Climate change, genetics or human choice: why were the shells of mankind's earliest ornament larger in the pleistocene than in the holocene?

Abstract

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.

Methodology/principal findings: 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.

Conclusion/significance: 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.

Figure 1. Sampling localities.

Sampling localities (A–J) from which specimens of Nassarius kraussianus were collected. The sample sizes were: A = 350, B = 104, C = 132, D = 112, E = 47, F = 183, G = 83, H = 54, I = 71, J = 94. Blue circles indicate localities dominated by haplotypes present in the western range of the species' distribution, and red circles indicate sites where haplotypes of the eastern lineage were mostly found. The location of Blombos Cave (where N. kraussianus fossils were found that were worn as beads by human beings during the Middle and Late Stone Age) is indicated.

Figure 2. Network of Nassarius kraussianus haplotypes.

A statistical parsimony haplotype network constructed from combined partial mtDNA COI and 16S rDNA sequences of N. kraussianus. Haplotypes shown in blue were mostly found in the western portion of the species' geographical range (localities A–F; Fig. 1) and those in red were mostly found in the eastern portion of its range (localities G–J). Haplotypes are represented as ovals, with sizes being proportional to a haplotype's frequency. Letters within ovals indicate in which sampling localities a particular haplotype was found and correspond to the letters in Fig. 1. Small white circles are interior node haplotypes not present in the samples. Grey areas around groups of haplotypes depict two-step nested clades.

Figure 3. Shell sizes of Nassarius kraussianus.

Shell sizes of the Middle Stone Age (MSA) specimens of N. kraussianus from Blombos Cave and the largest 75% of the shells of each of ten extant South African populations (A–J). Vertical bars represent means and whiskers are upper 95% confidence limits. Horizontal lines represent the shell size of the largest individual from each population. The insert shows how shell size was measured.