Homo rudolfensis

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.

Homo family tree showing H.* rudolfensis and H. *habilis at the base as offshoots of the human line:

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, 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. 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. 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.

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, Richard Leakey had reconstructed the skull KNM-ER 1470 with a flat face and a brain volume of 800 cc. For comparison, H.* habilis specimens average about 600 cc, and H. *ergaster 850 cc. Bromage and colleagues returned in 2008 with a revised skull reconstruction and brain volume estimate of 700 cc.

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.

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 100–120 cm in height and 20–37 kg 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 160 cm in height and 60 kg in weight, and females 150 cm and 51 kg.

For specimens that might be H.* rudolfensis: the femur KNM-ER 1472 which may also be H. habilis or H. ergaster was estimated at 155.9 cm and 41.8 kg, the humerus KNM-ER 1473 162.9 cm and 47.1 kg, the partial leg KNM-ER 1481 which may also be H. ergaster 156.7 cm and 41.8 kg, the pelvis KNM-ER 3228 which may also be H. ergaster 165.8 cm and 47.2 kg, and the femur KNM-ER 3728 which may be H. habilis or P. boisei 153.3 cm and 40.3 kg. 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.

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.

References

References

1. Leakey, R. E. F.. (1973). "Evidence for an Advanced Plio-Pleistocene Hominid from East Rudolf, Kenya". Nature.
2. Tobias, P. V.. (1980). ""''Australopithecus afarensis''" and ''A. africanus'': Critique and an alternative hypothesis". Palaeontologica Africana.
3. (1977). "Suid Evolution and Correlation of African Hominid Localities". Science.
4. (2012). "New single crystal 40Ar/39Ar ages improve time scale for deposition of the Omo Group, Omo-Turkana Basin, East Africa". Journal of the Geological Society.
5. (1978). "The Hominids of East Turkana". Scientific American.
6. (2015). "Handbook of Paleoanthropology". Springer.
7. Wood, B.. (1985). "Ancestors: The hard evidence". Alan R. Liss.
8. Valerii P. Alexeev, 1986, ''The Origin of the Human Race''. Progress Publishers, Moskou
9. Groves, C.P. 1989. ''A Theory of Human and Primate Evolution'', Clarendon Press, Oxford
10. Kennedy G.E. 1999. "Is 'Homo rudolfensis' a valid species?" ''Journal of Human Evolution'' '''36''': 119-121
11. Wood, B.. (1999). "''Homo rudolfensis'' Alexeev, 1986: Fact or phantom?". Journal of Human Evolution.
12. (1999). "The Human Genus". Science.
13. Cameron, D. W.. (2003). "Early hominin speciation at the Plio/Pleistocene transition". Homo.
14. (2012). "New fossils from Koobi Fora in northern Kenya confirm taxonomic diversity in early Homo". Nature.
15. (2013). "A Complete Skull from Dmanisi, Georgia, and the Evolutionary Biology of Early Homo". Science.
16. Tattersall, I.. (2001). "Classification and phylogeny in human evolution". Ludus Vitalis.
17. Kaplan, M.. (2012). "Fossils point to a big family for human ancestors". Nature News.
18. Argue, D.. (2017). "Encyclopedia of Evolutionary Psychological Science". Springer International Publishing.
19. (2006). "Diet in Early ''Homo'': A Review of the Evidence and a New Model of Adaptive Versatility". Annual Review of Anthropology.
20. (2014). "Handbook of Paleoanthropology". Springer.
21. (2015). "Early ''Homo'' at 2.8 Ma from Ledi-Geraru, Afar, Ethiopia". Science.
22. (2018). "Hominin occupation of the Chinese Loess Plateau since about 2.1 million years ago". Nature.
23. Holloway, R. L.. (1983). "Human Paleontological Evidence Relevant to Language Behavior". Human Neurobiology.
24. Hawk, J. D.. (31 March 2007). "KNM-ER 1470 is not a microcephalic".
25. (2008). "Craniofacial architectural constraints and their importance for reconstructing the early Homo skull KNM-ER 1470". The Journal of Clinical Pediatric Dentistry.
26. Wood, B.. (1993). "Rift on the record". Nature.
27. Schrenk, F.. (1997). "UR 501, the Plio-Pleistocene hominid from Malawi. Analysis of the microanatomy of the enamel". Comptes Rendus de l'Académie des Sciences, Série IIA.
28. (2015). "Spatial and temporal variation of body size among early ''Homo''". Journal of Human Evolution.
29. (2000). "''Australopithecus'' to ''Homo'': Transformations in Body and Mind". Annual Review of Anthropology.
30. (2016). "Bipedality and hair loss in human evolution revisited: The impact of altitude and activity scheduling". Journal of Human Evolution.
31. Pontzer, H.. (2012). "Ecological Energetics in Early ''Homo''". Current Anthropology.