ScienceWeek

ANTHROPOLOGY: ON THE EARLIEST HOMININS

The following points are made by David R. Begun (Science 2004 303:1478):

1) Fossil and molecular evidence are converging on a consensus that the human lineage diverged from that of the chimpanzee between ~6 and 8 million years ago (Ma). Recent fossil discoveries in Africa are consistent with this conclusion and are beginning to paint a picture of the pattern of speciation that led to the origin of our lineage. Haile-Selassie et al (1) announce a new late Miocene African hominid (2) species, the third in the past 3 years (1,3,4). Ardipithecus kadabba is a nearly 6-million-year-old hominin (2) that Haile-Selassie and colleagues elevate to the species level from Ardipithecus ramidus kadabba. The new discovery renews discussion of the number of taxa that lived shortly after the human lineage diverged from that of chimpanzees, and may shed light on the very nature of the first humans (2). Although Haile-Selassie and co-workers argue that all known African hominids between ~7 and 4.4 Ma may belong to one genus and see little taxonomic diversity near hominin origins, they may in fact have evidence of more diversity than previously recognized.

2) The new evidence, discovered in the Middle Awash region of Ethiopia, is a handful of teeth including premolars and canines that are indeed distinct from A. ramidus (1,5). But the relationship of this older and smaller sample of specimens to A. ramidus is uncertain. A. ramidus from the Middle Awash study area (locality of Aramis) has been dated at 4.4 Ma, whereas the morphologically distinctive A. kadabba sample from Asa Koma and several other localities also in the Middle Awash study area is older, 5.6 to 5.8 Ma (one specimen, an enigmatic foot phalanx, is dated to 5.2 Ma). On the basis of their interpretation of a gradual transformation from apelike to humanlike canine/premolar morphology, and the expectation of high levels of diversity within genera or even species, Haile-Selassie et al argue that A. kadabba represents an early phase in the development of human anterior teeth. They suggest that A. kadabba represents a transition between chimpanzees (with large, dagger-like canines) and A. ramidus, Australopithecus, and Homo (with unimpressive canines that wear down from the tips).

3) Two other late Miocene hominins, Orrorin tugenensis and Sahelanthropus tchadensis, show predictable variability in this evolving lineage. This is a plausible scenario. However, Ardipithecus, Orrorin, and Sahelanthropus offer evidence of striking diversity. Orrorin has modern-looking thickly enameled molars and robust jaws but has postcranial morphology that is distinct from that of Pliocene hominins (the epoch following the Miocene when clearly bipedal humans are found) (3). Sahelanthropus is also quite distinct from Pliocene hominins in craniofacial and dental morphology (4). Do these samples represent stages in early hominin evolution or discrete lineages? In other words, how many taxa are represented by these fossil samples?

References (abridged):

1. Y. Haile-Selassie, G. Suwa, T. D. White, Science 303, 1503 (2004)

2. The terms hominid, hominin, and even human are in flux among paleoanthropologists. The author refers to all great apes and humans as hominids (a family), and all taxa more closely related to living humans than to chimpanzees as hominins (a tribe). In this context, human is synonymous with hominin

3. B. Senut et al., C. R. Acad. Sci. Paris Ser. IIA 332, 137 (2001)

4. M. Brunet et al., Nature 418, 145 (2002)

5. T. White, G. Suwa, B. Asfaw, Nature 371, 306 (1994)

Science http://www.sciencemag.org

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A NEW HOMININ FOSSIL DATED AT 3.5 MILLION YEARS AGO

The terms hominine, hominin, hominoid, hominid, are not interchangeable, but their classification criteria are variously in a state of flux. In general, the hominoids are a primate superfamily, the hominid family comprises the great apes within the hominoid superfamily, the hominini are a "tribe" within the hominids characterized by a number of features including bipedalism, and the hominini are further partitioned into three genera, Homo, Paranthropus, and Australopithecus (until recently there were only two genera, with Paranthropus included in Australopithecus). In general, the term "hominin" refers to any human-related fossil group.

Concerning research in human evolution, most paleoanthropologists agree that what is important is to achieve an understanding of the evolutionary transitions and transformations of the human lineage, and any classification scheme must be secondary to this objective. In other words, in this context, classification must ultimately reflect phylogeny (the actual evolutionary relationships), and as knowledge of phylogeny changes, so must the extant classification schemes.

In general, the term "Australopithecus" refers to a now extinct genus believed to have existed between 4.4 and 1 million years ago, with this genus believed to have been the precursor of the genus Homo. All australopithecines are apparently characterized by an ape-like form, rather than the human-like form of the genus Homo. (Australopithecine fossils have been found only in Africa: the prefix "australo-" in this case means "south", the word "australopithecus" meaning "southern ape".)

The fossil Australopithecus afarensis, which provided the first record of human footprints, of hominids walking upright, was discovered at Laetoli in East Africa, and has been dated at 3.6 million years ago. A. afarensis, probably weighed 25 to 50 kilograms (60 to 120 lbs.) as an adult.

