(40)Ar/(39)Ar dating of tuffs and lavas of the late Pleistocene volcanic and sedimentary sequence of Olduvai Gorge, north-central Tanzania, provides the basis for a revision of Bed I ...chronostratigraphy. Bed I extends from immediately above the Naabi Ignimbrite at 2.038 ± 0.005 Ma to Tuff IF at 1.803 ± 0.002 Ma. Tuff IB, a prominent widespread marker tuff in the basin and a key to understanding hominin evolutionary chronologies and paleoclimate histories, has an age of 1.848 ± 0.003 Ma. The largest lake expansion event in the closed Olduvai lake basin during Bed I times encompassed the episode of eruption and emplacement of this tuff. This lake event is nearly coincident with the maximum precessional insolation peak of the entire Bed I/Lower Bed II interval, calculated from an astronomical model of the boreal summer orbital insolation time-series. The succeeding precessional peak also apparently coincides with the next youngest expansion of paleo-Lake Olduvai. The extreme wet/dry climate shifts seen in the upper part of Bed I occur during an Earth-orbital eccentricity maximum, similar to episodic lake expansions documented elsewhere in the East African Rift during the Neogene.
Development of the African Middle Stone Age (MSA) before 300 thousand years ago (ka) raises the question of how environmental change influenced the evolution of behaviors characteristic of early
We ...use temporally well-constrained sedimentological and paleoenvironmental data to investigate environmental dynamics before and after the appearance of the early MSA in the Olorgesailie Basin, Kenya. In contrast to the Acheulean archeological record in the same basin, MSA sites are associated with a dramatically different faunal community, more pronounced erosion-deposition cycles, tectonic activity, and enhanced wet-dry variability. As early as 615 ka, aspects of Acheulean technology in this region imply that greater stone material selectivity and wider resource procurement coincided with an increased pace of land-lake fluctuation, potentially anticipating the adaptability of MSA hominins.
Previous research suggests that the complex symbolic, technological, and socio-economic behaviors that typify
had roots in the middle Pleistocene <200 ka, but data bearing on human behavioral origins ...are limited. We present a series of excavated Middle Stone Age sites from the Olorgesailie Basin, southern Kenya, dated ≥295 to ~320 ka by
Ar/
Ar and U-Series methods. Hominins at these sites made prepared cores and points, exploited iron-rich rocks to obtain red pigment, and procured stone tool materials from ≥25-50 km distance. Associated fauna suggests a broad resource strategy that included large and small prey. These practices imply significant changes in how individuals and groups related to the landscape and one another, and provide documentation relevant to human social and cognitive evolution.
Mass extinctions manifest in Earth's geologic record were turning points in biotic evolution. We present 40 Ar/ 39 Ar data that establish synchrony between the Cretaceous-Paleogene boundary and ...associated mass extinctions with the Chicxulub bolide impact to within 32,000 years. Perturbation of the atmospheric carbon cycle at the boundary likely lasted less than 5000 years, exhibiting a recovery time scale two to three orders of magnitude shorter than that of the major ocean basins. Low-diversity mammalian fauna in the western Williston Basin persisted for as little as 20,000 years after the impact. The Chicxulub impact likely triggered a state shift of ecosystems already under near-critical stress.
The manufacture of flaked stone artifacts represents a major milestone in the technology of the human lineage. Although the earliest production of primitive stone tools, predating the genus Homo and ...emphasizing percussive activities, has been reported at 3.3 million years ago (Ma) from Lomekwi, Kenya, the systematic production of sharp-edged stone tools is unknown before the 2.58–2.55 Ma Oldowan assemblages from Gona, Ethiopia. The organized production of Oldowan stone artifacts is part of a suite of characteristics that is often associated with the adaptive grade shift linked to the genus Homo. Recent discoveries from Ledi-Geraru (LG), Ethiopia, place the first occurrence of Homo ∼250 thousand years earlier than the Oldowan at Gona. Here, we describe a substantial assemblage of systematically flaked stone tools excavated in situ from a stratigraphically constrained context Bokol Dora 1, (BD 1) hereafter at LG bracketed between 2.61 and 2.58 Ma. Although perhaps more primitive in some respects, quantitative analysis suggests the BD 1 assemblage fits more closely with the variability previously described for the Oldowan than with the earlier Lomekwian or with stone tools produced by modern nonhuman primates. These differences suggest that hominin technology is distinctly different from generalized tool use that may be a shared feature of much of the primate lineage. The BD 1 assemblage, near the origin of our genus, provides a link between behavioral adaptations—in the form of flaked stone artifacts—and the biological evolution of our ancestors.
The origin of the Middle Stone Age (MSA) denotes the transition from a highly persistent mode of stone toolmaking, the Acheulean, to a period of increasing technological innovation and cultural ...indicators associated with the evolution of
Here we use
Ar/
Ar and U-series dating to calibrate the chronology of Acheulean- and early MSA-rich sedimentary deposits in the Olorgesailie Basin, South Kenya Rift. We establish the age of late Acheulean tool assemblages from 615 to 499 ka, after which a large technological and faunal transition occurred, with definitive MSA lacking Acheulean elements beginning most likely by ~320 ka, but at least by 305 ka. These results establish the currently oldest repository of MSA in eastern Africa.
