Woody Cover and Hominin Environments in the Past 6 Million Years
Total Page：16
File Type：pdf, Size：1020Kb
ARTICLE doi:10.1038/nature10306 Woody cover and hominin environments in the past 6 million years Thure E. Cerling1, Jonathan G. Wynn2, Samuel A. Andanje3, Michael I. Bird4, David Kimutai Korir3, Naomi E. Levin5, William Mace1, Anthony N. Macharia1, Jay Quade6 & Christopher H. Remien1 The role of African savannahs in the evolution of early hominins has been debated for nearly a century. Resolution of this issue has been hindered by difficulty in quantifying the fraction of woody cover in the fossil record. Here we show that the fraction of woody cover in tropical ecosystems can be quantified using stable carbon isotopes in soils. Furthermore, we use fossil soils from hominin sites in the Awash and Omo-Turkana basins in eastern Africa to reconstruct the fraction of woody cover since the Late Miocene epoch (about 7 million years ago). 13C/12C ratio data from 1,300 palaeosols at or adjacent to hominin sites dating to at least 6 million years ago show that woody cover was predominantly less than 40% at most sites. These data point to the prevalence of open environments at the majority of hominin fossil sites in eastern Africa over the past 6 million years. 13 There is long-standing debate as to the importance of woody versus their discrimination against CO2 during photosynthesis, the stable herbaceous cover in the evolution of humans over the past 6 million carbon isotopic composition of soils can be used as a direct indicator years (Myr)1–5. Despite uncertainty as to the nature of the last common of the fraction of woody cover in tropical ecosystems17–19. ancestor (LCA) that we share with modern chimpanzees6, it is widely We present a method using stable carbon isotopes to quantify the recognized that the LCA inhabited wooded environments, and that fraction of woody cover in tropical ecosystems using new data from hominin habitats became less wooded after this divergence some eastern Africa that builds on earlier observations: the fraction of C4 5–8 Myr ago2,3,5,6. Woody plants provide shade, shelter and food biomass is related to the fraction of woody cover17,18,20. We then apply resources, and as such could have an important role in the evolution this relationship to well-dated fossil sites in the Awash and Omo- ofterrestrialmammals, includinghumans.For example, shade provided Turkana basins in Ethiopia and Kenya, which contain sedimentary by this cover may have influenced thermoregulatory and endurance archives that are critical to understanding hominin evolution in the running adaptations, or nesting and hunting behaviours of early Pliocene and Pleistocene epochs. The in-depth analysis of the rela- hominins7–11. tionship between stable carbon isotopes and woody cover in modern In broader ecosystem-scale terms, the role of ‘savannah’ ecosystems soils permits the reconstruction of the woody cover, or shade, avail- in hominin evolution remains a subject of debate1–5, although it is able to hominins in eastern Africa during the past 6 Myr. widely recognized that savannahs may have influenced a variety of hominin adaptations such as bipedalism and dietary adaptations to Calibrating a ‘palaeo-shade’ proxy novel foods2,3,6–13. Consideration of the role of savannahs in human We report results of woody cover measurements and the 13C/12C ratio evolution began in 192514 with the introduction of what is often (d13C) values of surface horizons from tropical soils for 76 locations in described as the ‘savannah hypothesis’ and continues today in efforts Kenya, Ethiopia, Malaysia, Botswana20, Zambia20, Australia17 and to reconcile the fossil record of human origins with diverse palaeo- Brazil21 (Supplementary Tables 1 and 2). Sites were selected because environmental proxies1–3,5,15. However, an imprecise and often overly of their undisturbed nature (that is, from National Parks or Reserves) simplistic application of the definition of savannahs hinders progress or because of minimal agricultural disturbance. ‘Gap’ samples are from in the debate over the timing and nature of their role in human soils that are not directly under woody cover and ‘canopy’ samples are evolution. To move past this persistent problem, we develop a rela- from soils directly beneath woody cover. The d13C values of the gap tionship between the modern carbon isotope ratio in soils and the and the canopy samples are well correlated for the entire data set amount of woody cover in tropical environments and show that (Fig. 1; see also Supplementary Information). In closed settings with this can be used as a calibration for estimating woody cover of past woody cover of approximately .80%, both the canopy and gap areas 13 17 environments. By using this relationship we can focus on the degree to show d C values characteristic of predominantly C3 vegetation . which habitats were wooded, thereby circumventing any need to Likewise, open settings, with woody cover approximately ,20%, show 13 apply a functional definition of savannah to past environments where d C values characteristic of predominantly C4 vegetation, both in