io
n
ersit
Omo River
onal Pa
from t
recovered by Kamoya Kimeu from the site of KHS (Kamoya?s Hom-
inid Site; Fig. 2) in Member I of the Kibish Formation (Butzer, 1969;
Butzer et al., 1969). According to the original description, ??excava-
tion of site KHS yielded some material in situ and established the
provenance of the Omo I skeleton in terms of the stratigraphy of
the Kibish desposts?? (Leakey, 1969: 1132). Both the cranial remains
Homo erectus (e.g., Day, 1969; Rightmire, 1976; Stringer, 1978;
Day and Stringer, 1982; White et al., 2003). The anatomical differ-
ences between the two crania led to considerable debate in the
subsequent literature about the actual stratigraphic provenance
of the two fossils.
The absolute age of the Omo hominids was dif?cult to establish,
largely because of the technological limitations of isotopic dating in
the late 1960s. In the same year as the original description, papers
describing the geology of the Kibish Formation and the hominid
Contents lists availab
m
.e l
ARTICLE IN PRESS
Journal of Human Evolution xxx (2008) 1?6* Corresponding author.seums, under the direction of Richard Leakey, recovered three fossil
hominid specimens from the Kibish Formation of southern Ethiopia
(Leakey, 1969). Omo III consists of a few fragments of the anterior
part of the neurocranium and the upper face and has received rela-
tively little attention (Day,1969). However, the other twospecimens,
Omo I and Omo II (Fig. 1), have occupied a critical but controversial
role in our understanding of the timing and geography of modern
human origins throughout the forty years since their initial discov-
ery and description (e.g., Stringer and McKie, 1996).
Omo I consists of numerous fragments of the cranium, dentition,
and mandible, as well as much of the postcranial skeleton. It was
Day, 1969; Stringer, 1978; Day and Stringer, 1982; Rightmire,
1976; but see Bartsiokas, 2002).
Omo II consists of a nearly complete neurocranium. It was re-
covered as a surface ?nd by Dr. Paul Abell (Fig. 3) at the site of
PHS (Paul?s Hominid Site), also in the upper part of Member I of
the Kibish Formation (Butzer, 1969; Butzer et al., 1969). In the ini-
tial description, Day (1969) attributed the Omo II neurocranium to
Homo sapiens along with the Omo I specimen. However, in con-
trast to the Omo I cranial remains, which have generally been con-
sidered anatomically modern, the Omo II neurocranium has been
regularly described as more primitive, with many similarities toExpedition to the Omo River, a team
In 1967, as part of the InternatiE-mail address: j?eagle@notes.cc.sunysb.edu (J.G.
0047-2484/$ ? see front matter  2008 Published by
doi:10.1016/j.jhevol.2008.05.007
Please cite this article in press as: John G. Fle
(2008), doi:10.1016/j.jhevol.2008.05.007and mandible, and many postcranial elements. Omo II, from Paul?s Hominid Site (PHS), is a virtually com-
plete calvaria. Only a limited fauna and a few stone artifacts attributed to the Middle Stone Age were re-
covered in conjunction with the fossil hominids. The available dating techniques suggested a very early
age, over 100 ka, for Member I, from which the Omo I and Omo II fossils were recovered. However, in
subsequent decades, the reliability of the dates and the provenance of the Kibish hominids were repeat-
edly questioned. The papers in this volume provide a detailed stratigraphic analysis of the Kibish Forma-
tion and a series of new radiometric dates that indicate an age of 196  2 ka for Member I and 104  1
for Member III, con?rming the antiquity of the lower parts of the Kibish Formation and, in turn, the fos-
sils from Member I. Studies of the postcranial remains of Omo I indicate an overall modern human mor-
phology with a number of primitive features. Studies of an extensive lithic record from Members I and III
indicate a Middle Stone Age technology comparable to assemblages of similar age elsewhere in Ethiopia.
Studies of the mammalian, avian, and ?sh faunas indicate overall similarities to those found in the region
today, with a few distinctive differences.
