Energetics and the Origin of Modern Humans

Abstract

Energy investment in reproduction is of primary evolutionary importance, and changes in energy budgets due to shifts in anatomy, activity, or development can affect reproductive rates. The energetic consequences of differences between archaic and modern humans can therefore provide insight into how modern humans were ultimately able to out-compete their archaic counterparts. Here we investigate three major areas of differentiation between archaic and modern humans that likely had significant energetic ramifications, and for which reliable fossil evidence exists: body size, locomotor anatomy, and juvenile growth. Using these data, we explore potential energetic disparities between archaic and modern humans in terms of proportions of energy budgets of reproductively active adults dedicated to maintenance (basic life functions and physical activity) versus offspring production. In comparison to modern humans, adult archaic humans would have had higher maintenance costs corresponding to their larger bodies (higher resting and activity energy requirements) and shorter lower limbs (reduced locomotor efficiency). Moreover, faster growth rates in archaic juveniles could have meant slightly higher daily energy requirements for dependent offspring in archaic versus modern humans. Overall, assuming similar caloric returns from foraging in both groups, modern humans' smaller bodies, longer lower limbs, and slower development may have allowed them to produce a greater number of viable offspring over the reproductive lifespan than could archaic humans, possibly at a ratio of ˜6:5. These reproductive differences may have facilitated higher population densities among modern humans, likely contributing to the rise of modern humans and disappearance of archaic forms.