In , researchers announced the results of more than 15 years of analysis of the species and introduced the world to a nearly complete skeleton called Ardi. They retained primitive features—such as long, curved fingers and toes as well as longer arms and shorter legs—that indicate they spent time in trees.
While the timeline of the evolution of upright walking is well understood, why hominids took their first bipedal steps is not. In , Charles Darwin offered an explanation in his book The Descent of Man : Hominids needed to walk on two legs to free up their hands.
But after the unveiling of Ardi in , anthropologist C. As climatic changes made African forests more seasonal and variable environments, it would have become harder and more time-consuming for individuals to find food. This would have been especially difficult for females raising offspring. At this point, Lovejoy suggests, a mutually beneficial arrangement evolved: Males gathered food for females and their young and in return females mated exclusively with their providers.
To be successful providers, males needed their arms and hands free to carry food, and thus bipedalism evolved. This scenario, as with all bipedalism hypotheses, is really hard to test. But earlier this year, researchers offered some support when they found that chimpanzees tend to walk bipedally when carrying rare or valuable foods. Another theory considers the efficiency of upright walking. In the s, Peter Rodman and Henry McHenry, both at the University of California, Davis, suggested that hominids evolved to walk upright in response to climate change.
Over time, physiological changes occurred in their upper bodies, backbones, and pelvic areas, causing their weight and centers of balance to shift to a lower point in the body. This gave the hominids a steadier stance as well as the ability to stand upright with greater ease than their quadruped cousins. When our ancestors developed the need to reach higher and stand, these new physical traits came in handy—just as evolving a long neck proved favorable for the giraffe.
Keeping Cool Walking on two feet did more than help early hominids conserve energy, as some hypotheses suggest—it also protected them from overheating. According to evolutionary biologist Peter Wheeler, early bipeds were generally exposed to less direct sunlight on the savannah than quadrupeds of the same size. In fact, when the sun shone directly overhead, the heat load upon a hominid on two feet would have been 60 percent less than that upon a knuckle-walker.
Additionally, bipedalism raised hominids' bodies above the ground, enabling their skin to come in better contact with cooler and faster-moving breezes. This allowed for further heat dissipation through convection, and, says Wheeler, it meant that biped hominids needed to consume only about three pints of water per day, whereas quadrupeds needed five.
Aquatic Apes Although most paleoanthropologists, despite their many differences, tend to agree that our ancestors became bipeds on dry land, a few suggest an alternate possibility. Aquatic Ape Theory, posed by marine biologist Alister Hardy in the s, postulates that several human traits, from relatively minimal body hair to the ability to sweat moisture and salt, can be explained only through the idea that early hominids once lived in semi-aquatic environments.
The hypothesis claims that our ancestors had to wade regularly through shallow lake- or riverside waters in order to reach shellfish, aquatic plants, and other potential food sources.
With their heavy upper bodies, quadrupeds would have had a more difficult time adjusting to walking upright on the savannah than in buoyant water. Weapons and Tools Some of the oldest and most popular suggestions for why we developed into bipeds state that our upright posture relates directly to our need to use weapons and tools.
They were found a very long distance from each other, and their association must be treated with some caution. Not long after Orrorin made the news, another possible species of hominin was announced.
They were originally dated to between 6 and 7 Ma based on faunal remains found at the site Vignaud et al. The best-known specimen is a relatively complete cranium called TM The researchers argued that because it appeared to have a relatively small canine, in combination with a narrow and less prognathic protruding face, it must have been a very early hominin.
However, the skull is heavily distorted and cracked, which has obscured some important diagnostic characters. The result has two very particular features of note, the position and the angulation of the foramen magnum. In the original, this structure is hard to position, but in the reconstruction its position and angulation are more hominin-like, indicating an affinity for bipedal locomotion.
A few more specimens of S. Weaving all these various threads of evidence together into something cohesive can be an overwhelming task. Some of the specimens discussed above have only been recently announced, and most of them are still being worked on by the teams that discovered and described them, making it difficult for other researchers to independently assess them.
As a result, the evolutionary relationships between these different species are still in a state of flux. Various opinions have thus been expressed, and it really boils down to how one views variation within and between named fossil species. After the announcement of S. This could still be the case, but others have suggested that we are overestimating the level of species diversity in early hominin fossils and that Ardipithecus , Sahelanthropus , and Orrorin could very likely all belong to the same genus White In terms of a broader evolutionary context, again, it is still early days.