 2008 Published by Elsevier Ltd.
leontological Research
he Kenya National Mu-
and the postcranial remains have been considered by virtually all
researchers to represent anatomically modern Homo sapiens (e.g.,Homo sapiens
Modern humansHominid Site (KHS), consists of much of a skeleton, including most of the cranial vault, parts of the faceKeywords:
Ethiopia
Cranial and skeletal remains of modern humans, Homo sapiens, were discovered in the Kibish Formation
in 1967 by a team from the Kenya National Museums directed by Richard Leakey. Omo I, from Kamoya?sPaleoanthropology of the Kibish Format
John G. Fleagle a,*, Zelalem Assefa b, Francis H. Brow
aDepartment of Anatomical Sciences, T8-023 Health Sciences Center, Stony Brook Univ
bNational Museum of Natural History, Smithsonian Institution, Washington, DC, USA
cCollege of Mines and Earth Sciences, University of Utah, Salt Lake City, UT 84112, USA
dDepartment of Anthropology, Stony Brook University, Stony Brook, NY 11794, USA
a b s t r a c t
Journal of Hu
journal homepage: wwwFleagle).
Elsevier Ltd.
agle et al., Paleoanthropologyn, southern Ethiopia: Introduction
c, John J. Shea d
y, Stony Brook, NY 11794-8081, USA
le at ScienceDirect
an Evolution
sevier .com/locate/ jhevolsites, including a series of radiometric dates, were published by
of the Kibish Formation, southern Ethiopia: Introduction, J Hum Evol
Butzer and Thurber (1969) and Butzer et al. (1969). In the latter pa-
per, the authors presented the results of several radiometric dates
on shells from a Nile oyster (Theria elliptica) bank in unit f (just
above the fossiliferous horizon) of Member I, yielding two 14C dates
of ??greater than 39,900 yr.?? and a Th230/U234 date of ??around
130,000.?? In a footnote, the authors noted that:
detailed work. The ??age?? reported here should perhaps be
considered only as con?rming the antiquity of the shells as
indicated by radiocarbon and as some substantiation of the
geologic interpretation. It should not be considered too seri-
ously as an ??age?? (Butzer et al., 1969: 19; see also Schwarcz
and Blackwell, 1992). The antiquity of Member I was also con-
Fig. 1. The fossil crania Omo I and Omo II from the Kibish Formation (photo courtesy of Michael Day).
ARTICLE IN PRESS
J.G. Fleagle et al. / Journal of Human Evolution xxx (2008) 1?62molluscs have been shown to have open chemical systems
with respect to uranium and its daughter products. Thus the
accuracy of ??ages?? calculated from Th230/U234 ratios is dif?cult
to determine, and perhaps impossible, even with much moreFig. 2. The 1967 excavation at the KHS site where Om
Please cite this article in press as: John G. Fleagle et al., Paleoanthropology of
(2008), doi:10.1016/j.jhevol.2008.05.007?rmed by dates from Member III. These included an Etheria
unit dated at greater than 37,000 yr by 14C and around 30,000
by Th230/U234. Thus, the fossils fromMember I were clearly older
than the analytical limits of radiocarbon dating (30?40,000 yr)o I was recovered (photo by Paul Abell).
the Kibish Formation, southern Ethiopia: Introduction, J Hum Evol
ARTICLE IN PRESS
HumJ.G. Fleagle et al. / Journal ofat the time and there were suggestions that they may be over
100,000 years old, but that date was not supported very enthu-
siastically. The very limited fauna offered no useful information
about the age of the hominids (Leakey, 1969).
These doubts over the actual age of the Kibish fossils were ech-
oed by subsequent reviews of the material for several decades. In
a review of African hominids in 1978, F.C. Howell noted:
Human cranial remains and some associated postcrania, were
recovered (in 1967) from a situation considered to represent
the lower part (Member 1) of the Kibish Formation, lower
Omo Basin, southwestern Ethiopia (R.E.F. Leakey, Butzer, and
Day, 1969). This unit of the Kibish Formation was accumulated
when Lake Turkana stood some 60 m higher than its present
(?375 m) level, and ?ooded the whole of the lower Omo valley
(Butzer et al., 1972). At least two (of three) specimens are
thought to derive from different localities related to the upper
sedimentary units (5 and/or 6, or 7 units) of that member. As
they have comparable N and U values they are considered to
be broadly contemporaneous. An age as old asw130,000 years
has been suggested for the specimens on the basis on Th/U
measurements (Butzer et al., 1969). However, the reliability of
this method has still to be adequately demonstrated; even if
this determination appears reasonable, it is nonetheless uncon-
?rmed. The radiocarbon determinations (on shell) from overly-
ing members, with a minimum age 37,000 years, have also
been questioned. The mammal fauna associated with one
(No. 1) of the specimens, with few species represented and
none of them extinct, is frankly unhelpful and also unconvinc-
ing of any very remote antiquity (Howell, 1978: 216).
Fig. 3. Richard Leakey (above) and Paul Abell (right) examining the Omo II cranium
recovered by Abell in 1967 (photo by Bob Campbell).
