Significance Clumped and stable isotope data of paleosol carbonate and fossil tooth enamel inform about paleoenvironments of Early Pleistocene hominins. Data on woodland- vs. grassland-dominated ecosystems, soil temperatures, aridity, and the diet of Homo rudolfensis and Paranthropus boisei ca. 2.4 Ma show that they were adapted to C 3 resources in wooded savanna environments in relatively cool and wet climates in the Malawi Rift. In contrast, time-equivalent Paranthropus living in open and drier settings in the northern East African Rift relied on C 4 plants, a trend that became enhanced after 2 Ma, while southern African Paranthropus persistently relied mainly on C 3 resources. In its early evolutionary history, Homo already showed a high versatility, suggesting that Pleistocene Homo and Paranthropus were already dietary generalists.

Abstract New geochemical data from the Malawi Rift (Chiwondo Beds, Karonga Basin) fill a major spatial gap in our knowledge of hominin adaptations on a continental scale. Oxygen (δ18O), carbon (δ13C), and clumped (Δ 47 ) isotope data on paleosols, hominins, and selected fauna elucidate an unexpected diversity in the Pleistocene hominin diet in the various habitats of the East African Rift System (EARS). Food sources of early Homo and Paranthropus thriving in relatively cool and wet wooded savanna ecosystems along the western shore of paleolake Malawi contained a large fraction of C 3 plant material. Complementary water consumption reconstructions suggest that ca. 2.4 Ma, early Homo (Homo rudolfensis) and Paranthropus (Paranthropus boisei) remained rather stationary near freshwater sources along the lake margins. Time-equivalent Paranthropus aethiopicus from the Eastern Rift further north in the EARS consumed a higher fraction of C 4 resources, an adaptation that grew more pronounced with increasing openness of the savanna setting after 2 Ma, while Homo maintained a high versatility. However, southern African Paranthropus robustus had, similar to the Malawi Rift individuals, C 3 -dominated feeding strategies throughout the Early Pleistocene. Collectively, the stable isotope and faunal data presented here document that early Homo and Paranthropus were dietary opportunists and able to cope with a wide range of paleohabitats, which clearly demonstrates their high behavioral flexibility in the African Early Pleistocene.

Dietary adaptations are responsible for significant behavioral and ecological differences among humans and other primates (1, 2). Early hominin diets in Africa before 4 Ma were dominated by C 3 resources and diversified over time (3). Increasing contributions of C 4 plants were triggered by biomes gradually shifting to more open C 4 grasslands since the Late Miocene (4⇓⇓⇓–8). In eastern Africa, dietary versatility was very likely an integral part of early hominin adaptations to open landscapes (7, 9⇓⇓–12), yet it is unclear whether this assumption holds for African Early Pleistocene hominin evolution in general.

Here, we present multiproxy carbon (δ13C), oxygen (δ18O), and clumped (Δ 47 ) isotope data focusing on paleoenvironmental patterns and diets of Paranthropus boisei and Homo rudolfensis. Both hominin taxa coexisted around 2.4 Ma in the Western (Malawi) Branch, the southern part of the East African Rift System (EARS), a region representing a large geographical gap in our knowledge of Early Pleistocene hominin adaptations on a continental scale (13⇓⇓–16). Paleosol development characterizes the lacustrine and deltaic deposits of the Chiwondo Beds (Karonga Basin, northern Malawi) (16). These deposits yielded remains of H. rudolfensis at Uraha (17) and Mwenirondo (15), as well as Paranthropus remains at Malema, which were assigned to P. boisei due to, for example, tooth morphology and palatal height (18). Collectively, these hominin localities of the Chiwondo Beds are less than 50 km apart and are situated today in the Zambesi Ecozone of the African Savannas just south of the boundary to the Somali-Masai Ecozone, and hence outlining the southernmost extent of the Intertropical Convergence Zone. Given this unique location, the Chiwondo Beds data are crucial for understanding hominin–environment interactions and are broadening our view on habitat flexibility and dietary adaptation in early hominins.

The Karonga Basin lies in the southeastern African hominin corridor region, connecting eastern and southern African endemic faunal zones (19). Hominins thriving in these faunal zones indicate different behaviors: Eastern African Paranthropus robustus had a mixed to C 4 -dominated diet around 2.4 Ma, while P. boisei, younger than 2 Ma, provides robust evidence for C 4 -dominated consumption in this region. This clearly distinguishes them from age-equivalent southern African P. robustus, which shows mixed or C 3 -dominated diets (20⇓⇓–23). Early Homo had a highly variable diet, including C 3 and C 4 resources (3, 6).

