Levantine overkill: 1.5 million years of hunting down the body size distribution

Abstract

Multiple large-bodied species went extinct during the Pleistocene. Changing climates and/or human hunting are the main hypotheses used to explain these extinctions. We studied the causes of Pleistocene extinctions in the Southern Levant, and their subsequent effect on local hominin food spectra, by examining faunal remains in archaeological sites across the last 1.5 million years. We examined whether climate and climate changes, and/or human cultures, are associated with these declines. We recorded animal abundances published in the literature from 133 stratigraphic layers, across 58 Pleistocene and Early Holocene archaeological sites, in the Southern Levant. We used linear regressions and mixed models to assess the weighted mean mass of faunal assemblages through time and whether it was associated with temperature, paleorainfall, or paleoenvironment (C3 vs. C4 vegetation). We found that weighted mean body mass declined log-linearly through time. Mean hunted animal masses 10,500 years ago, were only 1.7% of those 1.5 million years ago. Neither body size at any period, nor size change from one layer to the next, were related to global temperature or to temperature changes. Throughout the Pleistocene, new human lineages hunted significantly smaller prey than the preceding ones. This suggests that humans extirpated megafauna throughout the Pleistocene, and when the largest species were depleted the next-largest were targeted. Technological advancements likely enabled subsequent human lineages to effectively hunt smaller prey replacing larger species that were hunted to extinction or until they became exceedingly rare.

Introduction

Throughout the Pleistocene large mammals (and other vertebrates) went extinct while smaller ones mostly survived (Faith et al., 2018; Klein, 1984; Koch and Barnosky, 2006; Smith et al., 2018). Whether these size-selective extinctions were caused by Homo sapiens (Martin, 1984; Sandom et al., 2014; Smith et al., 2019a, b), sudden climatic events (Cooper et al., 2015, 2021; Louys and Roberts, 2020; Meltzer, 2020; Stewart et al., 2021), or both (Koch and Barnosky, 2006; Nogués-Bravo et al., 2008), has been debated for decades. Most studies, however, focused on the end of the Pleistocene and early Holocene, while earlier extinctions are seldom studied.

After recovering from the Cretaceous-Paleogene extinction, mean body size of the largest terrestrial mammals remained remarkably stable throughout the Cenozoic but declined in Africa during the Quaternary (Smith et al., 2018). This decline coincided with the emergence of the genus Homo. Smith et al. (2018) found that climate did not increase extinction risk for large mammals at any point during the Cenozoic (Smith et al., 2018). While the Late Pleistocene saw the extinctions of many large mammals globally, in Africa, mean mammal body mass started declining before the start of the Late Quaternary, possibly as a result of long-term hunting by early humans and finally by Homo sapiens (Smith et al., 2018). Some studies, however, propose that Early and Mid-Pleistocene extinctions are correlated with changes in climate or vegetation (Faith et al., 2018; Louys and Roberts, 2020; Potts et al., 2020). Large mammal extinctions started in Africa before the emergence of Homo sapiens but occurred in Australia and the New World following modern human colonization (Faith et al., 2018; Smith et al., 2019a, b; Wroe et al., 2013). However, declines in the abundance of large animals associated with early hominins have also been reported in European localities, spanning over one million years (Rodríguez et al., 2011; Yravedra, 2001).

The Southern Levant, situated between modern day southern Syria via Israel to Sinai, has a spatiotemporally dense and continuous Paleolithic archaeological record offering a unique opportunity to detect faunal changes, including those predating the appearance of Homo sapiens (Bar-Yosef, 1980; Stutz, 2014). It is thus a suitable model to test long-term changes in the body mass of mammalian assemblages, in view of paleoclimates and changing human lineages, to decipher whether climate and/or humans are responsible for animal body size declines. The excellent archaeological record can further illuminate whether size declines are observed since hominins first colonized the region, or whether they start with the emergence of Homo sapiens (Louys et al., 2021), or are concentrated in the last glacial and its aftermath. We tested whether the size, and size changes, in hominin prey through the Pleistocene and early Holocene were related to time, the prevailing human lineages and cultures, paleoenvironment, and temperatures.