the only structure can be inferred. canopy and gap samples. Thus, d13C values in these soils reflect the Stable carbon isotopes in palaeosols are a key means of reconstruct- amount of woody cover on the timescales of carbon turnover (,10 yr 22 13 ing ancient environments, particularly those in the tropics in the past in the tropics ), and C4-rich ecosystems have a different soil d C 6 Myr or longer. Woody plants, almost all of which use the C3 pho- signature than C3-rich ecosystems whether under a tree canopy or tosynthetic pathway, would have provided mammals with shade and not. We invert the relationship in Fig. 2 to reconstruct the fraction of 16 13 shelter from the direct sun . Tropical grasses, on the other hand, use woody cover (fwc)fromd C values of organic matter and soil carbon- 16 the C4 photosynthetic pathway . Because these two pathways differ in ate in fossil soils (see methods in Supplementary Information): 1University of Utah, Salt Lake City, Utah 84112, USA. 2University of South Florida, Tampa, Florida 33620, USA. 3Kenya Wildlife Service, PO Box 40241-00100 Nairobi, Kenya. 4James Cook University, PO Box 6811, Cairns QLD 4870, Queensland, Australia. 5Johns Hopkins University, Baltimore, Maryland 21218, USA. 6University of Arizona, Tucson, Arizona 85721, USA. 4 AUGUST 2011 | VOL 476 | NATURE | 51 ©2011 Macmillan Publishers Limited. All rights reserved RESEARCH ARTICLE –10 20 Meru grassland –15 Number of days 0 20 30 40 50 60 –20 C canopy 13 δ Meru 25 woodland –25 1:1 line Number of days –30 0 –30 –25 –20 –15 –10 20 30 40 50 60 δ13C gap Figure 1 | Correlation of d13C between gap and canopy samples for 76 Meru 50 forest tropical soils used in this study. Best-fit line is using the major axis regression 13 13 2 where d Ccanopy 5 0.793d Cgap 26.4; r 5 0.89. ÈÉÂÃÀÁ 2 f ~ sin {1:06688{0:08538 d13C Number of days wc om 0 This relationship is not a simple linear mixing line between C3 plants 20 30 40 50 60 (ca. -24 to -35%)andC4 plants (approximately 211% to 214%; Daily high surface ground temperature (°C) ref. 16) because C3 herbaceous plants may occur in both open and Figure 3 | Surface soil temperatures from soil temperature profiles. closed areas, and canopy areas may contain both C3 trees and both Calculated maximum daily soil-surface temperatures for a 12-month interval mixed C3–C4 understory. for forest, woodland and grassland sites in the Meru National Park region, Shade provided by woody cover also affects soil temperatures and Kenya (see Supplementary Tables 1 and 2). the proportion of C3 and C4 biomass in the herbaceous understory. Figure 3 shows maximum daily soil surface temperature for three sites oxygenase activity relative to carboxylase activity. Humidity and soil in Meru National Park as calculated from sub-surface soil-temperature moisture are also higher in shaded areas. Thus in the areas with higher loggers for the period June 2009 through to May 2010. Soil ground shade cover due to a woody canopy, C3 photosynthesis may be surface temperatures varied between 25 uCand35uC in riparian forest, favoured even in the gap areas. Conversely, C4 photosynthesis in open between 28 uC and 48 uC in woodland, and between 30 uCand60uCin areas is favoured by lower daily-integrated shade where there is higher grassland. Over a 1-year period, 130 days had daily maximum ground temperature, higher light intensity, with lower relative humidity and surface temperatures exceeding 45 uC in the grassland (unshaded) soil moisture. Taken together, the abundance of C3 biomass in gaps in environment. C4 photosynthesis is an adaption to both high leaf C3-dominated ecosystems, and the abundance of C4 biomass under the temperature and low atmospheric CO2 (ref. 16); C4 photosynthesis canopy in C4-dominated portions of the landscapes, is related to the at higher leaf temperatures is favoured over C3 photosynthesis light intensity and the surface ground temperature, and their effects on because of increasing photorespiration in C3 plants related to increased photorespiration, humidity and soil moisture. –10 A structural classification for palaeo-vegetation It is useful to define formal terminology of vegetation structural categories like ‘forest’, ‘woodland’, ‘grassland’ before proceeding with our reconstruction of ancient vegetation. Because of the strong rela- –15 13 tionship between d Candfwc (Fig. 2), we adopt a vegetation classifica- tion system that is based primarily on woody cover (the United Nations Educational, Scientific and Cultural Organization (UNESCO) clas- –20 sification of African vegetation23). The principal vegetation types are: C (SOM) 13 (1) forest: a continuous stand of trees at least 10-m tall with interlock- δ ing crowns. (2) Woodland/bushland/thicket/shrubland: where wood- –25 land is an open stand of trees at least 8-m tall with woody cover .40% and a field layer dominated by grasses; bushland is an open stand of bushes usually between 3- and 8-m tall with woody cover .40%; –30 thicket is a closed stand of bushes and climbers usually between 3- 0.00 0.50 1.00 and 8-m tall; and shrubland is an open or closed stand of shrubs up to Fraction woody cover 2-m tall.