Concerning the early hominids, Ardipithecus ramidus is the oldest, and is currently considered the "oldest human". This species was found in Ethiopia in 1994 and dated as 4.4 million years old. Australopithecus anamensis, found at Lake Turkana in Kenya, is dated as 4.1 to 3.9 million years old, and is considered to be an intermediate between A. ramidus and later hominids.

Lake Turkana (formerly called Lake Rudolf) is the fourth largest of the east African lakes, and is named after the Turkana people, a group living in northwestern Kenya. The lake lies mainly in northern Kenya, with its northern end extending into Ethiopia, and is situated in the eastern arm of the east Africa's Rift Valley. The lake covers an area of 6405 square kilometers at 375 meters above sea level. Lake Turkana is 248 kilometers long and only 16 to 32 kilometers wide. It is relatively shallow, with its greatest recorded depth at 73 meters. Since 1970, a number of valuable hominid fossils have been unearthed at localities on the eastern shore of Lake Turkana, these localities now called the "Koobi Fora sites". The present report concerns an important find on the opposite (western) shore of Lake Turkana.

The Pliocene time-frame extends from 5.2 to 1.64 million years ago, and the Pleistocene time-frame extends from 1.64 million years ago to 10,000 years ago.

The term "adaptive radiation" refers to the rapid evolution of one or a few forms into many different species that occupy different habitats within a new geographical area. In general, the term "genus" is a taxonomic category that stands between "family" and "species", with each genus containing a number of species that are presumably related to each other by descent from a common ancestor. There are various definitions in use for the term "species": a) the biological species concept: a species is an interbreeding or potentially interbreeding group of populations reproductively isolated from other groups; b) the evolutionary species concept: a species is a lineage evolving separately from others with its own unitary evolutionary role and tendencies; c) the population genetics concept: a species is a population of individuals bearing distinctive genes and gene frequencies and separated from other species by biological barriers preventing gene exchange. In the context of paleoanthropology, taxonomic categorizations of Pliocene hominids based on fossil remains are far from straightforward, and the criteria used for genus and species demarcations have been and continue to be controversial.

In this context, a "derived" characteristic is a characteristic not present in the closest known ancestor, i.e., a characteristic that has evidently newly evolved. In contrast, a "primitive" characteristic is a characteristic that is present in the closest known ancestor.

The following points are made by M.G. Leakey et al (Nature 2001 410:433):

1) The authors point out that the eastern African hominin record between 4 and 3 million years ago has been represented exclusively by a single species, A. afarensis and its possible ancestor A. anamensis, which are commonly thought to belong to the lineage ancestral to all later hominins. This apparent lack of diversity in the middle Pliocene contrasts markedly with the increasingly varied phylogeny in the later hominin fossil record. To further study the time interval 4 to 3 million years ago, fieldwork by the authors in 1998 and 1999 focused on sites of this age at Lomekwi in the Nachukui Formation west of Lake Turkana.

2) The authors report the discovery of new fossils west of Lake Turkana, the new fossils differing markedly from those of contemporary A. afarensis, indicating that hominin taxonomic diversity extended back into the middle Pliocene. A 3.5-million-year-old cranium, showing a unique combination of derived facial and primitive neurocranial features, is assigned by the authors to a new genus of hominin. The authors suggest their findings point to an early diet-driven adaptive radiation, provide new insight into the association of hominin craniodental features, and have implications for our understanding of Pliocene-Pleistocene hominin phylogeny. The new fossil (KNM-WT 40000) is named by the authors as Kenyanthropus platyops (platyops from the Greek "platus", meaning flat, and "opsis", meaning face; thus referring to the characteristically flat face of this species.) The genus name Kenyanthropus is chosen "in recognition of Kenya's contribution to the understanding of human evolution through the many specimens recovered from its fossil sites.")

In a commentary on the Leakey et al report, the following points are made by Daniel E. Lieberman (Nature 2001 410:419):

1) The author (Lieberman) suggests that the new fossil is almost certainly a new species and with a "dizzying mosaic" of features. None of these characteristics is in itself new, but the combination of features is not found in any other known species. The fossil resembles chimpanzees and one of the australopithecine species, A. anamensis, in having a small earhole. The fossil also shares many other features of primitive hominins with A. afarensis and A. anamensis, such as cheek teeth with thick enamel, a small brain the size of that of a chimpanzee, and flat nasal margins.

2) The author (Lieberman) suggests that we can now say with confidence that hominin evolution, like that of many other mammalian groups, occurred through a series of complex radiations, in which many new species evolve and rapidly diversify. Apparently, between 3.5 and 2 million years ago, there were several human-like species that were well adapted to life in different environments, although in ways that we have yet to appreciate fully.

