The spread of herding to eastern and southern Africa

Pastoralism, or herding domestic animals, was the first form of food production to spread across Africa [21]. Herding began ~8000–7000 BP when sheep and goats of southwest Asian origin, along with cattle with potentially mixed African/southwest Asian ancestry, spread rapidly through the Sahara during the African Humid Period [22���24]. Increasing aridity after ~6000 BP pushed herders out of the desert and along major watersheds to the west and southeast, facilitating the spread of food production into sub-Saharan Africa.

Herding entered eastern Africa through northwestern Kenya ~5000 BP as Lake Turkana receded, exposing new pasturelands [25–27]. The appearance of sheep, goats, and cattle coincided with major cultural changes, including new lithic and ceramic technologies as well as the construction of monumental cemeteries [27,28]. It is unclear whether livestock were introduced primarily through demic diffusion or exchange, but evidence that people engaged in both fishing and herding suggests complex population interactions during this transitional time [28–30].

Cattle did not spread further south for several hundred years, potentially limited by novel livestock disease vectors south of Lake Turkana [31,32]. By ~3300 BP, specialized sheep, goat, and cattle pastoralism became widespread in southern Kenya and northern Tanzania [28,33,34]. Herders entering the South-Central Rift Valley either from the Turkana Basin or elsewhere did not fully displace forager economies, leading to a mosaic of herders and foragers during the Pastoral Neolithic (PN) era [32,35]. Diverse PN cultures thrived for nearly two millennia until the arrival of Iron Age farming ~1600BP, after which time new relationships between foragers, herders, and farmers formed [36,37].

Domesticated caprines entered southern Africa ~2000 BP, although the route(s) and mechanism(s) of this process are contested [15,38–42]. As with eastern Africa, these non-indigenous species must have been introduced via contact with northern populations. This assumption is supported by linguistic [43] and genetic evidence for a connection between eastern and southern Africa [44–49]. What is in question is the timing and extent of contact, the role of migrants in transmitting herding practices, and the degree of cultural diffusion and local adoption.

In southern Africa, competing models for how herding spread must account for the rapid movement of sheep and their low proportions at sites that otherwise yield Later Stone Age (LSA) artifacts. Sheep/goats appeared on the western and southern coasts of South Africa ~2000 BP, virtually simultaneously with their arrival in Namibia ~2200 BP and northern Botswana ~2000 BP [15,38,42,50]. Yet domestic species represent <10% of the fauna at most first millennium AD herding sites [15]. Pottery also spread around this time, although the connection between these processes is debated [15,42]. Cattle appeared several centuries later after 1300 BP, with the exception of a cow horn core dated to 1529–1391 cal BP in Namaqualand [39]. Those seeking to explain this pattern typically fall into three camps: either sheep spread rapidly through down-the-line trade among foragers who independently became “hunters with sheep” [41,51], sheep spread through the demic diffusion of herders ancestrally related to the Khoekhoen [42], or a combination of these processes [38].

Genetic evidence suggests migration and admixture were factors in the spread of herding to both eastern and southern Africa. Based on ancient DNA (aDNA) analysis of 41 foragers and early food producers from Kenya and Tanzania, Prendergast et al. [52] propose a multi-step model for the introduction of herding and farming involving multiple population movements. Pastoral Neolithic herders exhibit ancestry related to both eastern African foragers and northern Africans, with the timing of admixture estimated to have occurred ~4500 BP. This is also when alleles for Lactase Persistence are thought to have entered the region, ostensibly through migration or contact with pastoralists from Ethiopia or Sudan [53,54].

Some eastern Africans herders were seemingly involved in the transmission of herding father south. A ~1200-year-old individual from a herder context at Kasteelberg in South Africa exhibits ancestry related to both southern and eastern African populations, with 40% of her ancestry related to an infant from the PN herder site of Luxmanda in Tanzania [49]. These findings support the hypothesis that a non-Bantu-related population carried eastern African ancestry to southern Africa by 1200 BP. Three ~2000-year-old individuals from forager contexts at Ballito Bay and Doonside, located on the eastern coast of South Africa, do not exhibit eastern African ancestry [48]. However, their genetic distinctiveness from recent southern Africans suggests modern day Khoesan exhibit up to 30% ancestry related to eastern African sources. Admixture is estimated to have occurred >1500 years ago, around the time herding spread, and is consistent with a migration of eastern African pastoralists who admixed with Later Stone Age hunter-gatherers [47,48]. Gene flow is also inferred from Y chromosome (Henn et al. 2008; Batini et al. 2011) and nuclear DNA patterns among contemporary Africans [47,55,56], as well as high frequencies of the most common eastern African allele for Lactase Persistence (14010G>C) among South African Nama herders [45].