Evidence for Quaternary climate change in East Africa has been derived from outcrops on land and lake cores and from marine dust, leaf wax, and pollen records. These data have previously been used to ...evaluate the impact of climate change on hominin evolution, but correlations have proved to be difficult, given poor data continuity and the great distances between marine cores and terrestrial basins where fossil evidence is located. Here, we present continental coring evidence for progressive aridification since about 575 thousand years before present (ka), based on Lake Magadi (Kenya) sediments. This long-term drying trend was interrupted by many wet–dry cycles, with the greatest variability developing during times of high eccentricity-modulated precession. Intense aridification apparent in the Magadi record took place between 525 and 400 ka, with relatively persistent arid conditions after 350 ka and through to the present. Arid conditions in the Magadi Basin coincide with the Mid-Brunhes Event and overlap with mammalian extinctions in the South Kenya Rift between 500 and 400 ka. The 525 to 400 ka arid phase developed in the South Kenya Rift between the period when the last Acheulean tools are reported (at about 500 ka) and before the appearance of Middle Stone Age artifacts (by about 320 ka). Our data suggest that increasing Middle- to Late-Pleistocene aridification and environmental variability may have been drivers in the physical and cultural evolution of Homo sapiens in East Africa.
The transport of moisture in the tropics is a critical process for the global energy budget and on geologic timescales, has markedly influenced continental landscapes, migratory pathways, and ...biological evolution. Here we present a continuous, first-of-its-kind 1.3-My record of continental hydroclimate and lake-level variability derived from drill core data from Lake Malawi, East Africa (9–15° S). Over the Quaternary, we observe dramatic shifts in effective moisture, resulting in large-scale changes in one of the world’s largest lakes and most diverse freshwater ecosystems. Results show evidence for 24 lake level drops of more than 200 m during the Late Quaternary, including 15 lowstands when water levels were more than 400 m lower than modern. A dramatic shift is observed at the Mid-Pleistocene Transition (MPT), consistent with far-field climate forcing, which separates vastly different hydroclimate regimes before and after ∼800,000 years ago. Before 800 ka, lake levels were lower, indicating a climate drier than today, and water levels changed frequently. Following the MPT high-amplitude lake level variations dominate the record. From 800 to 100 ka, a deep, often overfilled lake occupied the basin, indicating a wetter climate, but these highstands were interrupted by prolonged intervals of extreme drought. Periods of high lake level are observed during times of high eccentricity. The extreme hydroclimate variability exerted a profound influence on the Lake Malawi endemic cichlid fish species flock; the geographically extensive habitat reconfiguration provided novel ecological opportunities, enabling new populations to differentiate rapidly to distinct species.
The accuracy of 40Ar/39Ar geochronology relies in large part on precise and accurate calibration of the ages and K-Ar isotopic compositions of standards. A widely used standard for Quaternary ...samples, the ∼1.2 Ma Alder Creek sanidine (ACs), has published ages spanning a range of ∼2%. New measurements of ACs co-irradiated with the Fish Canyon sanidine (FCs) standard and sanidines from astronomically dated Miocene tuffs in Crete and Morocco yield results that enable both (i) a direct calibration of ACs relative to FCs, and (ii) stepwise calibrations between these two standards employing the Miocene intermediaries. Results are summarized by the parameter RFCsACs, defined as the ratio of (40Ar*/39ArK) of ACs to FCs, which embodies the fundamental age relationship between these standards that is independent of systematic variables such as decay constants or absolute ages of standards. Our new measurements, executed using three mass spectrometers and various irradiation and analytical protocols, yield a weighted mean RFCsACs = 0.041702 ± 0.000014 (σ). This result can be combined with previously published determinations of R values for ACs relative to the Miocene tuffs and to FCs to yield a recommended interlaboratory value ofRFCsACs = 0.041707 ± 0.000011. The weighted-mean age of ACs using this interlaboratory value, based on astronomically-calibrated ages of FCs and the Miocene intermediary sanidines, is tACs = 1.1848 ± 0.0006 Ma (±0.05%). Applying this result to the most precise published 40Ar/39Ar data for the Matuyama-Brunhes geomagnetic polarity reversal yields tMBB = 780.1 ± 0.8 ka. In addition, these new data for ACs support previous conclusions that U-Pb zircon ages from the Alder Creek rhyolite incorporate ∼13 ka of pre-eruptive residence time.
•Alder Creek sanidine (ACs) is calibrated to astronomically dated Miocene tuffs.•These provide stepwise and direct intercalibration with Fish Canyon sanidine (FCs).•New data yield a weighted mean intercalibration factor RFCsACs=0.041702±0.000014(σ).•Combining with published data yields an interlaboratory RFCsACs=0.041707±0.000011.•These results yield a weighted mean age of tACs = 1.1848 ± 0.0006 Ma (±0.05%).
Only one partial skeleton that includes both forelimb and hindlimb elements has been reported for Australopithecus afarensis. The diminutive size of this specimen (A.L. 288-1 "Lucy") has hampered our ...understanding of the paleobiology of this species absent the potential impact of allometry. Here we describe a large-bodied (i.e., well within the range of living Homo) specimen that, at 3.58 Ma, also substantially antedates A.L. 288-1. It provides fundamental evidence of limb proportions, thoracic form, and locomotor heritage in Australopithecus afarensis. Together, these characteristics further establish that bipedality in Australopithecus was highly evolved and that thoracic form differed substantially from that of either extant African ape.
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