Professor of Geosciences, Aix-Marseille University; Director, French Accelerator Mass Spectrometry Facility "ASTER", CEREGE - Centre Européen de Recherche et d Enseignement des Géosciences de l Environnement, Europôle méditerranéen de l Arbois, Aix en Provence
|Since the advent of the Accelerator Mass Spectrometry (AMS) technique, the atmospherically produced cosmogenic nuclide 10Be has proved to be a powerful tool for investigating various fields of earth sciences such as atmospheric, continental, oceanic, and subduction processes. However, considering the precision and the detection limits of the developed analytical methods, one of its potentially most exciting applications, linked to its radioactive decay with a half-life of 1.36 ± 0.07 Ma, remains the possibility of dating marine and continental sediments over the time range 0.2 to 14 Ma, beyond the range (0.2-5 Ma) of the 26Al/10Be burial dating method.|
Mainly produced in the atmosphere through nuclear reactions (spallation reactions) on oxygen (O) and nitrogen (N), the particle-reactive 10Be is rapidly transferred to the Earth's surface in soluble form by precipitation where it is either ultimately removed from water on settling particles and deposited in marine and lacustrine sediments, or efficiently retained onto continental sediment components where it decays.
However, because the absolute 10Be concentration depends on parameters such as the scavenging efficiency and the specific surface area of the sedimentary particles, it cannot be used directly to determine an age. It is first necessary to normalize 10Be concentrations in order to account for the variable environmental conditions affecting temporarily the chemical and grain size composition of the sediments. Although 10Be and stable 9Be have different sources - 10Be from cosmogenic production in the atmosphere and 9Be from detrital input, of which a small fraction is dissolved - studies up to now exclusively performed on marine sediments have shown that the dissolved beryllium isotopes are homogenized before deposition and demonstrated that their ratio is a useful chronologic tool. Sequential leaching procedures extracting only that portion of the beryllium that originated from solution, i.e, the authigenic Be that is adsorbed on particles and/or precipitated directly from solution, have thus been developed for dating purposes.
Providing: 1/ that the initial concentration relative to the normalizing element (the initial ratio) can be accurately estimated; 2/ that the selected samples have remained "closed" to entry or loss of the cosmogenic isotope and its normalizing element, the classical equation N(t) = N0e-»t (where N(t) is the measured ratio; N0 the initial ratio; » the radioactive decay constant and t the age of the sediments) can be applied to the measured authigenic 10Be/9Be ratios to calculate the ages of the studied sediments.
Applied to the silicated continental sediments deposited in favorable environmental conditions since the upper Miocene in the north Chad basin, the authigenic 10Be/9Be dating technique allowed constraining between 6.8 and 7.2 million years the deposition of the Anthracotheriid Unit from which a nearly complete cranium,holotype of Sahelanthropus tchadensis (Toumaï),was unearthed.
Professor, Chair of Human Paleontology, Collège de France, Paris; Member, International Institute of Palaeoprimatology and Human Palaeontology, University of Poitiers
|The idea of an ascendance for our species is quite recent (about 150 years ago).|
But which was our ancestral group, when and where did it arise? ...If these questions are more constraints they are still always unsolved.
In the 80 s, early hominids are known in South and East Africa, the oldest being in East Africa led to propose an East Side Story , the bipedal hominid original savanna hypothesis (Coppens, 1983).
From 1994 the M.P.F.T.2 digging in Djurab desert (Northern Chad) unearthed successively a new australopithecine nicknamed Abel (dated to 3.5 Ma), the first ever found West of the Rift Valley (Brunet et al., 1995) and later a new hominid, the earliest yet found (nicknamed Toumaï) Sahelanthropus tchadensis Brunet et al., 2002 from the late Miocene, dated to 7 Ma (Vignaud et al., 2002; Lebatard et al.,2008). This new milestone suggests that an exclusively southern or eastern African origin of the hominid clade is unlikely to be correct.
