Re: ul
Cre Wallace
The origin of the genus Homo in Africa signals the beginning of the shift from increasingly bipedal apes to primitive, large-brained, stone tool-making, meat-eaters that traveled far and wide. This early part of the human genus is represented by three species: Homo habilis, Homo rudolfensis, and Homo erectus. H. habilis is known for retaining primitive features that link it to australopiths and for being the first stone tool makers. Little is known about H. rudolfensis except that it had a relatively large brain and large teeth compared to H. habilis and that it overlapped in time and space with other early Homo. Our understanding of the paleobiology and evolution of the larger-brained H. erectus is enhanced due to its rich fossil record. H. erectus was the first obligate, fully committed biped, and with a body adapted for modern striding locomotion, it was also the first in the human lineage to disperse outside of Africa. The early members of the genus Homo are the first to tip the scale from the more apish side of our evolutionary history toward the more human one.
Of the earliest members of the genus Homo, the best-known species are Homo habilis, Homo rudolfensis, and Homo erectus (Table 1). Listed roughly in order of their earliest appearance from oldest to youngest, these three species are the focus of this review. The first two species are the most primitive. Compared to what is known about H. erectus, little is known about how they differ in anatomy and behavior from one another and from preceding australopiths. Although it was anatomically and behaviorally primitive compared to modern humans, H. erectus signifies a major shift in hominin evolution, most notably through increased brain and body size and increasingly complex tools and behaviors.
The exceptionally large-toothed and strong-jawed robust australopiths (Paranthropus, which also descended from an earlier Australopithecus species), with their distinct dietary niche, one of tough nuts, seeds and fruits and fibrous vegetation, continued to thrive in some of the same localities as these species of early Homo, but they remained evolutionarily separate. Early Homo is distinguishable from contemporaneous robust australopiths by their smaller teeth and jaws and by larger cranial capacity.
As with so many mammalian extinctions in the Pleistocene fossil record, it is unclear why H. erectus did not survive to the present day, except that later species of Homo had much bigger brains, much more sophisticated technology, and either indirectly or directly out-competed H. erectus at being big-brained, bipedal, stone tool-making hominins.
Because the fossil record is denser for this species, the variation in H. erectus is better understood than in any of the earlier hominins. A growing proportion of researchers employ a separate species name for the earliest members of H. erectus that tend to be smaller and are found mostly in Africa; they call these Homo ergaster (Tattersall 2007; for discussion see Dunsworth and Walker 2002). This scheme implies that H. ergaster in Africa is ancestral to, or is a sister group to, H. erectus in Asia. However, in this review, large-brained, large-bodied members of the genus Homo found across the Old World are considered to display regional, not species-level, variation, and are all described as H. erectus (following Antn 2003). Under this hypothesis, regional variation in H. erectus cranial features and body size, as in H. sapiens, are the result of drift, gene flow, and selection working differently in geographically dispersed populations.
The evolutionary lineage leading to later Homo, including Neanderthals and H. sapiens, split from a population of H. erectus, probably living in Africa. With future fossil discoveries across the Old World, this geographic interpretation for H. erectus origins may change, but the phylogenetic hypothesis for its relationship to humans is unlikely to be overturned soon.
Leakey, Tobias, and Napier argued that what they had found was not as large-brained or bodied as H. erectus, a species which was poorly known at the time, but it was not as primitive and apelike as the australopiths either. Thus, they laid some ground rules for what constituted the genus Homo. First of all, brain size had to be above 600 cubic centimeters, which is greater than in all known australopiths, as understood both then and now (Holloway 2000). Plus, the bones of the skull had to be smooth and rounded, lacking the crests displayed on australopiths.
Beyond cranial morphology, the second criterion for belonging to the genus Homo, they argued, should be stone tool-making ability, which indicates an increase in behavioral and cognitive complexity associated with the larger brain. Although no one has discovered a fossil hominin skeleton holding a stone tool, those found near fossil hominins are attributed to them.