Please cite this article in press as: John G. Fleagle et al., Paleoanthropology
(2008), doi:10.1016/j.jhevol.2008.05.007Similarly, in describing ??The Omo Specimens,?? Wolpoff (1980:
256) stated:
Date determination is a continuing problem for the African re-
mains. The oldest Upper Pleistocene East African specimens with
any hope of fairly accurate dating come from the Kibish formation
atOmo (inEthiopia).. Radiometricdatingof the laterhominid re-
mains . has been beset with problems. The faunal date of
60,000 years BP appears likely, making the Omo specimens
roughly contemporary with the early Wu?rm glaciation in Europe.
Numerous Levallois (prepared core) ?akes are found in the de-
posits, although there is no direct associationwith the hominids.
Wolpoff?s date of 60,000 years BP apparently comes from a com-
ment that Michael Day made in a UNESCO symposium published
in 1972: ??The fauna associated with the remains is said to be of
Middle Pleistocene type. On this basis, it seems likely that the dat-
ing of these remains should be Upper Middle Pleistocene, possibly
60,000 years B.P.?? (Day, 1972: 35). However, we have no idea where
Day obtained this assessment.
A decade later, Wolpoff (1989: 65) again summarized the age of
the Kibish fossils:
The Omo radiometric dates have been continuously disputed
ever since their ?rst publication because radiocarbon determi-
nations based on shells are notoriously inaccurate, and recent
Uranium/Thorium dates are problematic. Various faunal and
stratigraphic ?dates? have been suggested as replacements for
these radiometric estimates (Day, 1972; Stringer, 1989) and ac-
cording to these the age of the three fossils could range between
40,000 and 130,000 years. However, which of the various date
estimates may be correct cannot be established, and the fact
that there is no particular reason to accept any of them as valid!
Likewise, Smith (1992: 240?241) stated:
Unfortunately, although the morphology of these remains is not
open to serious question, the geological ages claimed for the
adult remains are very questionable. The Omo I skeleton was
found partially in situ in member 1 of the Kibish formation.
This level was dated to 130 ka ago by uranium-thorium applied
tomollusc shell (Day and Stringer,1982). However, shell-derived
uranium-thorium dates are generally considered dubious (H.P.
Schwarcz, personal communication). Errors usually involve un-
derestimates of age because of uranium uptake into the shell
(Aitkin, 1990) but it may be also possible that leaching might
produce erroneously older ages (as is the case with bone).
The associated fauna and a greater than 37 ka conventional
radiocarbon date for the overlying member 3 deposits in the
Kibish Formation (Day and Stringer, 1982) do indicate some an-
tiquity for Omo I, but it is impossible to con?dently determine
the precise antiquity of this specimen at the present time.
Similar doubts were expressed by Smith et al. (1989, p. 45).
In contrast, on the basis of the same data, Bra?uer (1989: 127)
suggested:
There is good reason to assume a high age for the anatomically-
modern Omo I hominid, whichwas found at the base of member
I of the Kibish Formation. The hominid level is situated much
lower in the stratigraphy (cf. the section in Day and Stringer,
1982) than upper level of member III for which a radiocarbon
date of 37,000 BP was obtained. Butzer (pers. comm.) even as-
sumes that the entire member III lies beyond the range of con-
ventional radiocarbon dating. Therefore, an age much greater
than 40,000 years appears very reasonable. According to Butzer
(pers. comm.), it is most probable that members I, II and III all
an Evolution xxx (2008) 1?6 3belong to oxygen isotope stage 5 (a non-glacial period), and
are thus older than 75,000 years. Summarizing all the available
of the Kibish Formation, southern Ethiopia: Introduction, J Hum Evol
ARTICLE IN PRESS
umevidence, including the Uranium/Thorium date of about
130,000 BP for member I, an age of at least about 100,000 years
seems well established for Omo I.
