Nonlinear detection of paleoclimate-variability transitions possibly related to human evolution

Abstract

Potential paleoclimatic driving mechanisms acting on human evolution present an open problem of cross-disciplinary scientific interest. The analysis of paleoclimate archives encoding the environmental variability in East Africa during the past 5 Ma has triggered an ongoing debate about possible candidate processes and evolutionary mechanisms. In this work, we apply a nonlinear statistical technique, recurrence network analysis, to three distinct marine records of terrigenous dust flux. Our method enables us to identify three epochs with transitions between qualitatively different types of environmental variability in North and East Africa during the (i) Middle Pliocene (3.35-3.15 Ma B.P.), (ii) Early Pleistocene (2.25-1.6 Ma B.P.), and (iii) Middle Pleistocene (1.1-0.7 Ma B.P.). A deeper examination of these transition periods reveals potential climatic drivers, including (i) large-scale changes in ocean currents due to a spatial shift of the Indonesian throughflow in combination with an intensification of Northern Hemisphere glaciation, (ii) a global reorganization of the atmospheric Walker circulation induced in the tropical Pacific and Indian Ocean, and (iii) shifts in the dominating temporal variability pattern of glacial activity during the Middle Pleistocene, respectively. A reexamination of the available fossil record demonstrates statistically significant coincidences between the detected transition periods and major steps in hominin evolution. This result suggests that the observed shifts between more regular and more erratic environmental variability may have acted as a trigger for rapid change in the development of humankind in Africa.

Fig. 1.

Map displaying the locations of the marine sediment cores analyzed in this study (orange): ODP 659 (East Atlantic) (34), 721/722 (Arabian Sea) (1, 2), and 967 (Eastern Mediterranean Sea) (66). In addition, locations of additional complementary records of Plio-Pleistocene climate evolution (red) in the Atlantic [ODP site 662 (61)] and the Mediterranean [composite sequence of marine sediments at the Sicilian and Calabrian coast (69)] as well as the EARS are shown.

Fig. 2.

(A) Terrestrial dust flux records from the three considered ODP sites (1, 2, 34, 66). (B,C) Results of RN analysis of the three dust flux records including 90% confidence bands (vertical shadings) of a stationarity test (SI Text). Comparing both measures for all records reveals significant and synchronous large-scale regime shifts in dust flux dynamics (horizontal shadings). (D) Time intervals with geological evidence for large lakes in East Africa, comprising collected information from different areas in the EARS (3) and additional results from the Afar basin (65, 70). (E) Major known steps of human evolution in East Africa (simplified from ref. 3). Red bars indicate epochs where the possible emergence and/or extinction of known hominin species coincides with detected climate transitions (SI Text).

Fig. 3.

Comparison of Middle Pliocene (3.5–3.0 Ma B.P.) global ice volume, sea-surface temperature, and dust flux reconstructions: (A) δ¹⁸O benthic stack record LR04 (57); (B) benthic δ¹⁸O record from ODP site 659; (C) alkenone-based SST reconstruction from ODP site 662 (61); (D) terrigenous dust flux at ODP site 659 (34); (E) terrigenous dust flux at ODP site 721/722 (1, 2); (F) alkenone-based SST reconstruction from ODP site 722 (61); (G,H) planktonic δ¹⁸O and SST reconstruction from ensembles of planktonic foraminifera for a composite sequence from Southern Italy (69). The dark gray vertical bar indicates the MIS M2 (preceding an epoch with cold SST in the Mediterranean Sea and reduced dust flux into the Atlantic Ocean); the light gray bar indicates the interval of reduced dust flux into the Arabian Sea.