Stature, body mass, and brain size: A two-million-year odyssey

Physical size has been critical in the evolutionary success of the genus Homo over the past 2.4 million-years. An acceleration in the expansion of savannah grasslands in Africa from 1.6 Ma to 1.2 Ma witnessed concomitant increases in physical stature (150-170 cm), weight (50-70 kg), and brain size (750-900 cm3). With the onset of 100,000 year Middle Pleistocene glacial cycles (“ice ages”) some 780,000 years ago, large-bodied Homo groups had reached modern size and had successfully dispersed from equatorial Africa, Central, and Southeast Asia to high-latitude localities in Atlantic Europe and North East Asia. While there is support for incursions of multiple Homo lineages to West Asia and Continental Europe at this time, data does not favour a persistence of Homo erectus beyond ∼400,000 years ago in Africa, west and Central Asia, and Europe. Novel Middle Pleistocene Homo forms (780,000-400,000 years) may not have been substantially taller (150-170 cm) than earlier Homo (1.6 Ma-800,000 years), yet brain size exceeded 1000 cm3 and body mass approached 80 kg in some males. Later Pleistocene Homo (400,000-138,000 years) were 'massive' in their height (160-190 cm) and mass (70-90 kg) and consistently exceed recent humans. Relative brain size exceeds earlier Homo, yet is substantially lower than in final glacial H. sapiens and Homo neanderthalensis. A final leap in absolute and relative brain size in Homo (300,000-138,000 years) occurred independent of any observed increase in body mass and implies a different selective mediator to that operating on brain size increases observed in earlier Homo.

► Body size (stature, mass) and brain size in the earliest members of the genus Homo were ∼40% greater than in earlier and contemporary australopiths. ► Available data supports a significant increase in body size and absolute brain size in later Lower Pleistocene Homo (1.4-1.2 Ma) with a further significant increase during the early Middle Pleistocene (800-500 ka). ► Absolute brain size increase in later Middle-Upper Pleistocene Neandertals and Homo sapiens cannot be tied to any obvious increases in absolute body size (stature, mass) and instead suggest selection for increased relative brain size. ► The onset of the last glacial cycle in MIS 5 (116-70 ka) witnessed further significant increases in absolute body mass and brain size, but likely not stature. Terminal Pleistocene (40 ka-9800 cal BP) Homo from Eurasia sample some of the largest individuals whom have ever lived.