Leakey and colleagues also suggested that the face and mandible of the genus Homo should be smaller than australopiths, more like H. erectus and H. sapiens and that the postcranial skeleton should resemble H. sapiens (few postcranial elements of H. erectus were known at the time). The partially preserved foot skeleton (OH 8) showed that H. habilis had fully adapted feet for walking bipedally, unlike preceding australopiths, which still had some primitive traits (like a slightly divergent big toe) despite their regular upright locomotion.
These standards were useful at a time when the fossil record was sparse and, thus, told a simpler story. But now that the fossil record has grown and the picture has become much more complex, these first standards for early Homo are no longer as effective for sorting out which fossils belong to the genus Homo and which belong to other genera.
Part of the difficulty lies in the parts preserved. None of the late Pliocene candidates for earliest Homo preserves the means for estimating brain size, so there is no way of tracking brain size expansion in the fossil record at this time. Moreover, the only two candidates for earliest Homo that preserve postcranial elements have already been attributed to Australopithecus despite the presence of some derived anatomical trends compared to australopiths (e.g., the humanlike humerofemoral length index of the Bouri remains; Asfaw et al. 1999). The only candidate for earliest Homo to be recovered in loose association with stone tools (not the tools themselves, but butchered bones) has been attributed to Australopithecus. This behavioral criterion may be the weakest of all of the original standards since the earliest tools on record, from as early as 2.6 million years ago in Gona, Ethiopia (Semaw et al. 1997, 2003), predate the candidates for earliest Homo by at least 100,000 years. What is more, the earliest stone tools do not exactly track brain size evolution since they predate the earliest evidence for significantly larger brains by at least 700,000 years.
To recognize that a tooth or jaw from the late Pliocene and onward is a member of the genus Homo and not Australopithecus or Paranthropus, it must be more humanlike and less apelike than those genera. This suite of traits includes smaller teeth overall, relatively smaller molars and premolars compared to the incisors, further reduced canines than earlier hominins, thick enamel, and a parabolic dental arcade. That is, the teeth form a horseshoe shape rather than a v-shape or a u-shape, as in earlier hominins and in nonhuman apes. As described below, particular features of the dentition, the jaws, the cranium, the cranial capacity, and the postcranial morphology distinguish H. habilis, H. rudolfensis, and H. erectus from one another.
Despite its small brain size, KNM-ER 1813 shares features with other, larger-brained cranial representatives of H. habilis (e.g., OH 24) that characterize the species: the roundness of the brain case, the small size of the orbits, and the degree of prognathism or projection below the nose, which is still more pronounced in H. habilis than in later Homo. Like its australopith ancestors, H. habilis still had large teeth, some specimens rivaling australopiths, but the molars are narrower and the third molars are reduced in size (Wood 1992).
With larger bodies, larger brains, and smaller teeth, most H. erectus fossils are distinct from the other two species of early Homo. H. erectus teeth are smaller than H. habilis and H. rudolfensis, and although the early specimens have large teeth, tooth size decreases through time in the species. The upper incisors of many specimens are shovel-shaped, and this is seen in some modern humans.
Although he was not fully grown, the skeleton of the Nariokotome boy (KNM-WT 15000) has provided much insight into the paleobiology of H. erectus (Dean and Smith 2009). According to the pattern of ossification of the ends of his long bones (also known as the fusion of his growth plates), his age at death is estimated to have been at 13 years old. However, the microscopic growth increments in his teeth indicate that he was only 8 years old when he died. Together, this evidence suggests that H. erectus grew up faster than H. sapiens (but slower than apes) and therefore achieved benchmarks of growth earlier than modern humans. Moreover, the H. erectus growth curve probably lacked the slowdown, followed by an adolescent growth spurt, typical in humans. The Nariokotome boy, at 8 years old, was probably as behaviorally independent and mature as a young human adult, not an 8-year-old human, so H. erectus had a life history unlike anything alive today (Dean and Smith 2009).
7fc3f7cf58