Humans are social creatures, but little is known about when, how, and why different populations connected in the past. Answering these questions is crucial for interpreting the biological and cultural diversity that we see in human populations today. DNA is a powerful tool for studying genetic interactions between populations, but it can't address any cultural exchanges within these ancient meetings. Now, scientists from the Max Planck Institute for the Science of Human History have turned to an unexpected source of information -- ostrich eggshell beads -- to shed light on ancient social networks. In a new study published in Nature, researchers Drs. Jennifer Miller and Yiming Wang report 50,000-years of population connection and isolation, driven by changing rainfall patterns, in southern and eastern Africa.

Digital microscope images of archeological ostrich eggshell beads.
Digital microscope images of archeological ostrich eggshell beads. © Jennifer Miller

Jennifer M. Miller, Yiming V. Wang. Ostrich eggshell beads reveal 50,000-year-old social network in Africa. Nature, 2021; DOI: 10.1038/s41586-021-04227-2

Humans evolved in a patchwork of semi-connected populations across Africa12; understanding when and how these groups connected is critical to interpreting our present-day biological and cultural diversity. Genetic analyses reveal that eastern and southern African lineages diverged sometime in the Pleistocene epoch, approximately 350–70 thousand years ago (ka)34; however, little is known about the exact timing of these interactions, the cultural context of these exchanges or the mechanisms that drove their separation. Here we compare ostrich eggshell bead variations between eastern and southern Africa to explore population dynamics over the past 50,000 years. We found that ostrich eggshell bead technology probably originated in eastern Africa and spread southward approximately 50–33 ka via a regional network. This connection breaks down approximately 33 ka, with populations remaining isolated until herders entered southern Africa after 2 ka. The timing of this disconnection broadly corresponds with the southward shift of the Intertropical Convergence Zone, which caused periodic flooding of the Zambezi River catchment (an area that connects eastern and southern Africa). This suggests that climate exerted some influence in shaping human social contact. Our study implies a later regional divergence than predicted by genetic analyses, identifies an approximately 3,000-kilometre stylistic connection and offers important new insights into the social dimension of ancient interactions.

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Fig. 4: Bead-derived social connection and comparison climate proxies from eastern and southern Africa.
Fig. 4: Bead-derived social connection and comparison climate proxies from eastern and southern Africa. - a, Annual mean rainfall map of Africa (map modified from Wikimedia Commons under the CC BY-SA 4.0 licence), showing the position of palaeoclimate data (yellow squares) used in c: Lake Tanganyika; Lake Malawi; Core GIK16160-3; Core 64PE304-80; Zambezi River catchment area (red) and major tributaries (white). The two black boxes indicate the modelled climate data area for eastern Africa (22.5–40° E, −9 to 9° N) and southern Africa (8–35° E, 20–35° S) from the LOVECLIM transient climate model. The black dashed lines indicate the location of present-day Intertropical Convergence Zone (ITCZ) in January and July. b, Latitudinal north to south sequence of climate data series (from top to bottom): total precipitation for eastern Africa (22.5–40° E, −9 to 9° N) derived from the LOVECLIM transient climate model; moisture availability derived from the δD of leaf wax record from Lake Tanganyika is overlaid with the June–July–August (JJA) mean insolation at 0–20° N (red dashed line); lake level is derived from PCA analyses (PC1) in Lake Malawi; moisture availability derived from the δD of leaf wax record (ice volume corrected) from the Zambezi River catchment area overlaid with the December–January–February (DJF) mean insolation at 0–20° S; the neodymium (Nd) isotope signature of lithogenic fractions in marine sediments from the Mozambique Shelf margin; the total precipitation for southern Africa (8–35° E, 20–35° S) derived from the LOVECLIM transient climate model. HE3, Heinrich Event III; LGM, Last Glacial Maximum; VSMOW, Vienna Standard Mean Ocean Water. c, Social connections between eastern and southern Africa, derived through bead styles, over the past 50 kyr.