In contrast to the mostly open C 4 grasslands of the northern EARS, the Malawi Rift was dominantly covered by persistent wooded savannas throughout the Plio-Pleistocene (24). Despite exhibiting a woody cover exceeding 60% and only regional patches of open C 4 grasslands (24, 25), this part of the EARS was populated by early P. boisei and H. rudolfensis, pointing to a much broader dietary versatility of early East African P. boisei than previously assumed.

We utilize a multiproxy approach for reconstructing habitats and diets during the earliest phases of coexisting Homo and Paranthropus in the Malawi Rift. Our study of δ13C and δ18O values in tooth enamel of H. rudolfensis and P. boisei provides insight into dietary preferences, flexibility, and adaptation in variably open and closed savanna environments. In contrast, intratooth δ13C and δ18O time series from contemporaneous equids (Eurygnathohippus sp.) and bovids [Alcelaphinae (Alc.) Megalotragus sp.], in addition to δ13C and δ18O data from pedogenic carbonate of the hominin fossil sites, allow the reconstruction of vegetation patterns (25, 26). The Δ 47 data from paleosols of the hominin fossil sites are from the southern part of the EARS and provide insight into regional temperature differences during the time of early hominin evolution. Our results show that early hominins must have been able to adapt to different environmental settings and diets as early as 2.4 Ma, indicating high behavioral flexibility already in the early stages of hominin evolution, as first indicated by, for example, Turkana Basin Kenyanthropus platyops individuals at 3.4–3.0 Ma (3).