In addition to questions about the dating of the Kibish fossils,
the morphological differences between the Omo I and Omo II cra-
nial remains led to continued questioning about the provenance
of the bones. Most of the attention paid to the Kibish fossils concen-
trated on the date of Omo I in the context of the origin of modern
humans, and the Omo II specimen was often ignored (e.g., Smith
et al., 1989). However, those authors that addressed the morpho-
logical differences between the specimens took a wide range of
approaches. Most (e.g., Rightmire, 1976) accepted the original geo-
logical results and noted that theymust represent either a consider-
able amount of morphological variation within a taxon (Day, 1969;
see also Trinkaus, 2005) or that the Kibish fossils sampled two
different, contemporarypopulations (DayandStringer,1982). Others
were more creative. In a series of papers, Bra?uer and colleagues
(Bra?uer et al., 1997; Bra?uer, 2001) published diagrams showing the
Omo II specimen as older than the Omo I specimen, while Klein
(1999: 397) suggested that ??a starkmorphological contrast between
Omo-Kibish 1 and Omo-Kibish 2 may mean that one (or both) were
intrusive into the stratigraphic unit they derive from, and Omo 1
(moremodern)may bemuchmore recent.?? To our knowledge, there
is no geological or taphonomic evidence for either of these views.
Thus, by the end of the second millennium, the signi?cance of
the Kibish hominids for our understanding of modern human ori-
gins was surrounded by considerable confusion and speculation
about the age and provenance of the Omo I and Omo II specimens.
In addition, from the original ?eldwork, there was very little infor-
mation in the form of either other fauna or archaeological materials
that might help put the Kibish hominids in a broader archaeological
or paleontological context.
In 1999, we began a series of expeditions designed to clarify the
many ambiguities about provenance and age of the fossils from
1967, as well as to recover additional paleontological and archaeo-
logical material from the Kibish Formation. We conducted ?eld re-
search in southern Ethiopia under the auspices of the Authority for
Research and Conservation of Cultural Heritage in 1999, 2001, 2002,
and 2003. In addition, the participants spent many additional
months working in the National Museum of Ethiopia, Addis Ababa.
The papers in this special issue of Journal of Human Evolution
describe the results of this project.
Our recent studies of the stratigraphy and geochronometry of
the Kibish Formation have corroborated and expanded the original
studies by Butzer and colleagues (Brown and Fuller, 2008; McDou-
gall et al., 2005, 2008). Brown and Fuller (2008) con?rm Butzer?s
division of the Kibish Formation into four distinct members and
report maximum thicknesses for the individual members that
are similar to those reported by Butzer and colleagues. However,
they ?nd that the stratigraphy of Member III is more complex
and divisible into several distinct parts. The basal parts of each
member of the Kibish Formation appear to have been laid down
very rapidly, probably as a series of annual layers, with overlying
parts of each member deposited somewhat less rapidly. Thus,
the formation seems to record short intervals of deposition
separated by long stretches of time. McDougall et al. (2008; see
also McDougall et al., 2005) report a weighted mean age of
196 2 ka for tephra in Member I and a weighted mean age of
1041 ka for Member III. These are concordant with the dates
provided by Butzer and colleagues in 1969, and con?rm a consider-
able antiquity for the lower three members of the Kibish Forma-
tion. In addition, the dates for individual members of the Kibish
Formation coincide with the dates of sapropels in the Mediterra-
J.G. Fleagle et al. / Journal of H4nean Sea. Both the sapropels formed by out?ow from the Nile
and the Kibish Formation formed by the ?ow on the Omo River
Please cite this article in press as: John G. Fleagle et al., Paleoanthropology
(2008), doi:10.1016/j.jhevol.2008.05.007are driven by peaks of rainfall in the Ethiopian highlands associ-
ated with climate cycles of roughly 23 kyr in conjunction with
precessional cycles of the earth?s rotation around the sun (e.g.,
Maslin and Christensen, 2007).
Questions about the provenance and relative ages of the Omo I
and Omo II fossils are addressed by Brown and Fuller (2008) and by
Feibel (2008). Although the site from which Omo II was recovered
(PHS) was incorrectly mapped in the publications from earlier
decades (Butzer, 1969; Day and Stringer, 1982), all evidence sup-
ports the original reports by Butzer (1969) that both Omo I and
Omo II came from the upper part of Member I and are of approxi-
mately the same age.
While the cranial remains of Omo I and Omo II have been
discussed extensively in the literature, the numerous postcranial
remains of Omo I have received very little attention (Day, 1969;
Day et al., 1991). Pearson et al. (2008a) provide a more detailed
description and preliminary analysis of the Omo I postcranial re-
mains. This study includes both the specimens recovered in 1967
and additional elements recovered during recent ?eldwork. Their
descriptions agree with earlier notes on the the Omo I postcranial
remains, describing them as modern in overall appearance, but
they note various primitive features. Primate and human fossil
clavicles are rare, but the Omo I specimen includes a nearly com-
plete left clavicle. Voisin (2008) ?nds that it shows a modern hu-
man pattern of curvature. Pearson et al. (2008b) describe
additional hominid remains from the Kibish Formation, including
a nearly complete tibia and several cranial fragments. The new
tibia, also from Member I, is very similar to tibial remains of
Omo I.
