Figure 8.

Mean (+95% confidence interval) predicted grooming time (indexed as per cent of daytime devoted to social grooming) for all fossil hominin species, estimated from individual cranial volumes (for details see the electronic supplementary material). Cranial volumes for H. erectus and H. neanderthalensis are reduced by 15% to take account of their larger visual systems [76]. AMH: fossil anatomically modern human (Homo sapiens). Solid line: mean predicted grooming time for early Homo populations; dashed line: observed mean grooming time for chimpanzees (from electronic supplementary material, table S1); dotted line: estimated social time for early Homo if the extra time above that for australopithecines (mean = 11%) involved laughter-based chorusing (allowing time savings of 67%). Hatched bars indicate the uplift in grooming time above the australopithecine baseline required by later hominin taxa. Note that estimates of hominin time budgets indicate that all would have been at their limit with no spare capacity [83].