Results We report δ13C and δ18O data from tooth enamel of all three Chiwondo Bed hominin individuals, as well as age-equivalent bovid (n = 2) and equid (n = 3) specimens [sample IDs stated according to the Hominin Corridor Research Project (HCRP), with location ID followed by the individual sample number]. We complement these by δ13C and δ18O (n = 199 samples) as well as Δ 47 thermometry (n = 12 samples) data of pedogenic carbonate. All samples originate from unit 3-A2 (ca. 2.5–2.3 Ma) of the Chiwondo Beds (27) in the Karonga Basin at the northwest margin of Lake Malawi. In addition, we present modern soil temperatures that were monitored at three locations (full shade, partial shade, and full sun) throughout a complete year (n = 26,280 measurements) in the depth of typical pedogenic carbonate precipitation within the region (∼40 cm below the surface). Results are shown in Figs. 1–3 and listed in SI Appendix, Tables S2–S8. Fig. 1. Intratooth δ13C and δ18O variations in equid and bovid teeth vs. distance from the crown; hence, time of tooth growth. The hominin tooth enamel sample quantity was too small to determine intratooth patterns; each sample averages almost the entire interval of enamel development. Bovids and equids were coexistent with the early hominins, and fossils were collected at the same localities. Different patterns indicate different migratory behavior of the individuals and/or seasonal changes. Eur., Eurygnathohippus; gen. indet., genus indeterminate. Fig. 2. δ13C, δ18O, and ∆ 47 values for Karonga Basin paleosol carbonate from the 2.5- to 2.3-Ma-old hominin fossil sites of Malema (A) and Uraha (B). Definitions of biome abbreviations are provided in Fig. 4. The black line shows modern MAAT. Fig. 3. Mean monthly soil temperatures (circles) measured for this study (2016/2017) with day and night air temperatures (diamonds), mean precipitation (blue), and average sunshine hours (orange) (32). Soil temperatures (columns on the left represent the complete temperature span of the full year in the respective color) generally follow air temperature patterns and correlate with sunshine hours, especially in soil directly exposed to the sun; here, the soil is usually warmer than the air due to the effect of direct solar radiation. Bars at the left of the figure show the maximum range of measured soil temperatures at the different locations. Temp., temperature. Hominin and Herbivore Tooth Enamel δ13C and δ18O Data. H. rudolfensis. Each tooth shows only a small range in δ13C and δ18O values, but the two individuals show a clear geochemical distinction in their tooth enamel, with a mean difference of 3.8‰ in δ13C and 2.5‰ in δ18O (Fig. 1). The δ13C and δ18O values from the Uraha individual HCRP-U18-501 (n = 3) are −6.3‰, −6.0‰, and −5.3‰ (mean = −5.9‰, σ = 0.5‰, C 3 /C 4 ratio = ∼65:35) and 25.9‰, 26.5‰, and 26.9‰ (mean = 26.4‰, σ = 0.5‰), respectively. The δ13C and δ18O values from the Mwenirondo specimen HCRP-MR10-1106 (n = 3) are more negative, with δ13C values of −9.9‰, −9.8‰, and −9.5‰ (mean = −9.7‰, σ = 0.2‰, C 3 /C 4 ratio = ∼95:5) and δ18O values of 23.6‰, 23.9‰, and 24.1‰ (mean = 23.9‰, σ = 0.2‰). P. boisei. The molar from the Malema individual HCRP-RC11-911 (n = 3) shows δ13C values of −7.0‰, −6.9‰, and −6.8‰ (mean = −6.9‰, σ = 0.1‰, C 3 /C 4 ratio = ∼70:30). The δ18O values are 26.3‰, 26.8‰, and 27.2‰ (mean = 26.8‰, σ = 0.5‰). Bovids (Alcelaphinae). The Alcelaphini Megalotragus sp. molar HCRP-U18-401 (n = 9) from the H. rudolfensis fossil site has much more positive δ13C values compared with the hominins, with a range between −2.6‰ and 0.8‰ (mean = −1.7‰, σ = 1.3‰, C 3 /C 4 ratio = ∼30:70). The δ18O values range between 26.3‰ and 28.4‰ (mean = 27.3‰, σ = 0.7‰). The δ13C and δ18O values of Alc. genus and sp. indeterminate HCRP-RC11-595 (n = 19) from the P. boisei locality at Malema are generally even more positive than the Megalotragus sp. data; δ13C values range from 0.7 to 2.0‰ (mean = 1.5‰, σ = 0.4‰, C 3 /C 4 ratio = ∼2:98). Only a slight positive shift of 1.3‰ is present in this very narrow range. The δ18O values show a curved pattern between 27.5‰ and 29.5‰ (mean = 28.5‰, σ = 0.5‰). Equids (Eurygnathohippus sp.). Similar to Alcelaphinae, a very narrow range in δ13C values and a larger variability in δ18O values characterize the stable isotope data of the three sampled Eurygnathohippus sp. teeth from different individuals, which show a clear geochemical distinction, especially in δ13C (Fig. 1). The premolar of HCRP-U17-390 (n = 11) (location U17 is the same age and spatially correlated with H. rudolfensis from U18) has δ13C values between −1.2‰ and 0.2‰ (mean = −0.2‰, σ = 0.5‰, C 3 /C 4 ratio = ∼15:85). The δ18O values