The evolution of mammalian brain size.

Relative brain size has long been considered a reflection of cognitive capacities and has played a fundamental role in developing core theories in the life sciences. Yet, the notion that relative brain size validly represents selection on brain size relies on the untested assumptions that brain-body allometry is restrained to a stable scaling relationship across species and that any deviation from this slope is due to selection on brain size. Using the largest fossil and extant dataset yet assembled, we find that shifts in allometric slope underpin major transitions in mammalian evolution and are often primarily characterized by marked changes in body size. Our results reveal that the largest-brained mammals achieved large relative brain sizes by highly divergent paths. These findings prompt a reevaluation of the traditional paradigm of relative brain size and open new opportunities to improve our understanding of the genetic and developmental mechanisms that influence brain size.

Age-related variation in limb bone diaphyseal structure among Inuit foragers from Point Hope, northern Alaska.

UNLABELLED: Age-related deterioration of limb bone diaphyseal structure is documented among precontact Inuit foragers from northern Alaska. These findings challenge the concept that bone loss and fracture susceptibility among modern Inuit stem from their transition away from a physically demanding traditional lifestyle toward a more sedentary Western lifestyle. INTRODUCTION: Skeletal fragility is rare among foragers and other traditional-living societies, likely due to their high physical activity levels. Among modern Inuit, however, severe bone loss and fractures are apparently common. This is possibly because of recent Western influences and increasing sedentism. To determine whether compromised bone structure and strength among the Inuit are indeed aberrant for a traditional-living group, data were collected on age-related variation in limb bone diaphyseal structure from a group predating Western influences. METHODS: Skeletons of 184 adults were analyzed from the Point Hope archaeological site. Mid-diaphyseal structure was measured in the humerus, radius, ulna, femur, and tibia using CT. Structural differences were assessed between young, middle-aged, and old individuals. RESULTS: In all bones examined, both females and males exhibited significant age-related reductions in bone quantity. With few exceptions, total bone (periosteal) area did not significantly increase between young and old age in either sex, nor did geometric components of bending rigidity (second moments of area). CONCLUSIONS: While the physically demanding lifestyles of certain traditional-living groups may protect against bone loss and fracture susceptibility, this is not the case among the Inuit. It remains possible, however, that Western characteristics of the modern Inuit lifestyle exacerbate age-related skeletal deterioration.