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The movement and dispersal of human populations during the Pleistocene is a complex question, and one that has been addressed extensively in the literature. During Pleistocene glacial periods, landscapes and coastlines altered dramatically, with old routes closing and new routes opening. One of the best understood of these glacial periods, the Last Glacial Maximum (between 22 and 18 ka), is of particular interest with regards to the colonization of the New World. Despite decades of study, archaeologists still do not fully understand the timing and tempo of that process. This study contributes to this discussion by presenting an estimation of habitable land area in the New World at the LGM when sea level was 120 m lower than present day using a previously published method for mapping ancient coastlines and calculating land areas in Google Earth. Areas defined as uninhabitable include those areas covered by ice sheets, glaciers, and ice fields, and areas of extreme aridity. Combined habitable land areas of North and South America are estimated as 34,149,094.81 km2. Added to data from the previous study, a new estimate of global habitable land area at the LGM is calculated to be 111,108,807.21 km2. Data on paleocoastlines are then used to predict locations where archaeological traces of early human dispersals may potentially be found.

The estimation of human population size during the Pleistocene is complex, and one which has been dealt with extensively in the literature. However, because many of these previous estimations are based in part on archaeological site distributions, they are more a reflection of present-day geography than of what the Earth looked like in the past. We address this issue by calculating an estimation of habitable land area during the Last Glacial Maximum (between 22 and 19 kya) when sea level was 120 m lower than today using the polygon creation function in Google Earth. We then subtract areas of land that were likely uninhabitable during the LGM – either due to glacier cover, extreme aridity, elevation, or areas at high latitudes. From this, the combined habitable land areas of Eurasia, Africa and the Australian landmass are estimated as 76,959,712.4 km 2 . This estimation is then coupled with population density data for medium-to large-bodied carnivores, and ethnographic population density data for hunter–gatherers culled from the literature. Total human census population size in the Old World during the Last Glacial Maximum is estimated at 2,117,000–2,955,000 based on carnivore densities and 3,046,000–8,307,000 for hunter–gatherer densities.

The sequencing of the Neanderthal genome answered once and for all the question of whether these hominins played a role in the origins of modern humans—they did, and a majority of humans alive today retain a small portion of Neanderthal genes. This finding rejects the strictest versions of the Recent African Origin model and has been celebrated by supporters of Multiregional Evolution (MRE). However, we argue that MRE can also be rejected and that other, intermediate, models of modern human origins better represent the means by which modern humans became the only extant human species. We argue this because we reject one of the major tenets of MRE: global gene flow that prevents cladogenesis from occurring. First, using reconstructions of Pleistocene hominin census size, we maintain that populations were neither large nor dense enough to result in such high levels of gene flow across the Old World. Second, we use mammalian divergence and hybridization data to show that the emergence of Homo is recent enoug...