Isotopic Ecology of Fossil Fauna from Olduvai Gorge at Ca
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Research Article Olduvai fossil isotopic ecology Page 1 of 14 Isotopic ecology of fossil fauna from Olduvai Gorge AUTHOR: at ca 1.8 Ma, compared with modern fauna Nikolaas J. van der Merwe' AFFILIATION: Light stable isotope ratios (ô'^C and 5"0) of tooth enamel have been widely used to defermine the diets 'Department ot Archaeology, and water sources of fossil fauna. The carbon isotope ratios indicate whether the plants at the base of the University ot Cape Town, Cape food web used Cj or C^ photosynthetic pathways, while the oxygen isotope ratios indicate the composition Town, South Atrica ot the local rainfall and whether the animals drank water or obtained it from plants. The contrasting diets of two early hominin species - Homo habilis and Paranthropus boisei - of ca 1.8 Ma (million years ago) in CORRESPONDENCE TO: Tanzania were defermined by means of stable carbon isotope analysis of their tooth enamel in a previous Nikolaas van der Merwe study. The diets of two specimens ot R boisei, from Olduvai and Peninj, proved to be particularly unusual, EMAIL: because 80% of their carbon was derived trom C^ plants. It was suggested that their diet consisted primarily nikolaas.vandermerwe(g>uct. ot plants, with particular emphasis on papyrus, a C^ sedge. The dominance ot C^ plants in the diet of P. boisei ac.za is a finding supported in another study of 22 specimens from Kenya. The isotopic ecology and diets of fossil fauna that were present at the same time as the two fossil hominin species are described here, in order to POSTAL ADORESS: provide a fuller understanding of their contrasting diets and of the moisture sources of their water intake. Department ot Archaeology, This informafion was then compared with the isotopic composition of modern fauna from the same region University ot Cape Town, of Tanzania. The carbon isotope ratios tor both fossil and modern specimens show that the habitats in which Private Bag, Rondebosch 7701, South Atrica these faunal populations lived were quite similar - grassland or wooded grassland. They had enough bushes and frees to support a few species of browsers, but most of the animals were grazers or mixed feeders. DATES: The oxygen isotope ratios of the fossil and modern tauna were, however, very different, suggesting strongly Received: 12 Apr 2013 that the source of moisture for the rain in the Oiduvai region has changed during the past 1,8 million years. Revised: 24 Jul. 2013 Accepted: 22 Aug. 2013 Introduction KEYWORDS: The purpose of this article is to provide isotopic information on the ecology and diets of fossil tauna trom GIduvai carbon isotopes: oxygen Gorge Beds I and II at ca 1.8 Ma, in order to provide an ecological context for the diets of two species of early isotopes: Oiduvai tossils: hominins. Recent isotopic analyses of the tooth enamel ot Homo habilis and Paranthropus boisei (formerly moisture sources: Zinjanthropus boisei, also Australopithecus boisei) showed that the two species had distinctly different diets.'-^ vertebrate ecology Three specimens of Homo habilis from Olduvai had, respectively, 23%, 27% and 49% of carbon derived from C^ HOW TO CITE: plants, much like early hominins trom South Africa": their diets probably included grass-eating animals and/or Van der Merwe NJ. Isotopic insects. However, two specimens ot P. boisei trom Olduvai and Peninj had 77% and 81 % of carbon derived from C^ ecology ot tossil tauna trom plants. Because modem humans are limited to about 20-50% protein-rich toods tor their energy requirements,' it Olduvai Gorge at ca 1.8 Ma, was suggested that the diet o\P boisei included a large component ot C^ plants. As grasses, especially edible seeds, compared with modern tauna. are highly seasonal at latitudes applicable to Olduvai, it was also suggested that the C, sedge Cyperus papyrus, S AtrJ Sei. 2013:109(11/12), which was presumably available in the treshwater swamps at Lake Olduvai and Lake Natron (Peninj), may have Art. #2013-0105,14 pages. been a major component in the diet ot P boisei. The very high contribution ot C, plants to the diet ot this hominin http://dx.doi.org/10.1590/ has been confirmed by tbe isotopic analysis ot 22 individuals trom sites in Kenya that stretch over 700 km of the sajs.2013/20130105 Ritt Valley.^ The authors of the latter study suggested that P boisei had a diet that comprised mostly C, plants, without specitying whether these were grasses or sedges. An online comment on their article by Lee-Tborp^ supports the opinion that sedges were predominant in the diet. Recent publications provide turtber evidence to put these early hominin diets in context.