Herding’s spread into eastern and southern Africa followed different trajectories in terms of timing, pace, landscapes, and human populations involved. These differences are reflected in distinctive material cultural records, which may extend to differences in OES beads. And yet, these processes were part of a larger movement of non-African domesticates through sub-Saharan Africa. The introduction of sheep, goats, and cattle from the north involved, to varying extents, the same mechanisms of demic diffusion, local adoption, interaction, and innovation as economies shifted and herders and foragers established new relationships. Both regions were then further impacted by the later arrival of Bantu-speaking farmers. The persistence of foragers in both regions across these transitions suggests disparate groups were involved in complex interactions. With this in mind, personal ornaments such as OES beads may provide clues as to how goods and people moved through these landscapes and were influenced by the changes around them.

Previous studies on OES beads

OES beads are abundant at Holocene sites in eastern Africa but there has been little systematic study of bead size or attributes. A notable recent exception is Tryon et al. [2], where mean diameter by level is reported for beads spanning >40,000 to ~4200 BP at Kisese II Rockshelter in Tanzania. More often, sources report the presence and number of beads associated with Holocene deposits. For example, beads from LSA forager contexts are reported around Lake Turkana [57,58], Lake Victoria [59–61] and Enkapune Ya Muto in the Central Rift Valley [3]. Beads are also found in early Pastoral Neolithic contexts, such as the thousands of OES beads reported from pillar site cemeteries constructed around Lake Turkana by the earliest herders in the region [62]. OES beads can also be found, though in lower quantities, in PN contexts further south at sites such as Gogo Falls [61], Keringet Cave [63], Porcupine Cave [64], Gambles Cave II [65], Lukenya Hill (GvJm 22) [66], Lion Hill Cave, Akira, and Salasun [67], and Luxmanda [68]. There may be more examples, but most sources focus on other types of PN ornaments such as stone and seed beads [69].

Considerably more attention has been paid to OES bead variation in Holocene southern Africa. Jacobson [5,6] suggested that OES bead diameters vary over time based on observations by W.E. Wendt and P. Wiessner about archaeological surface scatters, and ethnographic work among the Kalahari San. To test the idea, Jacobson analyzed 18 bead assemblages from seven sites, divided into three types: pre-herding foragers with microlithic assemblages but no pottery; foragers or transitional early herders with small amounts of pottery; and herders with few stone artifacts but with abundant pottery. He found the greatest differences in mean bead diameter between pre-herding foragers and herder sites, with no beads larger than 7.5 mm found in any of the foraging sites, although smaller beads were found in both. He therefore suggested that sites with larger beads were likely to be later, though the absence of larger beads did not necessarily signify an earlier site.

Bead diameter was soon adopted by researchers looking for ways to differentiate forager from herder archaeological assemblages and to distinguish herders from foragers on their periphery who may have used livestock as part of their seasonal cycle. By looking at bead size change through time at sites in the Western Cape, Smith et al. [7] observed a clear and consistent distinction between the sites of Witklip (associated with foragers) and Kasteelberg A and B (associated with herders). Combined with other material cultural evidence from lithic toolkits, faunal assemblages, and ceramic indices, they used this to argue for maintenance of distinct cultural groups of foragers and herders after the arrival of herding. This implied bead size could be used as an indicator of cultural identity, and not just time.

Subsequent studies sought to test changes in bead diameter in other parts of southern Africa [8–13,70,71]. While most validated the increase in bead size over time, some researchers questioned the association with herders [14,72]. Sadr and colleagues [10] re-evaluated the assemblages studied by Smith et al. [7] and argued that differences in bead size were driven by cultural evolution in bead-making as opposed to the preferences of distinct groups. Orton et al. [9] found smaller beads at Rooiwal Midden that fell into forager size ranges, yet were dated to later herder time periods and associated with pottery. This echoes Jacobson’s caveat that an absence of larger beads does not necessarily indicate older assemblages. Kandel and Conard [8] confirmed that smaller beads were associated with pre-pottery forager assemblages, however, noted that one larger bead (7.4 mm) was directly dated to the pre-herding period (3907–4087 cal BP), and cautioned that the smaller vs. larger bead dichotomy may not be universal. A similar result was produced by Orton and Compton [73] who directly dated a 6.0 mm bead to 6913–7162 cal BP, well before herding spread to southern Africa. These studies highlight variability within forager and herder assemblages; what is considered small in some cases is average in others. The nature of changes in bead size seems to be regionally variable, but Jacobson’s observation about increase over time has been largely validated.