3) The author (Lieberman) concludes: "A challenge for the next decade will be for skeletal biologists, paleontologists, and molecular biologists to work together, to devise new analytical methods with which to tease trustworthy signals from [existing] data. My guess is that it will be quite a while before we can confidently determine the position of Kenyanthropus platyops in the human evolutionary tree."

Nature http://www.nature.com/nature

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DIET AND THE EVOLUTION OF THE EARLIEST HUMAN ANCESTORS

In general, the term "hominids" refers to any primate in the human family, and the "Pliocene epoch", the geological epoch with the time-frame 5.2 to 1.64 million years ago, is the epoch during which the first hominids appeared. There are presently 12 species of hominids recognized, based on fossil evidence, but some of the classifications are controversial. Of the 12 hominid species, 6 belong to the early hominids generally known as "australopithecines" and 6 species to the later hominid genus "Homo".

The geological epoch preceding the Pliocene, was the Miocene, with the time-frame 23.3 to 5.2 million years ago, and it is during the Miocene that the precursors to the hominids, the "hominoids", first appeared. In general, the hominoids are a primate superfamily that includes the gibbons, the great apes (Pongidae) and the hominids.

Perhaps the most important characteristic of the hominids, at least as revealed by extant fossils, is the apparent existence of "bipedalism", walking upright on the hind limbs. The appearance of bipedalism would have led to anatomical changes in all parts of the body system, and many of these changes are distinguishing characteristics in hominid fossils. The evidence for the evolution of bipedalism includes the oldest hominid skeletons, dated as 3.6 to 4.4 million years old, and a trackway of footprints in volcanic ash dated as 3.75 million years old.

Another important characteristic in the anthropology of the hominids concerns dentition and its relation to diet. In general, hominid dental features show a reduction of the anterior teeth (canines and incisors) that is interpreted as indicating a dietary shift of significance. Recent research with the scanning electron microscope has revealed tiny pits and scratches on the surfaces of tooth enamel ("microwear patterns") that provide evidence of dietary conditions. Grasses, for example, leave linear scratches on teeth; leaves produce a polished effect; and the bone crunching involved in eating meat gouges out tiny pits in the enamel.

Concerning the early hominids, Ardipithecus ramidus is the oldest, and is currently considered the "oldest human". This species was found in Ethiopia in 1994 and dated as 4.4 million years old.

Australopithecus anamensis, found in Lake Turkana in Kenya, is dated as 4.1 to 3.9 million years old, and is considered to be an intermediate between A. ramidus and later hominids.

The hominid Australopithecus afarensis has provided many fossils, all in Africa, and has been dated as 3.2 to 2.9 million years ago.

The fossil Australopithecus africanus has been dated as approximately 3 to 1.5 million years ago.

During the last 50 years, ever-increasing numbers of hominid fossils have been found, and their taxonomy and apparent place in the pattern of human evolution continues to involve detailed controversy.

The following points are made by M.F. Teaford and P.S. Ungar (Proc. Natl. Acad. Sci. 2000 97:13506):

1) The authors point out that since the discovery of A. afarensis, many researchers have emphasized the importance of bipedality in scenarios of human origins, but less attention has been focused on the role played by diet in the ecology and evolution of the early hominids. Recent work in a broad range of disciplines, such as paleo-environmental studies, behavioral ecology, primatology, and isotope analyses has renewed interest in early hominid diets. In addition, important new fossils from the early Pliocene raise major questions about the role of dietary changes in the origins and early evolution of the hominids. The authors state: "We need to focus not just on how the early hominids moved between food patches, but also on what they ate when they got there."

2) The authors present a review of the fossil evidence for the diets of the Pliocene hominids A. ramidus, A. anamensis, A. afarensis, and A. africanus. These hominids offer evidence for the first half of human evolution, from our split with the prehistoric apes to the earliest members of our own genus. The authors view the taxa under consideration as a roughly linear evolutionary sequence from A. ramidus to A. africanus, spanning the time-frame 4.4 to 2.5 million years ago. As such, these species provide a unique opportunity to examine changes in dietary adaptations of human ancestors over nearly 2 million years. The authors also trace what has been inferred concerning the diets of the Miocene hominoids in order to place changes in Pliocene hominid diets into a broader temporal perspective.

3) The authors report that from their analysis it becomes clear that the dietary capabilities of the early hominids changed dramatically in the time period between 4.4 to 2.3 millions years ago. Most of the evidence has come from five sources: analyses of tooth size, tooth shape, enamel structure, dental microwear, and jaw biomechanics. Taken together, the evidence suggests a dietary shift in the early australopithecines, a shift to increased dietary flexibility in conjunction with apparent climate variability. Moreover, changes in diet-related adaptations from A. anamensis to A. afarensis to A. africanus suggest that hard abrasive foods became increasingly important through the Pliocene, perhaps as critical items in the diet.

Proc. Nat. Acad. Sci. http://www.pnas.org

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