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ScienceWeek
ANTHROPOLOGY: ENDURANCE RUNNING AND HUMAN EVOLUTION
The following points are made by D.M. Bramble and D.E. Lieberman (Nature 2004 432:345):
1) Most research on the evolution of human locomotion has focused on walking. There are a few indications that the earliest-known hominids were bipeds[1,2], and there is abundant fossil evidence that australopithecines habitually walked by at least 4.4 million years (Myr) ago[3,4]. Many researchers interpret the evolution of an essentially modern human-like body shape, first apparent in early Homo erectus, as evidence for improved walking performance in more open habitats that came at the expense of retained adaptations in the australopithecine postcranium for arboreal locomotion [5].
2) Although the biomechanics of running, the other human gait, is well studied, only a few researchers have considered whether running was a mode of locomotion that influenced human evolution. This lack of attention is largely because humans are mediocre runners in several respects. Even elite human sprinters are comparatively slow, capable of sustaining maximum speeds of only 10.2 m/s for less than 15 s. In contrast, mammalian cursorial specialists such as horses, greyhounds, and pronghorn antelopes can maintain maximum galloping speeds of 15-20 m/s for several minutes. Moreover, running is more costly for humans than for most mammals, demanding roughly twice as much metabolic energy per distance travelled than is typical for a mammal of equal body mass. Finally, human runners are less manoeuvrable and lack many structural modifications characteristic of most quadrupedal cursors such as elongate digitigrade feet and short proximal limb segments.
3) However, although humans are comparatively poor sprinters, they also engage in a different type of running, endurance running (ER), defined as running many kilometers over extended time periods using aerobic metabolism. Although not extensively studied in non-humans, ER is unique to humans among primates, and uncommon among quadrupedal mammals other than social carnivores (such as dogs and hyenas) and migratory ungulates (such as wildebeest and horses).
4) In summary: Striding bipedalism is a key derived behavior of hominids that possibly originated soon after the divergence of the chimpanzee and human lineages. Although bipedal gaits include walking and running, running is generally considered to have played no major role in human evolution because humans, like apes, are poor sprinters compared to most quadrupeds. The authors assess how well humans perform at sustained long-distance running, and review the physiological and anatomical bases of endurance running capabilities in humans and other mammals. Judged by several criteria, humans perform remarkably well at endurance running, thanks to a diverse array of features, many of which leave traces in the skeleton. The fossil evidence of these features suggests that endurance running is a derived capability of the genus Homo, originating about 2 million years ago, and may have been instrumental in the evolution of the human body form.
References (abridged):
1. Haile-Selassie, Y. Late Miocene hominids from the Middle Awash, Ethiopia. Nature 412, 178-181 (2001)
2. Galik, Y. et al. External and internal morphology of the BAR 1002'00 Orrorin tugenensis femur. Science 305, 1450-1453 (2004)
3. Ward, C. V. Interpreting the posture and locomotion of Australopithecus afarensis: where do we stand? Yb. Physical Anthropol. 35, 185-215 (2002)
4. Aiello, L. & Dean, M. C. An Introduction to Human Evolutionary Anatomy (Academic, London, 1990)
5. Rose, M. D. in Origine(s) de la BipÚdie chez les Hominides (eds Coppens, Y. & Senut, B.) 37-49 (CNRS, Paris, 1991)
Nature http://www.nature.com/nature
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HUMAN EVOLUTION: A FUNCTIONAL HUMAN-APE GENETIC DIFFERENCE
The following points are made by Pete Currie (Nature 2004 428:373):
1) Ever since the famous occasion in the 19th century in which Bishop Wilberforce ridiculed the possibility that man was descended from apes, and the biologist T. H. Huxley (1825-1895) bravely chose primate ancestry rather than ignorance, the debate over our origins has claimed a special place in evolutionary theory. With the acceptance by most of us that we are indeed a product of natural selection, discussions surrounding the issue have cooled somewhat. But exactly how natural selection acted to produce the modern human form has remained hotly contested.
