Climate change, genetics or human choice: why were the shells of mankind's earliest ornament larger in the pleistocene than in the holocene?
BACKGROUND
The southern African tick shell, Nassarius kraussianus (Dunker, 1846), has been identified as being the earliest known ornamental object used by human beings. Shell beads dated from approximately 75,000 years ago (Pleistocene era) were found in a cave located on South Africa's south coast. Beads made from N. kraussianus shells have also been found in deposits in this region dating from the beginning of the Holocene era (<10,000 years ago). These younger shells were significantly smaller, a phenomenon that has been attributed to a change in human preference.


RESULTS
We investigated two alternative hypotheses explaining the difference in shell size: a) N. kraussianus comprises at least two genetic lineages that differ in size; b) the difference in shell size is due to phenotypic plasticity and is a function of environmental conditions. To test these hypotheses, we first reconstructed the species' phylogeographic history, and second, we measured the shell sizes of extant individuals throughout South Africa. Although two genetic lineages were identified, the sharing of haplotypes between these suggests that there is no genetic basis for the size differences. Extant individuals from the cool temperate west coast had significantly larger shells than populations in the remainder of the country, suggesting that N. kraussianus grows to a larger size in colder water.


CONCLUSIONS
The decrease in fossil shell size from Pleistocene to Holocene was likely due to increased temperatures as a result of climate change at the beginning of the present interglacial period. We hypothesise that the sizes of N. kraussianus fossil shells can therefore serve as indicators of the climatic conditions that were prevalent in a particular region at the time when they were deposited. Moreover, N. kraussianus could serve as a biomonitor to study the impacts of future climate change on coastal biota in southern Africa.
Teske
Peter R
PR
Molecular Ecology and Systematics Group, Department of Botany, Rhodes University, Grahamstown, South Africa. P.Teske@ru.ac.za
Papadopoulos
Isabelle
I
McQuaid
Christopher D
CD
Newman
Brent K
BK
Barker
Nigel P
NP
D000825:Animal Structures / Q000737:chemistry; D000818:Animals; D002755:Choice Behavior; D016384:Consensus Sequence; D004272:DNA, Mitochondrial / Q000235:genetics; D004275:DNA, Ribosomal / Q000235:genetics; D003576:Electron Transport Complex IV / Q000235:genetics; D005580:Fossils; D005843:Geography; D049690:History, Ancient; D006801:Humans; D055498:Jewelry / Q000266:history*; D008928:Mitochondria / Q000201:enzymology; D012336:RNA, Ribosomal, 16S / Q000235:genetics; D012623:Seawater; D013019:South Africa
D004272:DNA, Mitochondrial; D004275:DNA, Ribosomal; D012336:RNA, Ribosomal, 16S; D003576:Electron Transport Complex IV