Since 1994, our roots went deeper, from 3.6 Ma to 7 Ma today, with three new Late Miocene species: Ardipithecus kadabba Haile-Selassie, 2001 (5.2-5.8 Ma, Middle Awash, Ethiopia) and Orrorin tugenensis Senut et al., 2001 (ca. 6 Ma, Lukeino, Kenya) while the oldest ( 7 Ma) is the Chadian one. These discoveries have a scientific impact similar to that of A. africanus Dart, 1925.
S. tchadensis displays a unique combination of primitive and derived characters that clearly shows that it is not related to chimpanzees or gorillas, but clearly suggests that it is related to later hominids, and temporally close to the last common ancestor between chimpanzees and humans (Brunet et al., 2002 & 2005;Zollikofer et al., 2005).
In Chad, the Late Miocene sedimentological and paleobiological data are in agreement with a mosaic landscape (Vignaud et al., 2002). Today in Central Kalahari (Bostwana) the Okavango delta appears to be a good analog with a similar mosaic of lacustrine and riparian waters, swamps, patches of forest, wooded islets, wooded savannah, grassland and desertic area (Brunet et al.,2005). Among this mosaic the studies of the precise habitat of Toumaï are still in progress but probably, as for the others known late Miocene Hominids, a wooded one. Moreover these three late Miocene hominids are probably usual bipeds. So the models that involve significant role for savanna in the hominid origin must be reconsidered.
Now, it appears that the earliest hominids inhabited wooded environments and were not restricted to Southern or Eastern Africa but were rather living in a wider geographic region, including also Sahelian Africa: at least Central Africa (Chad, Niger, Sudan) and may be also North Eastern Africa (Libya and Egypt) (Brunet, 2008).
According to, the early hominid history is going to be reconsidered within completely new paradigms.
Moreover these two new early Hominids from Mio-Pliocene of Chad, are two new milestones which enlighten Charles Darwin prediction in his master book the Descent of Man (1871).
(2) The Mission Paléoanthropologique Franco-Tchadienne heads by Michel Brunet, is an international scientific transdisciplinary collaboration between Collège de France (Paris), University of Poitiers (France), University of N Djamena and CNAR (N Djamena, Chad) including more than sixty researchers from ten countries. This transdisciplinary research program is granted by French Ministry of Research : CNRS (EDD & ECLIPSE) & ANR, French Foreign Ministry (DGCID Paris, SCAC N Djamena) and USA NSF (RHOI).
Selected bibliographic references:
Brunet, M. & al. (1995) The first australopithecine 2 500 kilometres west of the Rift Valley (Chad). Nature 378: 273-274.
Brunet, M. & al. (1996) - Australopithecus bahrelghazali une nouvelle espèce d'Hominidé ancien de la région de Koro Toro (Tchad). C.R. Acad. Sc. Paris, 322, Iia : 907-913.
Brunet, M.& al. (2002) A new hominid from the Upper Miocene of Chad, Central Africa. Nature 418:145-151.
Brunet M. & al. (2005). New material of the Earliest Hominid from the Upper Miocene of Chad. Nature 434: 753-755.
Brunet M . (2006). D Abel à Toumaï, Nomade Chercheur d Os, Editions Odile Jacob.
Brunet M; (2008) Origine et Histoire des Hominidés& Nouveaux paradigmes.Leçon inaugurale du Collège de France, Fayard Editeur.
Coppens, Y. (1983) Le singe, l Afrique et l Homme. Jacob/Fayard Paris.
Dart, R. (1925) Australopithecus africanus, the man-ape of South Africa. Nature 115: 195-199.
Darwin Ch. (1871). The Descent of Man and Selection in Relation to Sex. Reprinted, 1981, Princeton University Press.
Lebatard A.E. & al (2008) Cosmogenic nuclide dating of Sahelanthropus tchadensis and Australopithecus bahrelghazali: Mio-Pliocene hominids from Chad. PNAS 105 (9): 3226-3231.
Vignaud, P. & al. (2002) Geology and palaeontology of the Upper Miocene Toros-Menalla hominid locality, Chad, Nature 418: 152-155
Zollikofer C.P.E. & al. (2005). Virtual Cranial Reconstruction of Sahelanthropus tchadensis. Nature 434: 755-759.