Ancient DNA analyses exclude humans as the driving force behind late Pleistocene musk ox (Ovibos moschatus) population dynamics

Abstract

The causes of the late Pleistocene megafaunal extinctions are poorly understood. Different lines of evidence point to climate change, the arrival of humans, or a combination of these events as the trigger. Although many species went extinct, others, such as caribou and bison, survived to the present. The musk ox has an intermediate story: relatively abundant during the Pleistocene, it is now restricted to Greenland and the Arctic Archipelago. In this study, we use ancient DNA sequences, temporally unbiased summary statistics, and Bayesian analytical techniques to infer musk ox population dynamics throughout the late Pleistocene and Holocene. Our results reveal that musk ox genetic diversity was much higher during the Pleistocene than at present, and has undergone several expansions and contractions over the past 60,000 years. Northeast Siberia was of key importance, as it was the geographic origin of all samples studied and held a large diverse population until local extinction at approximately 45,000 radiocarbon years before present ((14)C YBP). Subsequently, musk ox genetic diversity reincreased at ca. 30,000 (14)C YBP, recontracted at ca. 18,000 (14)C YBP, and finally recovered in the middle Holocene. The arrival of humans into relevant areas of the musk ox range did not affect their mitochondrial diversity, and both musk ox and humans expanded into Greenland concomitantly. Thus, their population dynamics are better explained by a nonanthropogenic cause (for example, environmental change), a hypothesis supported by historic observations on the sensitivity of the species to both climatic warming and fluctuations.

Fig. 1.

Bayesian Skyride derived from the geospatial analysis of 135 ancient and 14 modern musk ox mtDNA control region sequences. The x axis is in units of radiocarbon years in the past, and the y axis is equal to Ne*τ (the product of the effective population size and the generation length in radiocarbon years). The colors represent the relative contribution of each of the geolocations to the overall estimated effective population size. Colors indicate geographical origins of the samples: blue, Greenland; red, Northeast Siberia; orange, Taimyr; green, Urals; light blue, Canada. The bar graph shows the number of radiocarbon-dated samples in bins of 2,000 radiocarbon years. No relation is apparent between the absolute number of samples and the estimated effective population size or transition time.

Fig. 2.

Maximum clade credibility tree summarized from the geospatial Bayesian analysis of mtDNA control region sequence of 135 ancient and 14 modern musk ox, including the postmortem DNA damage model. The colors of the branches correspond to their probable geographic location as calculated using the geospatial analysis. Labels on the tips of the branches correspond to the age of the sample. Bayesian posterior probabilities above 0.9 are shown (color code as in Fig. 1).

Fig. 3.

Temporal distribution of the 210 ¹⁴C dated fossil musk ox used in the present study. For a number of individuals, indicated by the extended timelines, finite dates could not be calculated. Samples are grouped in five major regions: Northeast Siberia, Taimyr, Urals, Canada, and Greenland. The approximate earliest presence of anatomically modern humans in the regions is added for comparison. Radiocarbon accession numbers associated with specific specimens are given in Table S1.

Fig. 4.

Geographic origin of musk ox samples that yielded aDNA sequences.