Potentially, loss should be even greater for these datasets than for others due to these dates having been derived from bone and organic materials (e.g., dung), which may progressively be lost to weathering and/or dissolution even if site sediments are not lost to erosion. Even after correction for taphonomic bias, the CUSA and SA datasets form curves typical of this phenomenon.

Given the progressive loss of materials through time, radiocarbon ages are expected to decrease nonlinearly with age from recent modes, producing a heavily right-skewed distribution ( 15, 16). One pattern that emerges from comparative analyses of these three regions is that the shape of the EB frequency distribution is distinct from those of the CUSA and SA ( Fig. Spacings are also taphonomically corrected ( SI Materials and Methods). Decline dates are estimated as occurring within the last mode before extinction. Taphonomic correction adjusts frequency distributions of radiocarbon dates to account for the loss of sedimentary contexts through time due to erosion and weathering. For histogram creation, we first apply a jackknife method to identify optimal binning parameters for histogram creation ( 14) to create frequency distributions of calibrated radiocarbon dates, which are then corrected for taphonomic bias following Surovell et al. This method produces both a likely date for extinction onset and an associated approximate 95% CI (confidence interval). Decline dates are identified as the time before extinction for which waiting times between dates begin to increase, implying population declines. For spacings, individual spacing values are regressed onto the midtime ( t i+1 + t i)/2 of the spacing interval using a generalized linear model with estimated breakpoints (representing times of onset of extinction). The key property of spacings is that mean spacings are inversely proportional to population levels. We use two analytical techniques to identify dates of initial megafaunal decline: for technical precision, an approach based on spacings (time lags between consecutive ordered dates), and, for simple understanding, an approach based on histograms of observed dates. Similar approaches using paleoecological proxy records as possible indicators of human arrival have been applied elsewhere, particularly on islands ( 2– 7). We intend this exercise to be both a direct test of the timing of extinction as proposed by Martin ( 1) and as an independent means of estimating the timing of human arrival to each region. In this paper, we use databases of radiocarbon dates on extinct megafauna from Eastern Beringia (EB), the contiguous United States (CUSA), and South America (SA) to estimate the timing of initial population declines that ultimately resulted in extinction. If we accept that premise as true, then as Martin argues, it is possible to assess the timing of human arrival independently of direct archaeological evidence by examining when megafaunal decline occurred across time and space. For areas south of Beringia, these estimates correspond well with the first significant evidence for human presence and are consistent with the predictions of the overkill hypothesis.Ĭentral to this hypothesis is the idea that small numbers of humans can have large ecological impacts and that those impacts should be directly observable in the paleontological record. For the contiguous United States, extinction began at ca. For the United States and South America, extinctions commenced considerably later but were closely spaced in time. Analyses of spacings and frequency distributions of radiocarbon dates for each region support the idea that the extinction event first commenced in Beringia, roughly 13,300–15,000 BP. To that end, we compiled lists of direct radiocarbon dates on paleontological specimens of extinct genera from North and South America with the expectation that the initial decline of extinct megafauna should correspond in time with the initial evidence for human colonization and that those declines should occur first in eastern Beringia, next in the contiguous United States, and last in South America. Following Martin, we propose the hypothesis that the timing of human arrival to the New World can be assessed by examining the ecological impacts of a small population of people on extinct Pleistocene megafauna.

Overkill definition