Scientists used 3D geometric morphometric techniques to superimpose the configuations of 15 landmarks measured on skulls of different species in order to measure the degree of variation between them. Pictured are a Neanderthal landmark configuration (red) superimposed over a modern human one (green). The data used included Neanderthal fossils, Upper Paleolithic European modern human fossils, and recent human populations, as well as data from living African apes and Old World Monkeys. Over 1000 skulls were included.