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*ScienceWeek*

* 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

--------------------------------

Related Material:

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)

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

--------------------------------

Related Material:

ANTHROPOLOGY: DEFINING THE HUMAN GENUS

Notes by ScienceWeek:

Taxonomy is the field in biology devoted to the classification of living
organisms, and the term "taxon" is used to indicate the members of any
particular group in the classification scheme: class, family, genus, etc
[*Note #1]. The general problem is how to incorporate information about
evolutionary history and adaptation into taxonomic classification
schemes, with the problem exemplified by attempts to define our own
genus, Homo.

The definition of the genus Homo has always been somewhat contentious,
not least because it is tied, implicitly or explicitly, to the state of
"being human". A series of anatomical characteristics is found to be
apparently unique in Homo -- for example, an increase in cranial vault
height and cranial thickness, reduced lower facial prognathism (i.e.,
reduced projection of the jaw), and in dentition reduction in the size
of the premolars and molars and the length of the molar row -- but what
has always been emphasized by taxonomic definitions is the size of the
Homo brain as revealed by the size of the cranium.

According to the classical scheme, to be Homo is to be a large-brained
*hominine (the apparent human-related fossil group), a hominine
presumably more technologically accomplished than the ancestor group,
the *Australopithecines.

At the present time, as is evident in this report, the taxonomic
categorizations related to the human genus are far from fixed.

The following points are made by B. Wood and M. Collard (Science 1999
284:65):

1) The authors suggest that recent data, fresh interpretations of the
existing evidence, and the limitations of the paleoanthropological
record invalidate existing criteria for allocating fossil species to Homo.

2) The authors suggest that regardless of any formal definitions, in
current practice fossil hominin species are assigned to Homo on the
basis of one or more out of 4 criteria: a) absolute brain size at least
600 cubic centimeters; b) possession of language as inferred from
*endocranial casts; c) possession of a modern human-like precision grip
involving a well-developed and opposable thumb (pollex); d) the ability
to manufacture stone tools. The authors state: "It is now evident,
however, that none of these criteria is satisfactory."

3) The authors present a revised definition for the genus Homo based on
criteria considered verifiable and conclude that two species, *Homo
habilis and *Homo rudolfensis, do not belong in the genus. The authors
suggest the earliest taxon to satisfy the criteria is *Homo ergaster, or
early African *Homo erectus, which currently appears in the fossil
record at about 1.9 million years ago.

4) The authors suggest that a fossil species should be included in Homo
only if the following can be demonstrated:

a) the species is more closely related to H. sapiens than it is to the
australopiths;

b) the species has an estimated body mass more similar to that of H.
sapiens than to that of the australopiths;

c) the species has reconstructed body proportions that match those of H.
sapiens more closely than those of the australopiths;

d) the species has a *postcranial skeleton whose functional morphology
is consistent with modern human-like obligate bipedalism and limited
facility for climbing;

e) the species is equipped with teeth and jaws that are more similar in
terms of relative size to those of modern humans than to those of the
australopiths;

f) the species shows evidence for a modern human-like extended period of
childhood growth and development.

5) The authors conclude by suggesting that the adoption of the above
criteria would mean the genus Homo would have both phylogenetic and
adaptive significance. "Researchers can then explore whether this
adaptive shift in hominin evolution corresponds with changes in climate,
analogous evolutionary changes in other large mammal groups, particular
innovations in the hominin cultural record, substantial expansions in
geographic range, or changes in ecological tolerance as reflected in
reconstructions of hominin habitats."

Science http://www.sciencemag.org

--------------------------------

Notes by ScienceWeek:

Note #1: The conventional hierarchy of classification in biology is
Kingdom, Phylum, Class, Order, Family, Genus, and Species. In the
literature, organisms are usually referred to by genus and species in
binomial nomenclature, with the genus capitalized. Human beings are
genus Homo, species sapiens, binomially Homo sapiens. The convention in
binomial nomenclature is to initialize the genus; thus: H. sapiens.

hominine: 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 the genera Homo and Australopithecus. Concerning
research in human evolution, most paleoanthropologists agree that what
is important is to achieve an understanding of the evolutionary
transitions and transformations, 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.

Australopithecines: Members of the now extinct genus Australopithecus,
believed to exist between 4.4 and 1 million years ago, and believed to
be precursors of the genus Homo. All australopithecines are apparently
characterized by an ape-like form, rather than the human-like form of
the Homo genus.

endocranial casts: In general, an "endocast" (steinkern) is any fossil
formed after dissolution of an interior molding substance. An
"endocranial cast" is an endocast involving the cranium. The interior of
the endocast can often reveal details concerning the absent soft
interior substance (in this case, the brain).

Homo habilis: In 1964, an early fossil hominin (1.9 to 1.6 million years
before the present) was found in Olduvai, Tanzania, the brain apparently
intermediate in size between the earliest known Homo fossil *Homo
erectus and the Australopithecus group. This new fossil was denoted as a
new species by its discoverers and named Homo habilis. The original set
of H. habilis fossils included a relatively complete hand, its structure
apparently compatible with an ability to make and use tools. (Homo
habilis literally means "handy-man") Considerable controversy in the
paleoanthropology community concerning H. habilis has continued from
1964 until the present.

Homo rudolfensis: The original H. habilis species has more recently been
divided into H. habilis and H. rudolfensis, after a fossil of the latter
group was discovered in 1993 and related to an earlier find in 1967,
both dating at approximately 2.4 million years ago. One view is that
Homo habilis/rudolfensis evolved in Africa approximately 2 million years
ago and quickly expanded into Asia to become *Homo erectus/ergaster.

Homo ergaster: H. ergaster and *H. erectus are the two immediate
precursors of H. sapiens, with H. ergaster believed to have originated
in Africa and to have given rise to H. erectus in Asia. But as with
other hominid fossil groups, precise evolutionary sequences and
geographical loci continue to be debated.

Homo erectus: First discovered by Eugene Dubois in 1891 in Indonesia,
this fossil group is currently viewed as the closest precursor to H.
sapiens. Formerly called "Anthropithecus erectus" and "Pithecanthropus
erectus". Pithecanthropus erectus and Sinanthropus erectus ("Peking
man", discovered in 1927) were in 1951 subsumed under the single
category Homo erectus, which was then recognized as a widespread species
exhibiting significant geographical variation.

postcranial skeleton: In general, this refers to the skeleton behind the
cranium in a quadruped and below the cranium in a biped.

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