Oxygen Isotope Data from South African Early Hominin Sites

Oxygen isotope ratios in herbivore bone and enamel have been used to reconstruct the isotopic composition of environmental waters, which in turn carry climate information (e.g., Longinelli, 1984; Ayliffe and Chivas, 1990; Bryant et al., 1996; Passey et al., 2002; Schoeninger et al., 2003). Oxygen isotope ratio data are regularly produced when analyzing tooth enamel for carbon isotope ratios, and thus studies aimed at providing dietary information using carbon isotopes may also provide paleoclimatic information. Of particular interest here is an idea advanced by Kohn et al. (1996), and recently tested and developed by Levin et al. (2006), that differences in the oxygen isotope compositions of environmentally sensitive (ES) and environmentally insensitive (EI) taxa can be used as paleoaridity indicators. The rationale is that the oxygen isotope compositions of apatite from some mammalian taxa are primarily records of meteoric water (environmental water that originates more directly from precipitation) 518O values (EI taxa), while others are more sensitive to differences in aridity as their oxygen isotope compositions track the evaporative enrichment in 18O that occurs in leaves (ES taxa). Thus, in mesic environments there should be little difference between ES and EI taxa as there will be little evaporative enrichment in leaf 18O, but this difference will increase as relative humidity decreases (Fig. 16.4).

This is quite an exciting possibility, and should be more reliable than simple comparisons of 518O values between sites, as the latter might be influenced by many factors such as altitude, distance inland, changes in storm tracks, and taxonomic composition. We applied a version of this technique to fossil enamel from Makapansgat, Sterkfontein, and Swartkrans in order to test the hypothesis that Australopithecus experienced more humid conditions than early Homo. In order to do so we had to determine firstly which taxa are ES and which are EI. This exercise is not straightforward. We can either rely upon the principle of taxonomic uniformitarianism, and assume that fossil relations of modern ES and EI taxa are likely to be ES or EI themselves, or we can rely on the 518O values of the taxa themselves, and assume that those with relatively higher 518O values are most likely to be ES. Both of these methods produce similar results in the case of Makapansgat Member 3. For instance, taxonomic uniformitarianism suggests that giraffids and perissodactyls should be ES and EI taxa respectively (as in Levin et al., 2006); and thus the former should have higher 518O values than the latter, which is indeed the case. However, the difference (e, see caption of Fig. 16.4) between these taxa in Member 3 is 4.8% (ANOVA, F = 63.918; d.f. = 20, p < 0.0001; Table 16.1; Fig. 16.3), which is similar to the difference between these two groups, in the hot, arid Turkana region today (e = 5.0%, mean annual precipitation (MAP) = 178 mm; data from Levin et al., 2006). If one broadens the analysis to include gazelles and grazing suids, which were found to be ES and EI taxa respectively in Levin et al. (2006), then the difference is 4.9% (ANOVA, F = 96.474, d.f. = 33, p < 0.0001; Table 16.1; Fig. 16.3). In fact, there is virtually no way to group the oxygen isotope data for Makapansgat (see Sponheimer, 1999) that results in a difference between ES and EI taxa of less than 4%, which is still considerably greater than the difference found between these groups at Olorgesailie (MAP = 417 mm) and Tsavo

Makapansgat Limeworks

Table 16.1 Stable isotope data from tooth enamel used to investigate aridity at Makapansgat Limeworks Member 3 (~3 Ma) (Species identifications are from Reed, 1996)

Fig. 16.4 Regression plots showing the differences (e) between environmentally sensitive (ES) and environmentally insensitive (EI) taxa from a variety of locations with varying water deficits (i.e., the difference between potential evapotranspiration and mean annual precipitation). In general, e increases in step with aridity. It is worth noting that A (S13Cb-S13Ca) rather than e ( (1,000 + S13CB) / (1,000 + S13CA)-1) * 1,000) is often used express such differences as it is more convenient, but as it is not strictly correct we use the latter here (see Cerling and Harris 1999). As can be seen in (a), the relationship is fairly weak when using many taxa simultaneously, but improves greatly when EI taxa are limited to those in the Perissodactyla (b). The star indicates the difference between ES and EI taxa for Makapansgat M3 and the starburst indicates the difference between these groups at Sterkfontein M5 and Swartkrans M2 combined. These data would seem to indicate that early members of the genus Homo in South Africa experienced much more humid conditions than Australopithecus at Makapansgat.

