August, 2014 Scientific American article out!

I subscribed to Scientific American as a kid and eagerly awaited the next issue in my mailbox. I’ve been wanting to write this up for the general public for years and I really enjoyed having this opportunity. Best of all was that there are several new and fascinating studies that have just been published that illuminate the paleoenvironmental contexts of human evolution and diet. It’s proving to be a fascinating cross-disciplinary, international collaboration story.

There is a lot more work to do but it appears that an answer to the age-old question “How did I get here?” is no longer beyond our reach.

April 2012 Symposium at Lamont:

“Did Climate Change Shape Human Evolution?”

Lectures on YouTube

Environmental hypotheses of African faunal evolution propose that major faunal speciation, extinction, and innovation events during the Pliocene-Pleistocene were mediated by changes in African climate or shifts in climate variability. Marine and terrestrial sediment sequences demonstrate that subtropical African climate periodically oscillated between markedly wetter and drier conditions, paced by earth orbital precession cycles recurring roughly every 20 kyr (Lourens et al., 1996). Earth eccentricity cycles modulate the amplitude of precessional forcing of the African monsoonal rains (see upper right panel), and deep lake conditions are observed in several East African basins during some (not all) eccentricity maxima over the 5 Ma (deMenocal, 2011; Trauth et al., 2005; Kingston et al., 2007).

Above: Late Miocene (ca. 8-9 Ma) exposure of Mediterranean sapropel layers (dark) internedded with carbonate marls (light layers; Monte Gibliscemi, Sicily). Each sapropel layer represents a wet phase in North Africa when increased rainfall and Nile river runoff from the East African highlands stratified the surface Mediterranean, inhibited bottom water ventilation, and promoted preservation of organic-rich sediments (Photo courtesy of Frits Hilgen). Note the bundling of 4-5 sapropel layers (100 kyr eccentricity cycles). Sapropel layers occur throughout the late Neogene, indicating that these orbitally-forced wet-dry cycles were a persistent influence on North and East African climate.

Above: Precessional wet-dry cycles recorded by variations in eolian dust percentages preserved in marine sediments off West Africa (Site 659) and East Africa (Site 722/722) (figure after deMenocal et al. 2004). Note that the amplitude modulation of the dust cycles tracks orbital eccentricity (smooth line) indicating persistent wet-dry cycles throughout the 2.0-3.5 Ma interval shown here. Large, deep lake conditions (diatomite sediments) are found at several East African localities during some (not all) high-eccentricity intervals, as indicated indicated by the blue bars here (Trauth et al., 2005; Kingston et al., 2007).

Superimposed on these precessional wet-dry cycles is a long-term shift toward drier and more variable conditions that commenced about 2.5-3.0 Ma, attaining maximum aridity near 1.8-1.6 Ma. Evidence for these shifts to greater aridity comes from both terrestrial and marine sediments. At Ocean Drilling sites off west and East Africa, increased dust fluxes and dust cycle amplitude occur after 2.8, with evidence of shifts in orbital variability near 2.8, 1.8, and 1 Ma (deMenocal, 1995. 2004). Molecule-specific carbon isotopic measurements of terrestrial plant biomarkers preserved in marine sediments off northeast Africa have ben used to reconstruct orbital-scale vegetation changes in short time windows over the past 9 Ma (Feakins et al., 2005). The biomarker data show large-amplitude, orbital-scale vegetation variability, with the greatest C4 expansion occurring after ca. 3 Ma. Orbital-scale oscillations are almost as large as the late Neogene trend, indicate that large and repeated oscillations between more open and more closed landscapes were an important aspect of northeast African vegetation change. These marine records compliment soil carbonate isotopic records near East African fossil localities also documenting the expansion of C4 vegetation after 3 Ma (Cerling, 1992; Cerling and Hay, 1986; Wynn, 2004). 

Analysis of the best dated and most complete African mammal fossil databases reveals marked changes in African fauna during the Pliocene-Pleistocene, suggesting more varied and open habitats roughly near 2.8 Ma and near 1.8 Ma. These same intervals correspond with key junctures in early hominid evolution, including the emergence of the genus Homo, the robust Austrolpithecines, and the appearance of the first stone tools (Mode 1).

Pliocene-Pleistocene shifts in African climate, vegetation, and faunal abundances thus appear to be contemporary within dating uncertainties, although the fossil record is limited by sampling gaps and preservational biases. Further study of possible relations between African faunal and climatic change will benefit from the accelerating pace of important new fossil discoveries, emerging molecular biomarker methods for reconstructing African paleovegetation changes, tephra correlations between terrestrial and marine sequences, as well as continuing collaborations between the paleoclimatic and paleoanthropological communities.

