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August 28, 2009

Drawing a Bead on Ancient Symbolic Behavior

Taforalttout.nassarius Anatomically modern humans evolved in Africa nearly 200,000 years ago, but researchers have long debated why there seems to be a gap between when hominins started looking modern and when they began acting modern. Some of the most important indications of modernity, such as cave art and certain types of advanced tools, don’t show up in the archaeological record until about 40,000 years ago, mostly in Europe. That has led some scientists, notably archaeologist Richard Klein of Stanford University in Palo Alto, California, to argue that modern human cognition, including language and other complex symbolic behavior, needed the additional kick-start of a genetic mutation about 50,000 years ago.

Yet an increasing number of researchers have come to think that Homo sapiens was capable of modern behavior from the very beginning of its history. Whether those behaviors show up in the archaeological record, these researchers say, depends on a variety of factors unrelated to genetics, such as how big and widespread early human populations were and what environmental challenges they faced. A team led by archaeologist Francesco d’Errico of the University of Bordeaux in France will soon publish what it considers yet more evidence for this viewpoint online in the Proceedings of the National Academy of Sciences (PNAS). Although the paper is still unpublished, PNAS has released the embargo on it, which will be available here when published. D’Errico and colleagues describe 25 shell beads, including those shown above, claimed to have been used as personal ornaments and found in four sites in Morocco dated between 85,000 and 70,000 years ago.

If this sounds familiar, that’s probably because d’Errico and others have recently published numerous sightings and analyses of shell beads from sites in Morocco, Algeria, South Africa, and Israel as old as 100,000 years ago. Nearly all researchers, including Klein, agree that personal ornaments are solid indications of modern behavior. Wearing jewelry is a form of symbolic signaling of personal identity to other hominins. And the use of symbols, scientists also agree, is a hallmark of modern human behavior—even if there are indications that earlier hominins might also have ventured into this cognitive realm.

Yet Klein has questioned many of the claims made by d’Errico and others, arguing most recently in a 2008 review in Evolutionary Anthropology that the holes found in the shells, which presumably allowed them to be strung as beads, were the result of natural abrasion from the action of the sea. He also points out that there are very few such examples in Africa. Given the near total absence of shell beads in Africa between 70,000 and 40,000 years ago, Klein suggests that the relatively infrequent earlier sightings do not represent a real symbolic tradition that endured over time. The new PNAS paper cites Klein’s Evolutionary Anthropology critique and counters with a detailed analysis of the newly discovered shell beads from the four Moroccan sites: Taforalt, Rhafas, Ifri n’Ammar, and Contrebandiers, which are dated by various techniques, including optically stimulated luminescence, thermoluminescence, and Uranium-series isotopic measurements.

Nearly all of the shells, the team writes, are from the genus Nassarius, a sea snail that shows up in archaeological sites all over Africa, Asia, and Europe. Moreover, three of the four sites are located about 50 kilometers from a coast, reinforcing the suggestion that the shells were deliberately transported inland and used for symbolic purposes, the team says. The team also notes that fragments of gravel inside shells and signs of mechanical erosion from lying on the seabed suggest that many snails were dead before they were collected. Thus they weren’t transported inland to be eaten. Finally, at some sites, the team found signs of ochre pigment clinging to the outside of the shells, which many archaeologists consider evidence of symbolic behavior.

But if hominins could create personal ornaments as early as 100,000 years ago, why have researchers found so few of them between 70,000 and 40,000 years ago in Africa and the Near East? The team points out that after 70,000 years ago, the climate turned markedly colder in both the Northern and the Southern hemispheres. When the temperature was warmer, the team writes, modern human populations were growing and “marine shell beads may have been instrumental in creating and maintaining exchange networks between coastal and inland areas. …” In other words, ornaments were a means for hominin groups to communicate their identities to each other. But the arrival of harsher conditions, and the possible resulting population crashes, “may have disrupted these networks through the depopulation of some areas, thereby isolating hunter-gatherer populations to the extent that such social and exchange networks became untenable.”

The article is part of a PNAS Special Feature series on “Out of Africa,” edited by none other than Richard Klein himself, who told Science that the d’Errico et al. paper adds “balance” to the issue. Some papers are being published online ahead of time; the entire Special Feature is tentatively scheduled for publication in late September, according to the PNAS press office.

—Michael Balter

CREDIT: Copyright d'Errico/Vanhaeren

2 Comments

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