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ScienceWeek
PALEOANTHROPOLOGY: ON THE FLORES FOSSILS
The following points are made by M.M. Lahr and R. Foley (Nature 2004 431:1043):
1) The recently discovered Homo floresiensis fossils (1,2) probably left no descendants, are not very old, and were found on a remote island. Despite this, they are among the most outstanding discoveries in palaeoanthropology for half a century. The find is startling. It is of a pygmy-sized, small-brained hominin, which lived as recently as 18,000 years ago, and which was found on the island of Flores together with stone tools, dwarf elephants and Komodo dragons.
2) The Flores fossils add a new and surprising twig to the hominin family tree, which diverged from the chimpanzee lineage about 7 million years ago. The first African hominins existed 7-1.2 million years ago, were 1-1.5 meters tall, walked upright on two legs (i.e., were bipedal), and had chimpanzee-size brains. These early forms comprised as many as six genera and fourteen species, of which the australopithecines are the best known. By 2.5 million years ago, our own genus, Homo, had emerged, with its different body shape, slower growth, greater reliance on meat in the diet, and "encephalization" -- larger brains than expected for body size. These were the first hominins to make stone tools systematically and to colonize Eurasia. They include the familiar names of H. habilis, H. erectus, H. neanderthalensis and, finally, H. sapiens, which put in an appearance about 160,000 years ago. The new fossil is part of this Homo group.
3) Flores lies to the east of Java, and was probably never connected to the mainland. The presence of 800,000-year-old simple stone tools first attracted attention in 1998 (3), raising the controversial possibility that H. erectus had produced them and had crossed major sea barriers to reach Flores. Now we have the announcement of the discovery of an 18,000-year-old hominin skeleton from a cave, Liang Bua, on Flores. Although this date is more than 140,000 years after modern humans evolved in Africa, more than 25,000 years after H. sapiens reached Australia, and about 10,000 years after the last known Neanderthal, the skeleton is that of a new species -- Homo floresiensis. Its most remarkable features are its diminutive body (about a meter in height) and brain size (at 380 cm^(3), the smallest of any known hominin).
4) Homo floresiensis is a challenge -- it is the most extreme hominin ever discovered. An archaic hominin at that date changes our understanding of late human evolutionary geography, biology and culture. Likewise, a pygmy and small-brained member of the genus Homo raises questions about our understanding of morphological variability and allometry -- the relation between the size of an organism and the size of any of its parts. Brown et al(1) claim that the skeleton, designated LB1, represents a new species within the genus Homo. They believe that it may have been a female. They also conclude that it was a dwarfed descendant of Javanese H. erectus, and part of an endemic island fauna.(4,5)
References (abridged):
1. Brown, P. et al. Nature 431, 1055-1061 (2004)
2. Morwood, M. J. et al. Nature 431, 1087-1091 (2004)
3. Morwood, M. J., O'Sullivan, P. B., Aziz, F. & Raza, A. Nature 392, 173-176 (1998)
4. Conway Morris, S. Life's Solutions: Inevitable Humans in a Lonely Universe (Cambridge Univ. Press, 2003)
5. Wood, B. & Richmond, B. G. J. Anat. 197, 19-60 (2000)
Nature http://www.nature.com/nature
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ANTHROPOLOGY: ON HOMINID FOSSILS
The following points are made by Jeffrey H. Schwartz (Science 2004 305:53):
1) The period from 1 million to 500,000 years ago (~1 to 0.5 Ma) is well represented in the human fossil records of Europe and Asia. Sites containing such fossils include Ceprano, Italy (~0.9 to 0.8 Ma), the TD-6 level at Atapuerca's Gran Dolina, Spain (~0.78 Ma), Trinil, Indonesia (1 to 0.7 Ma), some parts of the Sangiran Dome, Indonesia (1.5 to 1 Ma), Lantian, China (~1 Ma), and probably Zhoukoudian, China (0.55 to 0.3 Ma).
2) By contrast, Africa has been unusually uninformative about this part of human evolution. Three partial mandibles unearthed more than 50 years ago at Tighenif (Ternifine) in Algeria (~0.7 Ma) are similar in dental morphology to specimens from Gran Dolina (1), but the former are rarely mentioned in the literature. The question thus remained: Where are the African fossils?
3) The recent discovery of the partial Daka skull (~1 Ma) at the Bouri site, Middle Awash, Ethiopia (2), provided part of the answer. Potts et al (3) recently reported that the archaeologically and faunally rich site of Olorgesailie, Kenya, has divulged its first hominid fossils: a partial frontal and more fragmentary temporal bone dated 0.97 to 0.9 Ma. Like the Daka specimen, these fragments (KNM-OL 45500) were assigned to the species Homo erectus.