On the basis of the 1967 expedition, Leakey and colleagues
reported that ??very few stone tools were collected, all of which
were surface ?nds with the exception of ?ake debris from the
KHS excavation?? (Leakey, 1969: 1132). As reported by Shea (2008)
and Sisk and Shea (2008), ?eldwork between 1999 and 2003
yielded a large number of stone tools from both surface collections
and from three separate excavations (see also Shea et al., 2007), one
at the KHS site, one in the lower part of Member III, and two sites in
Member I. The lithic assemblages from the Kibish Formation exhibit
radial-centripetal preparation of cores struck from small clasts,
relatively few retouched tools, and a large biface component. These
properties align them with Lupemban industries of the Equatorial
African Middle Stone Age. The Kibish assemblages are similar to
lithic assemblages of roughly equivalent age elsewhere in Ethiopia,
such as Aduma in the Middle Awash (Yellen et al., 2005) and
Gademotta (Wendorf and Schild, 1974).
The report on the 1967 expedition mentioned only a very lim-
ited fauna recovered from the Kibish Formation. However the re-
cent expeditions have yielded large numbers of specimens from
Members I, III, and IV, including mammals, birds, and ?sh. Assefa
et al. (2008) provide an analysis of the mammalian fauna from
Members I, III, and IV. All of the mammalian fossils from the Kibish
belong to extant taxa (at the species level, when so identi?ed), and
most are from taxa that currently live in the area today. However,
a few of the taxa are now found only in other parts of Africa and
suggest that the Kibish Formation formed under wetter conditions
than today, a ?nding that is concordant with the results of geolog-
ical research (McDougall et al., 2005, 2008; Brown and Fuller,
2008). Analyses of the mammalian fauna indicate a mosaic of envi-
ronments. Louchart (2008) describes the avian fossils from the
Kibish Formation. Most of the fossils are attributed to water birds
that are found in the region today. These include pelicans (two spe-
cies), darters, and herons, as well as the terrestrial guinea fowl.
Likewise, the ?sh fauna described by Trapani (2008) includes spe-
cies found in the Omo River today and is dominated by cat?sh
an Evolution xxx (2008) 1?6and Nile perch. However, many of the fossil specimens are larger
than any recorded from the modern ?sh fauna.
of the Kibish Formation, southern Ethiopia: Introduction, J Hum Evol
bothmicrostratigraphy and tephra correlations, support the original
the Omo I and Omo II crania, such as the higher frontal and more
ARTICLE IN PRESS
Humrounded occipital contour in Omo I, remains a subject of debate.
One possibility is that this diversity re?ects the range of morpho-
logical variation found in a single population at that time (e.g., Trin-
kaus, 2005). Alternatively, the two craniamay sample two different,
roughly contemporary populations. In any case, further study, in-
cluding taphonomic analysis, of the Omo hominid remains and
comparisons with other early modern human fossils is clearly
warranted. There is abundant archaeological material in Members
I, III, and IV of the Kibish Formation that offers many opportunities
for excavation and research into lithic technology and zooarchaeol-
ogy during the past 200,000 years of human evolution.
Acknowledgements
This project was conducted under the auspices of the Authority
for Research and Conservation of Cultural heritage, under the
Directorship of Ato Jara Haile. Fieldwork in Ethiopia was greatly
aided by the efforts of Tilahun G/Selassie, Eysias Sebba, Mame
Mala, Dagne Gebre, Getu Assefa, Abebaw Ejigu, Minase Girma,
Awoke Amzaye, as well as Essayas G/Mariam, Achamu Takelle, Ber-
hanu Hailu, Tamrat H/Mariam, Lomenang Lometo, Narka Losirio,
the remarkable Meri, Solomon Yirga, Ben Passey, Chad Fuller,
Adam Jagesh, Lawrence Bender, Mark Mathison, and many others.
Without the good will of the local Mursi and Nyangatom popula-
tions, the work could not have been done. The project received
funding from the National Science Foundation, the Leakey Founda-
tion, the Wenner-Gren Foundation, and the National Geographic
Society.
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J.G. Fleagle et al. / Journal of Human Evolution xxx (2008) 1?66Please cite this article in press as: John G. Fleagle et al., Paleoanthropology
(2008), doi:10.1016/j.jhevol.2008.05.007of the Kibish Formation, southern Ethiopia: Introduction, J Hum Evol