Improved age control on early Homo fossils from the upper Burgi Member at Koobi Fora, Kenya

Abstract

To address questions regarding the evolutionary origin, radiation and dispersal of the genus Homo, it is crucial to be able to place the occurrence of hominin fossils in a high-resolution chronological framework. The period around 2 Ma (millions of years ago) in eastern Africa is of particular interest as it is at this time that a more substantial fossil record of the genus Homo is first found. Here we combine magnetostratigraphy and strontium (Sr) isotope stratigraphy to improve age control on hominin-bearing upper Burgi (UBU) deposits in Areas 105 and 131 on the Karari Ridge in the eastern Turkana Basin (Kenya). We identify the base of the Olduvai subchron (bC2n) plus a short isolated interval of consistently normal polarity that we interpret to be the Pre-Olduvai event. Combined with precession-forced (~20 kyr [thousands of years]) wet-dry climate cycles resolved by Sr isotope ratios, the magnetostratigraphic data allow us to construct an age model for the UBU deposits. We provide detailed age constraints for 15 hominin fossils from Area 131, showing that key specimens such as cranium KNM-ER 1470, partial face KNM-ER 62000 and mandibles KNM-ER 1482, KNM-ER 1801, and KNM-ER 1802 can be constrained between 1.945 ± 0.004 and 2.058 ± 0.034 Ma, and thus older than previously estimated. The new ages are consistent with a temporal overlap of two species of early Homo that can be distinguished by their facial morphology. Further, our results show that in this time interval, hominins occurred throughout the wet-dry climate cycles, supporting the hypothesis that the lacustrine Turkana Basin was a refugium during regionally dry periods. By establishing the observed first appearance datum of a marine-derived stingray in UBU deposits at 2.058 ± 0.034 Ma, we show that at this time the Turkana Basin was hydrographically connected to the Indian Ocean, facilitating dispersal of fauna between these areas. From a biogeographical perspective, we propose that the Indian Ocean coastal strip should be considered as a possible source area for one or more of the multiple Homo species in the Turkana Basin from over 2 Ma onwards.

Associated ilium and femur from Koobi Fora, Kenya, and postcranial diversity in early Homo

“…Subsequent work (Feibel, 1988;Brown and Feibel, 1991) demonstrated that there are multiple erosional surfaces through this interval and that many of the associated sandstone bodies predate the KBS Tuff. This observation is explained by base-level response behavior in the Turkana Basin system, which has been demonstrated to link closely to monsoonal variability at the frequency of orbital precession (20,000 years; McDougall and Brown, 2006;Joordens et al, 2011Joordens et al, , 2013. Close investigation of the strata exposed around the KNM-ER 5881 locality (Fig.…”

“…The circumstances of discovery, with the original specimen found in 1980, and additional material recovered from the same locality in 2009, after a period of nearly 30 years, suggest the latter material was not yet exposed in 1980, and that these fossils were all found very close to the point of burial. The sandstone lies stratigraphically below the KBS Tuff (1.88 Ma; McDougall and Brown, 2006;Joordens et al, 2013) and above the base of the Olduvai Subchron (1.94 Ma; Horng et al, 2002;Joordens et al, 2013), yielding a depositional age for the fossils of approximately 1.9 Ma.…”

From Australopithecus to Homo : the transition that wasn't

“…The best estimate of the age of KNM-ER 1813 is 1.86 + 0.08 Ma [64], and for KNM-ER 1470 it is 2.058 + 0.034 Ma [65]. The KNM-ER 62000 maxilla, which reiterates ER 1470s distinctive facial morphology [66], is constrained in age between 1.95 and 1.98 Ma [65]. The find spot of KNM-ER 2598 occurs 4 m below the KBS Tuff [67]; its age is estimated to be 1.9 Ma [68].…”

Hominin taxic diversity: Fact or fantasy?

“…This date was subsequently adjusted to 2.03-2.09 Ma by Joordens et al (2013). We will thus use c. 2.0 Ma as the conservative FAD for this taxon, and 2.09 Ma as the less conservative estimate.…”