2) Stedman et al(1) describe what may be the first functional genetic difference between humans and apes. Remarkably, the timing of the appearance of this genetic alteration, or mutation, roughly coincides with the appearance of "human-like" characteristics in the hominid fossil record, and the authors present convincing arguments as to how the mutation could have been responsible for their acquisition.
3) The evolution of the genus Homo is associated with the appearance of several defining features or traits, which set the species within it apart from our more distant ancestors(2). Among these morphological innovations is a reduced reliance on powerful masticatory (jaw) muscles as a means of breaking down food. Large jaw muscles are a feature of a number of genera of the family Hominidae. These include living primates and extinct forerunners of Homo, such as Paranthropus and Australopithecus, which are believed to possess many features more allied to extant apes. By contrast, a relatively slight masticatory apparatus is associated with the fossil remains of species within Homo and modern humans, and the advent of this morphological transition is closely correlated with a dramatic increase in cranial capacity.
4) How exactly did this shift in cranial morphology come about? By analyzing gene sequences in the human genome, Stedman et al(1) have identified a new member of the class of genes that encode myosin heavy chain (MYH) that may be associated with this change. Myosin heavy chains are a critical protein component of the sarcomeres, the "engine room" of skeletal muscle from which contractile force is derived. There are numerous types of myosin heavy chain specialized for the different muscle-contraction rates specific for different muscles. The inactivation of individual MYH genes, either in gene "knockout" mutant mice or in human disease, often results in a dramatic reduction in the size of the muscles in which they are active(3-5).
5) The particular gene in question, MYH16, is specifically expressed in the jaw muscles of humans and monkeys. But, surprisingly, a mutation in the human gene prevents the accumulation of MYH16 protein. Stedman et al(1) found that, by contrast, all non-human primates for which genome sequence could be obtained have an intact copy of the gene, and have a high level of MYH16 protein in their jaw muscles. An analysis of the time at which the mutation arose during hominid evolution places it at about 2.4 million years ago, the period just before the evolution of the modern hominid cranial form. These findings suggest a seductive hypothesis: that a decrease in jaw-muscle size, produced by inactivation of MYH16, removed a barrier to the remodeling of the hominid cranium which consequently allowed an increase in the size of the brain.
References (abridged):
1. Stedman, H. H. et al. Nature 428, 415-418 (2004)
2. Wood, B. Nature 355, 783-790 (1992)
3. Acakpo-Satchivi, L. et al. J. Cell Biol. 139, 1219-1229 (1997)
4. Allen, D. L., Harrison, B. C., Sartorius, C., Byrnes, W. C. & Leinwand, L. A. Am. J. Physiol. Cell Physiol. 280, C637-C645 (2001)
5. Martinsson, T. et al. Proc. Natl Acad. Sci. USA 97, 14614-14619 (2000)
Nature http://www.nature.com/nature
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ON HUMAN EVOLUTION
The following points are made by Ian Tattersall (Scientific American 2001 December):
1) When we contemplate the extraordinary abilities and accomplishments of Homo sapiens, it is certainly hard to avoid a first impression that there must somehow have been an element of inevitability in the process by which we came to be what we are. The product, it's easy to conclude, is so magnificent that it /must/ stand as the ultimate expression of a lengthy and gradual process of amelioration and enhancement. How could we have got this way by accident? If we arrived at our exalted state through evolution, then evolution must have worked long and hard at burnishing and improving the breed, must it not?
2) Yet that seems not to be how evolution works; for natural selection is not -- it cannot be -- in itself a creative process. Natural selection can only work to promote or eliminate novelties that are presented to it by the random genetic changes (influenced, of course, by what was there before) that lie behind all biological innovations. Evolution is best described as opportunistic, simply exploiting or rejecting possibilities as and when they arise, and in turn, the same possibility may be favorable or unfavorable, depending on environmental circumstances (in the broadest definition) at any given moment. There is nothing inherently directional or inevitable about this process, which can smartly reverse itself any time the fickle environment changes.
Scientific American http://www.sciam.com
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