(MAP = 549 mm) in Kenya today (Fig. 16.3; data from Levin et al., 2006).

This suggests that Makapansgat at ~3 Ma was much drier than the area is today (600-700 mm), a result which is difficult to reconcile with regional and global paleoclimatic datasets. In addition, the fauna from Makapansgat Member 3 consists of a large number of mesic-associated taxa (e.g., Cephalophus, Redunca) (Reed 1997). Furthermore, when this paleoaridity indicator was applied to the Sterkfontein Valley fossil sites (a composite of data from similarly aged

Table 16.1 Stable isotope data from tooth enamel used to investigate aridity at Makapansgat Limeworks Member 3 (~3 Ma) (Species identifications are from Reed, 1996)

Specimen

Species

S13C

S18O

ES

M167

Ceratotherium simum

-3.3

-2.3

No

M2088

Ceratotherium simum

-2.7

-1.7

No

M8939

Ceratotherium simum

-4.3

-2.2

No

M8940

Ceratotherium simum

-3.6

-1.6

No

M2108

Diceros bicornis

-11.0

-1.1

No

M2109

Diceros bicornis

-10.8

-3.5

No

M642/2106

Diceros bicornis

-13.1

-5.6

No

M766

Gazella gracilior

-12.4

3.9

Yes

M767

Gazella gracilior

-10.7

3.1

Yes

M1188

Gazella vanhoepeni

-10.6

0.8

Yes

M529

Gazella vanhoepeni

-10.3

-0.2

Yes

M7805

Gazella vanhoepeni

-12.4

2.3

Yes

M7811

Gazella vanhoepeni

-12.8

1.7

Yes

M8823

Gazella vanhoepeni

-10.9

3.2

Yes

M9014

Gazella vanhoepeni

-11.5

1.9

Yes

M1113

Giraffa jumae

-10.8

0.6

Yes

M1798

Giraffa jumae

-9.6

1.1

Yes

M2085

Giraffa jumae

-10.9

1.6

Yes

M2085

Giraffa jumae

-12.6

4.4

Yes

M528

Giraffa jumae

-10.5

3.2

Yes

M8853

Giraffa jumae

-10.5

1.6

Yes

M936

Giraffa jumae

-11.9

5.4

Yes

M938

Giraffa jumae

-11.0

2.4

Yes

M193

Hipparion lybicum

0.2

-2.4

No

M2476

Hipparion lybicum

0.0

-1.5

No

M2480

Hipparion lybicum

-1.3

-1.3

No

M2505

Hipparion lybicum

-2.0

-3.4

No

MUE1

Hipparion lybicum

-0.8

-1.7

No

M2025

Notochoerus capensis

-1.0

-2.0

No

M8913

Notochoerus capensis

-0.6

-1.2

No

M1826/1890

Potamochoeroides shawi

-2.3

-4.9

No

M1859

Potamochoeroides shawi

-2.2

-2.4

No

M1886

Potamochoeroides shawi

-1.8

-6.0

No

M1876

Sivatherium marusium

-11.3

2.5

Yes

M2086

Sivatherium marusium

-10.8

1.5

Yes

ES stands for environmentally sensitive as defined in Levin et al. (2006)

Sterkfontein M5 and Swartkrans Member 2) using Antilopini and Hippotragini as ES taxa and perissodactyls as EI taxa (as in Levin et al., 2006), it predicts far wetter environments (e = 0.8%, ANOVA, F = 1.999, d.f. = 43, p = 0.1646; Table 16.2; Fig. 16.4) akin to what is expected today in the Masai Mara or Samburu, Kenya (MAP ~700 mm). This is puzzling once again as taxonomic (Vrba, 1985), ecomorpho-logical (Reed, 1997), and carbon isotope (Sponheimer and Lee-Thorp, 2003) studies of the Makapansgat fauna are all consistent with it having had the most mesic (or at least the most heavily wooded) environment of all the South African australopith sites. Either the fauna at these sites tell us very little about environments and climates, or there is a suite of unknown, confounding factors which precludes application of this paleoaridity indicator at these South African sites. We favor the latter interpretation, and thus we fear that we are

Table 16.2 Stable isotope data from tooth enamel used to explore aridity at Sterkfontein and Swartkrans