Despite an emerging picture of complexity across time and space, there has been limited consideration of results between studies. When these early bead data are plotted on a single graph, the inverse relationship between bead size and time is evident (Fig 1). However, most of these points represent average diameters by excavation level or strata, obscuring variation among individual beads. A few larger beads in an otherwise small-diameter assemblage may skew the results. Grouping by excavation level also renders it difficult to compare assemblages between sites. Furthermore, it remains unclear whether an increase in bead size occurred in other parts of the continent where herding was introduced, such as eastern Africa where beads have a similarly long history [1,33]. By combining published and new data on bead diameter change through time using a finer-grained approach, we revisit the question of bead diameter change in southern Africa and compare results to similar processes in eastern Africa. In doing so, we hope to contribute another line of evidence to debates on how herding spread, and the human interactions underlying these important transitions.

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Fig 1. Previously published OES bead diameter data for southern Africa.

Produced using data from [5–10,12,73–76].

https://doi.org/10.1371/journal.pone.0225143.g001

Bead patterns differ with the spread of herding in eastern and southern Africa

Patterns of OES bead diameter in eastern and southern Africa are patently different. Both regions acquired herding through contact with populations from the north, and both regions had prior, long-standing OES bead traditions. Yet regional and temporal patterns in OES bead diameters reflect this transition in unique ways. This begs the question of what beads can tell us about the introduction of herding to different parts of the continent.

In eastern Africa, OES beads size did not appear to change with the arrival of herding despite other shifts in material culture. This may be, at least partly, because livestock were adopted by local foragers who retained their bead-making traditions. It is also possible that migrants entering eastern Africa possessed similar bead traditions, or adopted local traditions soon after arriving through contact and/or integration with foragers. These possibilities are not mutually exclusive, and multiple factors likely contributed to herding’s spread. However, using beads to distinguish between different trajectories requires further research on pastoralist artifact assemblages farther north. OES beads have been reported from numerous sites in the Nile Valley including Gebel Ramlah [86], el Barga [87], el-Kadruka [88], Esh Shaheinab [89]; R12 [90], el-Kadada [91] and el Geili [92]—all of which predate herding’s spread into eastern Africa—but virtually no information has been published on beads or their diameters. Additionally, although pillar site cemeteries around Lake Turkana have yielded thousands of OES beads, these have also yet to be systematically studied or measured. However, the sheer volume of OES beads at the pillar sites imply they were important for identity signaling and exchange among early herders and likely foragers interacting along this frontier [27,28,62].

In eastern African sites where beads are known from both pre- and post-herding contexts, there is virtually no change in bead diameter over time. Although it is unclear for many of the measured sites whether beads were associated with herders or foragers, after ~3000 BP these groups were likely in contact, or at least aware of one another. If the development of PN traditions in the Rift Valley disrupted pre-existing social networks, this is not reflected in OES bead diameters.

In southern Africa, there is a detectable change in bead diameters that appears to correlate with herding’s spread. Although the exact timing is disputed, zooarchaeological data support an initial entrance of sheep/goats around 2000 BP, exactly when a subset of >6.5 mm OES beads appear. The introduction of new bead sizes seems coeval with the arrival of herding, but larger beads did not replace existing traditions or become dominant in the assemblages we sampled. This is consistent with other evidence of cultural continuity in the archaeological record [15,40]. Two interpretations follow: either the arrival of livestock stimulated a local change in bead traditions, or more likely, contact with foreign groups introduced new bead styles into southern Africa along with domesticates. Whether this process entailed infiltration of the bead-makers, or simply the movement of beads, is unknown. Continuity in smaller bead assemblages suggests either domesticates and foreign bead styles were acquired through exchange and adopted locally, or that migrants accompanying livestock integrated into local communities. Although we cannot reconstruct the underlying social processes, timing suggests the arrival of livestock meaningfully shifted social practices, networks, or both.

