq in), a surface area of 141.4 mm (5.57 in), it is the smallest M2 ever discovered for an early hominin. For comparison, those ofA. afarensis in the comparative sample Leakey and colleagues used ranged from about 160 to 225 mm2 (0.248 to 0.349 sq in),H. habilis andH. rudolfensis 165 to 250 mm2 (0.256 to 0.388 sq in), and the robustP. boisei (with the largest molars among hominins) about 240 to 380 mm2 (0.37 to 0.59 sq in). The reconstructed dimensions of KNM-WT 38350's M1 are 10.5 mm × 12 mm (0.41 in × 0.47 in) for a surface area of 126 mm2 (0.195 sq in), which is on the lower end of variation forA. anamensis,A. afarensis andH. habilis. The thick molarenamel is on par with that ofA. anamensis andA. afarensis. KNM-WT40000 retains the ancestral ape premolartooth root morphology, with a single lingualroot (on the tongue side) and two buccal roots (towards the cheeks), though the P4 of KNM-WT 38350 may only have a single buccal root; the ancestral pattern is frequent inParanthropus and variable inAustralopithecus. Individuals of more derived species typically have single-rooted premolars. The canine jugum is not visible (a line of bone in the maxilla corresponding to the canine tooth root), which may mean the canines were not that large. The cross-sectional area of the I2 (2nd upperincisor) is 90% the size of that of I1, whereas it is usually 50 to 70% in other great apes. The tooth roots of the incisors do not appear to be orientated out (there was probably no alveolar prognathism, the front teeth did not jut forward).[1]
Brain volume is uncalculable due to distortion of the braincase, but it was probably similar to that ofAustralopithecus andParanthropus.[1] A sample of fiveA. afarensis averaged 445 cc.[8] LikeParanthropus, there is no frontal trigon (a triangle formed by the conjunction of thetemporal lines behind the brow ridge). UnlikeH. habilis but likeH. rudolfensis, there is nosulcus (trench) behind the brow ridge. The degree ofpostorbital constriction, the narrowing of the braincase in thefrontal lobe region, is on par with that ofAustralopithecus,H. rudolfensis andH. habilis, but less thanP. boisei. Like the earlierA. anamensis andAr. ramidus, thetympanic bone retains the ancestral hominin ear morphology, lacking thepetrous crest, and bearing a narrowear canal with a small opening. Theforamen magnum, where the skull connects to thespine, was probably oval shaped as opposed to the heart-shaped one ofP. boisei.[1]
In 2015, French archaeologistSonia Harmand and colleagues identified the Lomekwianstone-toolindustry at the Lomekwi site.[9] The tools are attributed toKenyanthropus as it is the only hominin identified at the site, but in 2015, anthropologist Fred Spoor suggested that at least some of the indeterminate specimens may be assignable toA. deyiremeda as the two species have somewhat similar maxillary anatomy.[10] At 3.3 million years old, it is the oldest proposed industry. The assemblage comprises 83cores, 35flakes, 7 possible anvils, 7 possiblehammerstones, 5 pebbles (which may have also been used as hammers), and 12 indeterminant fragments, of which 52 were sourced frombasalt, 51 fromphonolite, 35 from trachyphonolite (intermediate composition of phonolite andtrachyte), 3 fromvesicular basalt, 2 from trachyte, and 6 indeterminant. These materials could have originated at aconglomerate only 100 m (330 ft) from the site.[9]
The cores are large and heavy, averaging 167 mm × 147.8 mm × 108.8 mm (6.57 in × 5.82 in × 4.28 in) and 3.1 kg (6.8 lb). Flakes ranged 19 to 205 mm (0.75 to 8.07 in) in length, normally shorter than laterOldowan industry flakes. Anvils were heavy, up to 15 kg (33 lb). Flakes seem to have been cleaved off primarily using the passive hammer technique (directly striking the core on the anvil) and/or the bipolar method (placing the core on the anvil and striking it with a hammerstone). They produced bothunifaces (the flake was worked on one side) andbifaces (both sides were worked). Though they may have been shaping cores beforehand to make them easier to work, the knappers more often than not poorly executed the technique, producing incomplete fractures and fissures on several cores, or requiring multiple blows to flake off a piece. Harmand and colleagues suggested such rudimentary skills may place the Lomekwian as an intermediate industry between simple pounding techniques probably used by earlier hominins, and the flaking Oldowan industry developed byHomo.[9]
It is typically assumed that early hominins were using stone tools to cut meat in addition to other organic materials.[11] Wild chimpanzees andblack-striped capuchins have been observed to make flakes by accident while using hammerstones to crack nuts on anvils, but the Lomekwi knappers were producing multiple flakes from the same core, and flipped over flakes to work the other side, which speak to the intentionality of their production.[9][12] In 2016, Spanish archaeologists Manuel Domínguez-Rodrigo and Luis Alcalá argued Harmand and colleagues did not convincingly justify that the tools were discoveredin situ, that is, the tools may be much younger and werereworked into an older layer.[13] If the date of 3.3 million years is accepted, then there is a 700,000-year gap between the next solid evidence of stone tools, atLedi-Geraru associated with the earliestHomoLD 350-1, theOldowan industry, reported by American palaeoanthropologist David Braun and colleagues in 2019. This gap can either be interpreted as the loss and reinvention of stone tool technology, orpreservation bias (that tools from this time gap either did not preserve for whatever reason, or sit undiscovered), the latter implying the Lomekwian evolved into the Oldowan.[11]
From 4.5 to 4 million years ago, Lake Turkana may have swelled to upwards of 28,000 km2 (11,000 sq mi), in comparison to today's 6,400 km2 (2,500 sq mi); the lake at what is now theKoobi Fora site possibly sat at minimum 36 m (118 ft) below the surface. Volcanic hills by Lomekwi pushed basalt into the lake sediments. The lake broke up and from 3.6 to 3.2 million years ago, the region was probably characterised by a series of much smaller lakes, each covering no more than 2,500 km2 (970 sq mi).[14][15] Similarly, thebovid remains at Lomekwi are suggestive of a wet mosaic environment featuring both grasslands and forests on a lakeside orfloodplain.Theropithecus brumpti is the most common monkey at the site as well as the rest of theTurkana Basin at this time; this species tends to live in more forested and closed environments. At the fossiliferousA. afarensisHadar site in Ethiopia,Theropithecus darti is the most common monkey, which tends to prefer drier conditions conducive to wood- or grassland environments. Leakey and colleagues argued this distribution meansKenyanthropus was living in somewhat more forested environments than more northerlyA. afarensis.[1]
Kenyanthropus,A. afarensis andA. deyiremeda all coexisted in the same time and region, and, because their anatomy largely diverges in areas relevant to chewing, they may have practisedniche partitioning and foraged for different food items.[10]
Forms such asArdipithecus,Sahelanthropus, andOrrorin have also been admitted to the pantheon, though this has clearly been facilitated by their great age. And in a nod to history, the venerable genusParanthropus has been grandfathered in for use by those who think it useful. But except for the widely dismissed revival ofPraeanthropus, there has been little real rethinking of the hugely minimalist hominid taxonomy, generic as well as specific, that Mayr foisted on us all those years ago...
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