^'" An assessment of the isotopic ecology at Peninj and Olduvai during the presence of P boisei cannot settle the argument about grasses and sedges in P boisei's diet, but it can illuminate whether the environment was dominated by C^ plants and their consumers. This assessment is ot particular importance at Olduvai around 1.8 Ma, for which the isotope values of the two hominins are available. In East Africa, C^ plants appeared in the Late Miocene and continued through the Pliocene,"'^ but did not become dominant until ca 1.8 Ma."' At Olduvai, the carbon isotopes in palaeosol carbonates indicate that C^ plants made up about 40-60% of tbe biomass during the time of Beds I and II," but a preliminary study ot the carbon isotopes in tooth enamel ot fossil tauna indicate a much higher C^ component.'^"' An assessment ot the isotopic ecology at Peninj was undertaken by measuring the carbon and oxygen isotopes in the tooth enamel ot 40 specimens ot fossil fauna from the Maritinane Type Section.^ These fossils were all grazers and the carbon isotopes were closely similar to those ot modern fauna of related species from the modern Serengeti. The environment was essentially open grassland with very tew trees. However, the fossil specimens were some 1.3 million years old, so the results do not provide information about the ecology of Olduvai at ca 1.8 Ma. It was noted that the S'^O values ot the Peninj tossils were distinctly negative, relative to the Vienna Pee Dee Belemnite (VPDB) standard, while those ot the modern fauna were all positive. It was suggested that the moisture source ot rain in this region had presumably changed during the past 1.3 million years: this suggestion is addressed in detail here. For this purpose, tooth enamel samples were obtained from 145 specimens of tossil fauna from Olduvai Gorge Middle Bed I (ca 1.785-1.83 Ma) and Lowermost Bed II (ca 1.75-1.83 Ma) and their 5'^C and 5"0 values were ©2013. The Authors. measured. These values were then compared with those of 77 modern animals from six wildlife reserves, primarily Published under a Creative Serengeti and Maswa, but also Lobo, Lukwati, Ugalla and Selous (Figure 1 ). Their carbon and oxygen isotope ratios Commons Attribution Licence. were measured to illuminate their diets and water sources. Soutii African Journal of Science Volume 109 \ Number 11/12 http://www.sajs.co.za 64 November/December 2013 Research Article Olduvai fossil isotopic ecology Page 2 of 14 orongoro Conservation area Olduvai-Gorge Figure 1 : Map of Tanzania showing the six wildlife reserves from which modem faunal specimens were coiiected, as well as Olduvai and Peninj. Tooth enamel and stable light isotopes earliest measurements on bone apatite were done on tossil bone,^" ^^ but The assessment of prehistoric diets and environments by means ot tooth enamel has proved to be the most dependable material tor isotopic isotopic analysis of bone has been developed over the past 30 years and analysis.^f^" Tooth enamel is highly crystalline and resists aiteration by is wideiy used in archaeoiogy"*-^' (for review see Van der Merwe^'). in carbonates in groundwater Contaminating carbonates may precipitate in the early development ot this method, the major interest involved "C/"C cracks in the tooth enamel, but this materiai is much more soluble than ratios (5"C values) in bone collagen, which provide a measure ot the tooth enamel and is readily removed with dilute acetic acid. proportion ot C3 and C^ plants at the base ot the tood web. However, collagen in bone has a iimited litetime, especially in hot and humid Tooth enamel is a biologicai apatite with about 3% carbonate. With environments where organic materiais deteriorate rapidly. The oldest appropriate pretreatment, the carbon and oxygen isotope ratios ot hominin collagen specimens that have been anaiysed isotopically were the carbonate can be measured with confidence to provide dietary those of Neanderthals trom cold, dry caves.^^'^^ The tossiiised taunal intormation. The carbon isotope ratio is an average ot the plants at the specimens trom Oiduvai contain no collagen, but carbon and oxygen base ot the tood web, acquired by herbivores trom eating the plants and isotopes can be measured in the mineral phase of their skeletons. The passed along the tood chain to omnivores and carnivores, with some South African Journai of Science Volume 109 ¡ Number 11/12 http://www.sajs.co.za 65 November/December 2013 Research Article Ofduvai fossil isotopic ecology Page 3 of 14 digestive differences between ditterent species.^' The oxygen isotope drink surface water that is closest to meteoric water and feed at night, ratio is a measure of the body water of an animal, acquired from plant when humidity increases and plant 5"0 decreases."^ water or from drinking water and is altered by the thermophysiology of the animal. The oxygen isotope ratio can contribute to dietary and Materials and methods environmentai reconstructions^ ^'Î^" and is particuiarly indicative ot Samples of tooth enamel used for isotopic analysis in this study were changes in humidity and aridity.^^'^^ removed from the tooth specimens at their location of storage.