Above: A snapshot of African evolutionary and paleoclimate changes (Figure from deMenocal, 2011). (A) Summary diagram of human evolution spanning the last 4.6 Ma [no phylogenetic relations are indicated; ranges compiled from the recent NRC Report (2011). First appearances and approximate durations of Mode 1 (Oldowan) and Mode 2 (Acheulean) stone tools are indicated (Bobe and Leakey, 2009). (B) Occurrences of Mediterranean sapropel deposits compiled from marine and land sediment sequences (Lourens et al., 1996). (C) Compilation of sedimentary evidence indicating deep lake conditions recorded in several East African paleolake basins (Trauth et al., 2005; Kingston et al., 2007). (D) Carbon isotopic analyses of plant-wax biomarker compounds measured at Site 231 in the Gulf of Aden, currently the most proximal ocean drilling site to hominin fossil localities (Feakins et al., 2005). The shift to higher values after 3 Ma indicates a greater proportions of C4 vegetation, or savannah grasslands. (data compiled in Wynn et al., 2004); Carbon isotopic values of soil carbonate nodules compiled from several studies (19, 20), also indicating grassland expansion after ~ 3 Ma, peaking between 1.8 and 1.6 Ma. (F) Relative abundance of African mammals indicative of seasonally arid grasslands in the lower Omo Valley (Ethiopia) showing an initial increase in grassland-adapted mammals after 2.5 Ma with peak values after 1.8 Ma (Bobe et al., 2004).

natureNEWS  (4/1/2011): The Drying of East Africa

Some papers on this topic

deMenocal, P.B. Climate and Human Evolution. Science, 331, 540-541. (2011).

Committee on the Earth System Context for Hominin Evolution (13 coauthors incl. P deMenocal).  “Understanding Climate's Influence on Human Evolution” National Research Council of the National Academies Report. National Academies Press, 115 pp. (2010).

Feakins, S., Brown, F.H., and deMenocal, P.B. Plio-Pleistocene Microtephra in DSDP Site 231, Gulf of Aden. J. African Earth Sciences, 48, pp. 341-352, 2007.

Feakins, S., Eglinton, T., deMenocal, P.B. A comparison of biomarker records of Northeast African vegetation from lacustrine and marine sediments ca. 3.4 Ma. Organic Geochemistry 38 (2007) 1607–1624.

Feakins, S.J., deMenocal, P.B., Eglinton, T.I., 2005. Biomarker records of Late Neogene changes in northeast African vegetation. Geology, v. 33; no. 12; p. 977–980; doi: 10.1130/G21814.1. 2005.

deMenocal, P.B. African climate change and faunal evolution during the Pliocene-Pleistocene. Earth and Planetary Science Letters (Frontiers). 220, 1/2, 3-24. 2004.

Zabel, M., Wagner, T., deMenocal, P. Terrigenous signals in sediments from Terrigenous Signals in Sediments of the Low-Latitude Atlantic – Indications to Environmental Variations during the Late Quaternary,  Part II: Lithogenic Matter. In "The South Atlantic in the Late Quaternary: Reconstruction of Mass Budget and Current Systems",Wefer, G., Mulitza, S. & Ratmeyer, V. (eds), Springer, Berlin, Heidelberg, New York. 2003.

deMenocal, P.B. and Brown, F.H. Pliocene tephra correlations between East African hominid localities, the Gulf of Aden, and the Arabian Sea. In Climatic and Environmental Change in the Neogene of Europe, Cambridge University Press, p. 23-52. 1999.

Partridge, T., P.B. deMenocal, S. Lorentz, M. Paiker, and J. Vogel, Orbital forcing of climate over South Africa: A 200,000-year rainfall record from the Pretoria Saltpan, Quat. Sci. Rev., 16 (10), 1125-1133. 1998.

deMenocal, P.B. and Rind, D. Sensitivity of subtropical African and Asian climate to prescribed boundary condition changes: Model implications for the Plio-Pleistocene evolution of low-latitude climate. In Johnson, T. and Odada, E., The Limnology, Climatology, and Paleoclimatology of West African Lakes: New York (Gordon and Breach), 57-77. 1996.

Partridge, T. C., Wood, B., and deMenocal, P. B., The influence of global climatic change and regional uplift on large mammalian evolution on East and southern Africa, in Vrba, E., Denton, G., Partridge, T. C., and Burckle, L., eds., Paleoclimate and Evolution With Emphasis of Human Evolution: New Haven, Yale Univ. Press, 330-355. 1995.

deMenocal, P. B., and Bloemendal, J., Plio-Pleistocene subtropical African climate variability and the paleoenvironment of hominid evolution: A combined data-model approach, in Vrba, E., Denton, G., Burckle, L., and Partridge, T., eds., Paleoclimate and Evolution With Emphasis on Human Origins: New Haven, Yale University Press, p. 262-288. 1995.

Partridge, T. C., Bond, G. C., Hartnandy, C. J. H., deMenocal, P. B., and Ruddiman, W. F., Climatic effects of late Neogene tectonism and volcanism, in Vrba, E., Denton, G., Burckle, L., and Partridge, T., eds., Paleoclimate and Evolution With Emphasis on Human Origins: New Haven, Yale Univ. Press, p. 8-23. 1995.

deMenocal, P.B. Plio-Pleistocene African climate. Science, 270, 53-59. 1995.

deMenocal, P., Ruddiman, W., and Pokras, E., Influences of high- and low-latitude processes on African terrestrial climate:  Pleistocene eolian records from equatorial Atlantic Ocean Drilling Program site 663. Paleoceanography, 8 (2); 209-242. 1993.

deMenocal, P., and Rind, D.  Sensitivity of Asian and African climate to variations in seasonal insolation, glacial ice cover, sea-surface termperature, and Asian orography. J. Geophys. Res., 98 (D4); 7265-7287. 1993.

Bloemendal, J. and deMenocal, P.  Evidence for a change in the periodicity of tropical climate cycles at 2.4 Myr from whole-core magnetic susceptibility measurements.  Nature, 342:897-899. 1989.