Specimen

Taxon

Site

Member

S13C

S18O

ES

SE 1258

Antilopini

Sterfontein

5

-10.8

-0.5

Yes

SE 1855.1

Antilopini

Sterfontein

5

-12.7

-1.7

Yes

S94-6124

Antilopini

Sterfontein

5

-9.6

-0.7

Yes

BP/3/16974

Antilopini

Sterfontein

5

-9.2

-2.8

Yes

S94-7958

Antilopini

Sterfontein

5

-7.4

-0.7

Yes

S94-7314

Antilopini

Sterfontein

5

0.7

3.7

Yes

S94-6124

Antilopini

Sterfontein

5

-9.6

-0.7

Yes

BP/3/16974

Antilopini

Sterfontein

5

-9.2

-2.8

Yes

SE 1258

Antilopini

Sterfontein

5

-10.8

-0.5

Yes

SE 1855.1

Antilopini

Sterfontein

5

-12.7

-1.7

Yes

S94-7958

Antilopini

Sterfontein

5

-7.4

-0.7

Yes

S94-7314

Antilopini

Sterfontein

5

0.7

3.7

Yes

S94-1787

Equidae

Sterfontein

5

-0.6

-2.9

No

S94-390

Equidae

Sterfontein

5

-2.0

-0.9

No

S94-349

Equidae

Sterfontein

5

-0.9

-2.1

No

S94-1750

Equidae

Sterfontein

5

0.8

-0.7

No

S94-329

Equidae

Sterfontein

5

-4.4

-0.4

No

STS 3006

Equidae

Sterfontein

5

-3.6

-2.5

No

STS 2102

Equidae

Sterfontein

5

-2.9

-1.8

No

STS 1972

Equidae

Sterfontein

5

-4.6

-3.1

No

STS 2313

Equidae

Sterfontein

5

-4.8

-4.4

No

S94-329

Equidae

Sterfontein

5

-4.4

-0.4

No

S94-1787

Equidae

Sterfontein

5

-0.6

-2.9

No

S94-390

Equidae

Sterfontein

5

-2.0

-0.9

No

S94-349

Equidae

Sterfontein

5

-0.9

-2.1

No

S94-1750

Equidae

Sterfontein

5

0.8

-0.7

No

SE 1125.1

Hippotragini

Sterfontein

5

-5.3

-0.6

Yes

SE 1125.1

Hippotragini

Sterfontein

5

-5.3

-0.6

Yes

SKX 811

Antilopini

Swartkrans

2

-12.9

0.6

Yes

SKX 1896

Antilopini

Swartkrans

2

-10.6

1.4

Yes

SKX 2736

Antilopini

Swartkrans

2

-11.5

3.6

Yes

SKX 12067

Antilopini

Swartkrans

2

-2.3

-0.5

Yes

SK 2574

Antilopini

Swartkrans

2

-4.5

-2.4

Yes

SK 6023

Antilopini

Swartkrans

2

-4.3

-2.0

Yes

SKX 5907

Antilopini

Swartkrans

2

-2.9

0.3

Yes

SKX 9385

Antilopini

Swartkrans

2

-3.3

-2.7

Yes

SKX 5962

Antilopini

Swartkrans

2

-4.2

-2.8

Yes

SK 3841

Antilopini

Swartkrans

2

-1.7

-0.9

Yes

SK 5922

Antilopini

Swartkrans

2

-2.4

-4.0

Yes

SKX 12273

Antilopini

Swartkrans

2

-3.8

-3.1

Yes

SK 3160

Equidae

Swartkrans

2

-6.4

0.5

No

SK 2626

Equidae

Swartkrans

2

-2.1

0.0

No

Sk 2626

Equidae

Swartkrans

2

-3.5

-5.0

No

SK 3990

Equidae

Swartkrans

2

0.0

0.4

No

SK 3992

Equidae

Swartkrans

2

-2.8

0.9

No

All of the specimens are believed to be roughly contemporaneous at about 1.8-1.5 Ma. ES stands for environmentally sensitive as defined in Levin et al. (2006).

All of the specimens are believed to be roughly contemporaneous at about 1.8-1.5 Ma. ES stands for environmentally sensitive as defined in Levin et al. (2006).

unable to meaningfully test hypotheses about the relative humidty/aridity experienced by A. africanus and Homo in South Africa at this time.

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