Koro-Toro is the name of about 40 fossil localities in the Chad Basin of the Djurab Desert in northern Chad. Two of those localities, Koro-Toro Locality 12 (KT12) and Locality 13 (KT13), have yielded fossil remains of a species of the ancient hominid Australopithecus called A. bahrelghazali (nicknamed Abel), the only species of australopithecus identified outside of the Rift Valley of Africa. Koro Toro is 150 kilometers (~90 miles) east of the Upper Miocene site of Toros-Menalla, where the seven million year-old fossil remains of Sahelanthropus tchadensis (nicknamed Toumaï) were discovered.
The known fossil remains of A. bahrelghazali are fairly minimal. The largest piece recovered to date is from KT12, and it is the anterior portion of a lower jawbone (mandible) with an associated premolar (P3). Two other individuals are only represented by teeth, recovered from locality KT13. Project director Michel Brunet originally identified this jaw as coming from an A. afarensis, and the jaw does have some morphological similarities to the type fossil called Lucy.
But K12's jaw is significantly distinctive, particularly in terms of its mandibular symphysis. The symphysis is located at the front point of the lower jaw, and in some animals (but not adult modern humans), it allows two halves of the mandible to rotate, and A. bahrelghazali's is more vertical and bulbous than that of A. afarensis. Modern human symphyses fuse after the first year of life.
Dates at Koro-Toro were estimated between 3-3.5 million years ago, based on the associated faunal remains. A more specific date was recently determined to be 3.58 million years old, contemporaneous with A. afarensis. This date was derived from a new method developed at the site, using cosmogenic beryllium isotope dating (10Be/9Be). The beryllium 10 isotope dating is analogous to other isotope dating techniques--beryllium has a half life of ~1.4 million years, and can be used successfully in beryllium-rich dating locations between 0.2-14 million years ago.
Environment of Koro Toro
Koro Toro is a flat, desertic region, with the the fossil localities varying in size from a few hundred square meters to more than 100,000 sq m. The geological formation that holds the fossil deposits in Koro-Toro consists of siliciclastic materials, dominated by weakly lithified sandstone, argilaceous mudstone and diatomite.
Environmental analysis of the animals recovered from the deposits indicate that when A. bahrelghazali was living at Koro-Toro, the region was rather open and dry, bordering a lake. The closest parallel to the environment today is the Okavango Delta of the central Kalahari basin in Botswana, where a mosaic of lake and shoreline swamps are interleaved with a semi-arid savanna and arid desert. Such environments provide a range of resources, and it is typical for human species of all sorts to find such complex places good to live in.
Cows, Fishes and Crocodiles
A fairly wide range of animal species have been identified at the site, including primates, bovids, carnivores, hippopotami, waterfowl, turtles, ducks, crocodiles and fish.
Nine separate species of bovid have been identified at KT12 and 13, including three new species: Kobus korotorensis (likely ancestral to east African species) and Kobus tchadensis and Parmularius pachyceras (likely ancestral to north African species).
Eleven fish taxa were identified at the Koro-Toro localities, including abafish, tigerfish, perciform fishes and puffer fishes.
In 2012, a study (Lee-Thorp and colleagues) involving radioisotope and microwear studies of the jaw and teeth of A. bahrelghazali revealed that its dietary input was predominantly C4 plants: that is to say, plants that convert carbon into a compound with four carbon atoms. C4 plants are primarily tropical sedges and grasses, requiring long growing seasons and lots of sunshine, such as maize, sorghum, sugarcane, millet, fonio, tef and papyrus.
The identification of a diet predominantly made up of plants is surprising for a number of reasons, firstmost because previous research suggested that the earliest human ancestor who was primarily a plant eater was P. boisei, from Olduvai and Lake Turkana and nearly a million years later.
And, although the current environment of Lake Chad includes multiple C4 plants, including several Cyperus species, including papyrus, C4 plants are challenging for hominins to digest, because they lack the necessary dental and digestive adaptations. It is possible that A. bahrelghazali focused attention on leaves and shoots of these plants, or that they consumed animals other animals that consumed C4 plants.
Archaeology at Koro-Toro
Localities KT12 and KT13 are two of over 300 fossil sites discovered by the on-going Mission Paléontologique Franco-Tchadienne, under the direction of Michel Brunet since 1994.
Brunet M, Guy F, Pilbeam D, Lieberman DE, Likius A, Mackaye HT, Ponce de León MS, Zollikofer CPE, and Vignaud P. 2005. New material of the earliest hominid from the Upper Miocene of Chad. Nature 434:752-755.
Brunet M. 2010. Short note: The track of a new cradle of mankind in Sahelo-Saharan Africa (Chad, Libya, Egypt, Cameroon). Journal of African Earth Sciences 58(4):680-683.
Geraads D, Brunet M, Mackaye HT, and Vignaud P. 2001. Pliocene Bovidae (Mammalia) from the Koro Toro Australopithecine sites, Chad. Journal of Vertebrate Paleontology 21(2):335-346.
Guy F, Mackaye H-T, Likius A, Vignaud P, Schmittbuhl M, and Brunet M. 2008. Symphyseal shape variation in extant and fossil hominoids, and the symphysis of Australopithecus bahrelghazali. Journal of Human Evolution 55(1):37-47.
Lebatard A-E, Bourlès DL, Duringer P, Jolivet M, Braucher R, Carcaillet J, Schuster M, Arnaud N, Monié P, Lihoreau F et al. 2008. Cosmogenic nuclide dating of Sahelanthropus tchadensis and Australopithecus bahrelghazali: Mio-Pliocene hominids from Chad. Proceedings of the National Academy of Sciences 105(9):3226-3231.
Lee-Thorp JA, Likius A, Mackaye HT, Vignaud P, Sponheimer M, and Brunet M. 2012. Isotopic evidence for an early shift to C4 resources by Pliocene hominins in Chad. Proceedings of the National Academy of Sciences Early Edition.
Otero O, Pinton A, Mackaye HT, Likius A, Vignaud P, and Brunet M. 2010. The early/late Pliocene ichthyofauna from Koro-Toro, Eastern Djurab, Chad. Geobios 43(2):241-251.