4) Potts et al. correctly assess the "Homo erectus" debate: "The entire sample of fossils from Africa, Asia, and Europe exhibits wide morphological variation that some researchers divide into multiple lineages and others place in a single, polytypic species." They opt for the latter hypothesis and conclude that "comparison of the KNM-OL 45500 with other crania . . . illustrates that metric and qualitative similarities cut across temporal and spatial groups of fossil specimens." Assuming that a vast array of specimens of differing morphologies constitute the same species, favorable comparisons between some of them in one or a few morphologies are expected, especially if primitive retentions and shared derived features are not sorted out.
5) But this does not clarify the question, "What is H. erectus?" One is left primarily with the traditional approach to the genus Homo: H. erectus is not H. habilis, H. heidelbergensis, or H. sapiens, whatever they are.
6) Recognizing that "Homo erectus" may be more a historical accident than a biological reality might lead to a better understanding of those fossils whose morphology clearly exceeds the bounds of individual variation so well documented in the Trinil/Sangiran sample. In the meantime, OL 45500 should remind us that hominid systematics must be an endeavor of testing long-entrenched hypotheses, especially when those who turn to these hypotheses acknowledge them as being problematic.(4,5)
References (abridged):
1. J. H. Schwartz, I. Tattersall, The Human Fossil Record, vol. 4, Craniodental Morphology of Australopithecus, Paranthropus, and Orrorin (Wiley-Liss, New York, in press)
2. B. Asfaw et al., Nature 416, 317 (2002)
3. R. Potts, A. K. Behrensmeyer, A. Deino, P. Ditchfield, J. Clark, Science 305, 75 (2004)
4. J. H. Schwartz, I. Tattersall, The Human Fossil Record, vol. 2, Craniodental Morphology of Genus Homo (Africa and Asia) (Wiley-Liss, New York, 2003)
5. J. H. Schwartz, I. Tattersall, Acta Anthropol. Sin. 19 (suppl.), 21 (2000)
Science http://www.sciencemag.org
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ON THE NEW HOMINID FOSSIL FROM CHAD
The following points are made by Bernard Wood (Nature 2002 418:133):
1) A single fossil can fundamentally change the way we reconstruct the tree of life. More than 75 years ago, Raymond Dart's description(1) of the Taung skull from southern Africa wrought such a transformation with regard to human evolution. Dart provided hard evidence to support Darwin's prediction that the roots of human evolutionary history run deepest in Africa. A fossil cranium recently discovered by Michel Brunet and his colleagues(2,3) marks a similar turning point in our understanding of human origins. The new fossils -- the cranium, a jaw fragment and several teeth -- belong to a primitive human precursor, or hominid, that is an astonishing 6-7 million years old. The transformation wrought here is more nuanced than Dart's, but it is as fundamental. Here we have compelling evidence that our own origins are as complex and as difficult to trace as those of any other group of organisms.
2) For almost 150 years(4) it has been suggested that modern humans are more closely related to the African apes than they are to the orangutan. Nowadays, evidence from both bones and teeth(5), and soft tissues (muscles, nerves, and so on), and from molecular and DNA analyses, support the view that modern humans and chimpanzees are particularly closely linked. When the DNA differences are calibrated by using palaeontological evidence, they indicate that the hypothetical ancestor of modern humans and the chimpanzee lived between about 5 and 7 million years ago.
3) The hominid fossil record outside Africa has stubbornly failed to break the 2-million-year barrier. Thus, if the "molecular clock" keeps reasonably good time, between 3 and 5 million years or so of our independent evolution took place on the African continent. Four regional "windows" provide fossil evidence relevant to our early evolutionary history. The southern African window was revealed by Dart in 1925 when the first (and only) hominid fossil from Taung, near Kimberley, was recognized; since then, neighboring cave sites have provided a rich fossil record that stretches back to around 3 to 3.5 million years ago. The East African window comprises sites along the Eastern, or Gregory, Rift Valley, from close to the Gulf of Aden in the north to northern Tanzania in the south. The sites are associated with sedimentary basins or the rivers that fed or drained them. Two of them, Middle Awash in Ethiopia and Lukeino in Kenya, have so far provided the oldest evidence of creatures that are plausible human ancestors.
References (abridged):
1. Dart, R. A. Nature 115, 195-199 (1925)
2. Brunet, M. et al. Nature 418, 145-151 (2002)
3. Vignaud, P. et al. Nature 418, 152-155 (2002)
4. Huxley, T. H. Evidence as to Man's Place in Nature (Williams & Norgate, London & Edinburgh, 1863)
5. Groves, C. P. in Systematics of the Great Apes (eds Swindler, D. R. & Erwin, J.) 187-217 (Liss, New York, 1986)
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