The team that discovered Ardipithecus has suggested that the evidence from Ethiopia and northern Kenya strongly point to a Ar. This is possible, but where Sahelanthropus and Orrorin might fit into the sequence remains to be seen. There is also the issue of Au. We find specimens as old as 4. So what can we definitively say about early hominins? We have possibly as many as four species and three genera between 7 and 4.
Twenty years ago, none of these species had been discovered or named, so we are doing well. The dating of these species coincides very well with the chimpanzee—modern human divergence dates predicted by molecular genetic work. All of them seem to exhibit adaptations to increased levels of bipedalism, but at least two genera Ardipithecus and Orrorin would have been very competent climbers as well.
We also see a reduction in canine size that might be associated with behavioral shifts in male competition and aggressive threat displays. Finally, and this we can be sure of, the hard work that goes into finding these specimens in often very remote places promises many more delights and surprises in the years to come. Aiello L, Dean C. An introduction to human evolutionary anatomy. London: Academic; Google Scholar. Begun D. The earliest hominins—is less more?
New material of the earliest hominid from the Upper Miocene of Chad. Dainton M. Did our ancestors knuckle-walk? Article Google Scholar. Haile-Selassie Y. Late Miocene hominids from the Middle Awash, Ethiopia. Late Miocene teeth from Middle Awash, Ethiopia, and early hominid dental evolution.
Harcourt-Smith WEH. The origins of bipedal locomotion. In: Henke W, Tattersall I, editors. The handbook of paleoanthropology. Dordrecht: Springer; Chapter Google Scholar. Fossils, feet and the evolution of bipedal locomotion. J Anat. Harrison T. Apes among the tangled branches of human origins. Hunt KD. The postural feeding hypothesis: an ecological model for the origin of bipedalism. S Afr J Sci. Pliocene hominids from the Hadar formation, Ethiopia — : Stratigraphic, chronologic, and paleoenvironmental contexts, with notes on hominid morphology and systematics.
Am J Phys Anthropol. Kingston JD. Shifting adaptive landscapes: progress and challenges in reconstructing early hominid environments.
Yearb Phys Anthropol. Kivell TL, Schmitt D. Independent evolution of knuckle-walking in African apes shows that humans did not evolve from a knuckle-walking ancestor.
Lothagam: the dawn of humanity in eastern Africa. New York: Columbia University Press; Pliocene footprints in the Laetoli Beds at Laetoli, northern Tanzania.
New four-million-year-old species from Kanapoi and Allia Bay, Kenya. Cosmogenic nuclide dating of Sahelanthropus tchadensis and Australopithecus bahrelgahazali , Mio-Pliocene hominids from Chads. The Maka femur and its bearing on the antiquity of human walking: applying contemporary concepts of morphogenesis to the human fossil record.
The great divides: the postcrania of our last common ancestors with African apes. CAS Google Scholar. Careful climbing in the Miocene: the forelimbs of Ardipithecus ramidus and humans are primitive. Combining prehension and propulsion: the foot of Ardipithecus ramidus.
The pelvis and femur of Ardipithecus ramidus : the emergence of upright walking. McHenry HM. Introduction to the fossil record of human ancestry. In: Hartwig WC, editor. The primate fossil record. Cambridge: Cambridge University Press; McNulty KP. Evo Edu Outreach. Reed KE. Early hominid evolution and ecological change through the African Plio-Pleistocene. J Hum Evol. Orrorin tugenensis femoral morphology and the evolution of hominin bipedalism. Evidence that humans evolved from a knuckle-walking ancestor.
Bioenergetics and the origin of hominid bipedalism. Ruvolo M. Molecular hhylogeny of the Hominoids: inferences from multiple independent DNA sequence data sets.
Mol Biol Evol. Sarmiento EE. Generalized quadrupeds, committed bipeds and the shift to open habitats: an evolutionary model of hominid divergence. Am Mus Nov. Comment on the paleobiology and classification of Ardipithecus ramidus. C R Acad Sci Paris. Strait D. The evolutionary history of the australopiths. The Ardipithecus ramidus skull and its implications for human origins. Origin of human bipedalism as an adaptation for locomotion on flexible branches. Tuttle RH. Evolution of hominid bipedalism and prehensile capabilities.
Wheeler PE. The influence of bipedalism on the energy and water budgets of early hominids.
0コメント