are between 27.5‰ and 28.7‰ (mean = 28.1‰, σ = 0.3). HCRP-RC11-528 (n = 12) from the P. boisei site has δ13C values between −2.4‰ and −1.3‰ (mean = −1.9‰, σ = 0.4‰, C 3 /C 4 ratio = ∼30:70) and δ18O values from 27.2 to 28.6‰ (mean = 28.1‰, σ = 0.5‰). HCRP-RC11-545 (n = 14) also shows more negative δ13C values than the other Eurygnathohippus sp., with a very narrow range from −5.7 to −5.0‰ (mean = −5.3‰, σ = 0.2‰, C 3 /C 4 ratio = ∼50:50), while the δ18O values are generally more positive, between 29.0‰ and 30.3‰ (mean = 29.8‰, σ = 0.4‰). Pedogenic Carbonate Δ 47 Temperatures. Pedogenic carbonate Δ 47 temperatures from the H. rudolfensis site U18 (Uraha) range from 19 to 38 °C (mean = 28 °C, σ = 8.3 °C; n = 6). The dataset seems to be bimodal, with values grouping at relatively high temperatures (mean = 35 °C, σ = 2.2 °C; n = 3) and another cluster of low temperatures (mean = 21 °C, σ = 1.8 °C; n = 3). A trend over time (i.e., across the sampled fossil horizon) is not present. Paleosol temperatures from the P. boisei site RC11 (Malema) show a smaller variation, with values between 22 °C and 29 °C (mean = 26 °C, σ = 2.9 °C; n = 6). Karonga Basin soil water δ18O values (δ18O SW ) were calculated using the δ18O values of the carbonate and their Δ 47 temperatures according to the methods of Kim et al. (28) and Kim and O’Neil (29), resulting in δ18O SW values between −5.4‰ and −1.5‰ (mean = 3.6‰, σ = 1.2‰ °C; n = 12). Pedogenic Carbonate δ13C and δ18O Values. Uraha. The δ13C values of pedogenic carbonate from a soil profile at the H. rudolfensis site U18 (n = 52) range between −11.0‰ and −7.0‰ (mean = −9.1‰, σ = 0.8‰). The estimated fraction of woody cover (f wc ) (5) ranges between 0.5 and 0.8. The corresponding δ18O values lie between 23.6‰ and 24.9‰ (mean = 24.3‰, σ = 0.3‰). Malema. The δ13C values of pedogenic carbonate from the P. boisei site RC11 (n = 147) cover a similar range as at the Uraha site, with δ13C values between −10.7‰ and −6.3‰ (mean = 8.5‰, σ = 1.1‰). The estimated f wc values range from 0.4 to 0.8. The δ18O values show a slightly larger variation than at the Uraha site, with values between 23.3‰ and 24.7‰, excluding one outlier with a value of 25.2‰ (mean = 24.1‰, σ = 0.4‰). Correlation Between Stable and Clumped Isotope Data. (Negative) correlation between datasets was determined by using the Pearson correlation coefficient. Statistically (negative) correlation is significant at the 95% level. We observe a statistically relevant positive correlation between δ18O and δ13C values in the Uraha (P = 0.5) and Malema (P = 0.8) datasets, suggesting a higher fraction of woody cover during periods of wetter climates. Reconstructed Δ 47 soil temperatures and δ18O and δ13C values show a strong negative correlation at the Malema site RC11 (P = −0.9 for δ18O and P = −0.8 for δ13C); hence, high (soil) temperatures coincide with a moister climate, which produces an even denser canopy cover serving as a buffer and resulting in less thermal loss at night. This is complemented by the small range in soil temperatures at this site (Fig. 5). Modern Soil Temperatures. Soil temperatures were measured hourly 40 cm below the surface under conditions of full shade, partial shade, and full sun (Fig. 3). Full shade. Over the course of 1 y (August 1, 2016 to July 31, 2017), soil temperatures in sandy soil under dense tree cover range from ca. 23 to 27 °C (mean = 26 °C, σ = 0.9 °C; n = 8,760). Temperature differences between day and night, as well as between dry and wet seasons, are relatively small (generally ca. 0.5 °C and 2 °C, respectively). Partial shade. Recorded soil temperatures (August 1, 2016 to July 31, 2017) show a range of 21 to 32 °C (mean = 27 °C, σ = 2.1 °C; n = 8,760) ∼50 m away from the full-shade location. Here, temperatures below 24 °C are limited to a few outliers (n = 98; i.e., 1%) during the wet season, probably due to extreme rainfall and resulting soil water saturation. Day/night differences at around 0.5 °C are comparable to the full-shade site, but the difference between dry and wet seasons is more pronounced, with almost 6 °C warmer soil temperatures during the end of the dry season. Full sun. Soil temperature loggers were put directly in the Chiwondo Bed sandy deltaic deposits at the open habitat of the Malema P. boisei site (RC11). The recorded temperatures (August 15, 2016 to August 14, 2017) are higher than at the locations of full shade and partial shade and range from 25 to 37 °C (mean = 31 °C, σ = 3.2 C; n = 8,760). Day/night temperature differences are >2 °C and increase during the rainy season, with maximum day temperatures of generally 36 °C contrasting with night temperatures, which regularly drop below 32 °C.

