Hominin brain size increase has emerged from within-species encephalization

Area of Science:

• Paleoanthropology
• Evolutionary Biology
• Human Evolution

Background:

• Rapid brain size increase is a hallmark of human evolution.
• Previous studies have not fully disentangled within-species vs. between-species changes or accounted for body size effects.
• Understanding these factors is crucial for reconstructing hominin evolutionary patterns.

Purpose of the Study:

• To differentiate the contributions of within-species and between-species changes to relative brain size increase in hominins.
• To incorporate the effect of body size into analyses of hominin brain evolution.
• To investigate the temporal patterns and potential acceleration of relative brain size increase.

Main Methods:

• Phylogenetic comparative methods applied to paleoanthropological data.
• Analysis of relative brain size changes across approximately 7 million years of hominin evolution.
• Statistical modeling to account for body mass and time effects.

Main Results:

• Relative brain size increase primarily resulted from changes within individual hominin species.
• Inter-species variation in brain size, after accounting for within-species trends, correlates with body mass but not time.
• A significant acceleration of the within-species trend was observed in more recent hominin lineages.

Conclusions:

• Within-species evolution, rather than solely species divergence, is the main driver of increased relative brain size in hominins.
• Body mass is a significant factor influencing species-level brain size variation.
• Human brain evolution has not been constant but has accelerated in its rate over evolutionary time.