These results also lend support for Jacobson’s [5,6] original use of larger OES beads as a chronological indicator. Because beads >6.5 mm only appeared in large numbers over the last 2000 years, their presence can be used with other lines of evidence to tentatively identify sites post-dating the arrival of herding in southern Africa. Bead size alone, however, should not be used to ascribe an assemblage or archaeological site to herders in southern (or eastern) Africa, as we do not yet understand how beads transferred between hunters and foragers, or the relationship between these groups in antiquity. Likewise, because smaller beads dominate throughout time, the absence of larger beads should not be used to infer a pre-herder assemblage, just as Jacobson warned. Although bead size should not be used as a sole chronological indicator, these results support some association between larger beads and occupations younger than ~2000 BP, in southern Africa.

Comparisons of eastern and southern African OES bead assemblages support other lines of evidence showing different trajectories in the introduction of livestock and herding. Although both scenarios likely involved population interaction and exchange, there is only an observable change in bead styles in southern Africa. Examining these different patterns more closely in conjunction with other archaeological data may reveal what interactions looked like and unique pathways to food production on the African continent. Since OES beads were used by foragers for more than 40,000 years in both regions, change—or lack thereof—in bead styles is informative about how foragers responded, coped, and mediated cultural contact and economic change. Given the ubiquity of OES beads at African archaeological sites bridging this transition, beads constitute important evidence for how the arrival of herding impacted diverse human landscapes.

A new way to explore interregional contact?

Patterns in OES beads may also provide an independent line of evidence connecting eastern and southern Africa in the Late Holocene. The hypothesis that sheep and goats were introduced to southern Africa by migrants who were the ancestors of Khoekhoen herders, has a long history of debate in southern African archaeology (summaries in [42,93,94]). Although studies have focused on the extent to which herding was introduced via migration or cultural diffusion, the origins of northern populations transmitting livestock also remains in question. Size similarities between larger OES beads in eastern African and later southern African contexts may point to a connection with the herding intensive PN cultures of the South-Central Rift Valley.

Multiple genetic studies now document gene flow from eastern to southern Africa around the time herding spread, providing unequivocal evidence of contact between these regions consistent with other lines of archaeological evidence. This is especially evident in the shared ancestry of girls buried at Luxmanda, Tanzania and Kasteelberg, South Africa [49]. What remains in question is the nature of this connection, and whether migration and admixture played a role in the initial spread of herding or occurred later as interregional population dynamics evolved. Importantly, the scale of admixture was small and inconsistent with a major migration or population replacement [44,46,47]. Rather, small groups from eastern Africa, or who were in contact with eastern Africans, appear to have interacted and potentially integrated with southern African foragers. This is supported by the genetic relatedness of Khoe and San groups despite evidence of some eastern African admixture, which Uren et al. [95] use to argue that herding primarily spread through cultural diffusion. Additional sequences from ancient southern African foragers and herders may help refine the timing and extent of contact and admixture.

Patterns in southern African OES beads seem to parallel genetic evidence for interregional contact, but add the perspective of cultural identity rather than genetic ancestry. OES beads similar in diameter to those from eastern Africa appeared around the same time as livestock, but did not replace existing smaller bead traditions. One explanation for this is that small numbers of migrant herders brought finished beads and/or produced beads using eastern African stylistic norms. These items then entered local knowledge and exchange networks along with livestock and herding practices. It is also noteworthy that both genetic and bead evidence suggests this was a unidirectional process—neither smaller beads, nor southern African genetic signatures, appear to flow back to eastern Africa. Whether beads capture an additional link between eastern and southern African populations around 2000 BP is an intriguing possibility that should be tested through future research.

Individual bead measurements versus summarized data

A final point that emerges from this research is the importance of reporting individual bead diameters instead of a single averaged calculation by excavation or stratigraphic level. Comparing Figs 1 and 3 demonstrates the masking effect of using averages by level. Previous arguments for increasing bead size through time in southern Africa have been skewed by the addition of larger beads toward the end of this sequence, obscuring the continuity of smaller bead traditions. When the data points are plotted individually, the picture that emerges is one where people incorporated new styles into existing mosaics as opposed to replacing earlier traditions. We suggest that future studies present individual bead diameters, and publish the raw measurements, to facilitate more effective research on bead variation in datasets from across the African continent and beyond.