Conclusions Stable carbon and oxygen isotope data of hominin fossil tooth enamel from the Malawi Rift show that early (ca. 2.4 Ma) H. rudolfensis and P. boisei included a large fraction of C 3 food resources in their diets. Abundant C 3 resources were provided by relatively cool and well-watered wooded savanna ecosystems in the vicinity of paleolake Malawi in the southern part of the EARS. Younger (<2 Ma) Paranthropus individuals from the Eastern Rift incorporated an increasing amount of C 4 resources in hotter, more arid, and open savanna settings, while P. robustus maintained a C 3 -dominated diet in the wetter and more mesic environments of southern Africa. Throughout the Early Pleistocene, Homo shows a high versatility in the diverse habitats of the EARS. H. rudolfensis and P. boisei were therefore dietary generalists, able to adapt to different paleohabitats, successfully utilizing a broad range of ecosystems, including freshwater environments near the tributaries to paleolake Malawi.

Methods Teeth of three hominins, three equids, and two bovids temporarily housed in the Senckenberg Research Institute and from the Cultural and Museum Centre Karonga were sampled using a high-speed, rotary, diamond-tip drill to obtain 2.5–5 mg of enamel powder from each of the up to 19 samples per tooth. To remove organic and potential diagenetic carbonate, enamel was pretreated with 2% sodium hypochlorite solution for 24 h, followed by treatment with 1 M Ca-acetate acetic buffer solution for another 24 h (44). Additionally, 199 pedogenic carbonate nodules were analyzed. Then, 600–1,200 μg of pretreated enamel and 100–160 μg of untreated pedogenic carbonate material were reacted with 99% H 3 PO 4 for 90 min at 70 °C in continuous flow mode using a Thermo Finnigan 253 mass spectrometer interfaced to a Thermo GasBench II. All analyses were performed at the Goethe University Frankfurt–Senckenberg Biodiversity and Climate Research Centre Stable Isotope Facility. Analytical procedures followed the technique of Spötl and Vennemann (44). Final isotopic ratios are reported versus VPDB (Vienna Pee Dee Belemnite; δ13C) and VSMOW (Vienna Standard Mean Ocean Water; δ18O); overall analytical uncertainties are better than 0.3‰. Homogenized powder of 12 selected soil nodules measured for stable isotopes was additionally used for clumped isotope analyses, which were performed in the same laboratory. Untreated carbonate powder (5–15 mg) was digested in ≥106% H 3 PO 4 at 90 °C for 15–30 min, using a semiautomated acid bath (45, 46). The produced CO 2 was cleaned by flowing through cryogenic traps at −80 °C before and after passage through a Porapak Q-packed gas chromatography column to remove traces of water and hydrocarbons (cf. refs. 45, 46). The purified CO 2 was analyzed using a Thermo Scientific MAT 253 gas source isotope ratio mass spectrometer dedicated to the determination of masses 44–49 in 10 acquisitions consisting of 10 cycles each, with an ion integration time of 20 s per cycle. Five to six replicates were run per carbonate sample. The best precision that can be achieved under these conditions is represented by the shot noise limit, which is 0.004‰ for n = 5 and 0.003‰ for n = 6. The Δ 47 values are reported in the absolute reference frame (47) and were processed according to the protocol of Fiebig et al. (48). Apparent carbonate crystallization temperatures were computed from measured Δ 47 values using the empirical calibration technique of Wacker et al. (46). Soil temperatures (quoted accuracy of ±0.1 °C) were measured hourly using Voltcraft DL-101T USB-Temperature loggers, which were buried within polyethylene-plastic bottles ca. 40 cm below the surface in narrow trenches (∼15 cm wide), which were subsequently backfilled with the soil removed during excavation. SI Appendix, Materials and Methods includes additional information on analytical procedures, data processing, dating, time interval recorded in enamel and pedogenic carbonate, isotope enrichment factors in mammals, stratigraphic information, and biome classifications used in this work.

Acknowledgments We thank the Cultural Museum Centre Karonga, our local Malawian field crew, and the Malawian Government for the long-term cooperation with the HCRP. We particularly thank H. Simfukwe (Cultural Museum Centre Karonga) for assistance and hospitality. T.L. thanks U. Treffert, N. Löffler, K. Methner (Senckenberg Biodiversity and Climate Research Centre), and S. Hofmann (Goethe University Frankfurt) for laboratory support and H. Thiemeyer (Goethe University Frankfurt) for support in the field. The paper was greatly improved by the thoughtful comments of two anonymous reviewers. T.L. acknowledges funding by Deutsche Forschungsgemeinschaft Grant LU 2199/1-1.

Footnotes Author contributions: T.L., O.K., F.S., and A.M. designed research; T.L. performed research; T.L., U.W., and J.F. contributed new reagents/analytic tools; T.L. analyzed data; and T.L., O.K., O.S., F.S., and A.M. wrote the paper.

The authors declare no conflict of interest.

This article is a PNAS Direct Submission.

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