Kraft and 11 co-authors, reported in late 2021 that they have found evidence that suggests early humans gained an energy budget by increasing their rate of energy acquisition, not by taking advantage of adaptive strategies. In their paper published in the journal Science, they describe their study of energy expenditure versus energy intake in early humans.
In this new effort, the scientists noted that humans long ago diverged in significant ways from the other great apes. They wondered how this happened and decided to look at energy intake and expenditure. People and other animals have to put in a certain amount of work (expenditure) to receive an energy intake. Climbing a tree to fetch a banana is a simple example. The amount of energy required to climb a tree far outweighs the potential benefit of eating a single banana. But if a single person is able to throw down multiple bananas, then the overall energy intake may surpass the effort of climbing a tree a single time. To learn more about how energy intake and expenditure might have led to modern human characteristics, the researchers studied two groups of modern people—hunter gatherers in Tanzania and forager-horticulturalists in a Bolivian rain forest.
In looking at both groups, they found that both spent more energy on subsistence but also achieved energy efficiencies compared to modern great apes. This was despite the fact that bipedalism and the use of tools are known to decrease the amount of energy expended to obtain food. The result was the acquisition of more food at a much higher rate than the great apes. The researchers suggest this indicates that humans are not cost economizers but are instead creatures that operate in high throughput ways that lead to large payoffs. They suggest that diverging from the great apes in such a way led to the production of so much food that early humans had much more time to do other things, such as socialize. They further suggest that such socializing, combined with the organizational activities involved in obtaining food led to the development of larger brains and from there, other uniquely human attributes.
Efficiency leads to leisure
Humans are animals—merely another lineage of great apes. However, we have diverged in significant ways from our ape cousins and we are perennially interested in how this happened. Kraft et al. looked at energy intake and expenditure in modern hunter-gatherer societies and great apes. They found that we do not spend less energy while foraging or farming, but we do acquire more energy and at a faster rate than our ape cousins. This difference may have allowed our ancestors to spend more time in contexts that facilitated social learning and cultural development. —SNV
Relative to other great apes, humans have large brains, long life spans, higher fertility and larger neonates, and protracted periods of childhood dependency and development. Although these traits constitute the unique human life history that underlies the ecological success of our species, they also require human adults to meet extraordinarily high energetic demands. Determining how human subsistence strategies have met such extreme energy needs, given time and energy expenditure constraints, is thus key to understanding the origins of derived human traits.
RATIONALE
Two major transitions in hominin subsistence strategies are thought to have elevated energy capture: (i) the development of hunting and gathering ~2.5 million years ago, which coincided with brain enlargement and extended postnatal growth, and (ii) the rise of agriculture ~12,000 years ago, which was accompanied by substantial increases in fertility and population densities. These transitions are associated with the exploitation of novel food sources, but it is not clear how the energy and time budgets of early human foragers and farmers shifted to accommodate expensive traits.
Some evolutionary reconstructions contend that economical locomotion, cooperation, the use of sophisticated tools, and eventually agriculture increased energy efficiency (i.e., energy gained versus energy spent), beyond that of other great apes. Alternatively, unique human subsistence strategies may reduce time and improve yield, increasing return rates (i.e., energy gained versus time spent).
To test these ideas, we compared subsistence costs (energy and time) and energy acquisition among wild orangutans, gorillas, and chimpanzees with high-resolution data on total energy expenditure, food acquisition, and time allocation, collected among Tanzanian hunter-gatherers (Hadza) and Bolivian forager-horticulturalists (Tsimane). Both populations actively forage (hunt, gather), whereas the Tsimane also practice slash-and-burn horticulture, which permits exploration of further changes in the energetics of subsistence associated with farming. We also assembled a global subsistence energetics database of contemporary hunter-gatherers and horticulturalists.
RESULTS
Relative to other great apes, human hunter-gatherers and horticulturalists spend more energy daily on subsistence, and they achieve similar energy efficiencies despite having more economical locomotion and using sophisticated technologies. In contrast, humans attain much greater return rates, spending less time on subsistence while acquiring more energy per hour. Further, horticulture is associated with higher return rates than hunting and gathering, despite minimal differences in the amount of time devoted to subsistence. Findings from our detailed study of the Hadza and Tsimane were consistent with those from the larger cross-cultural database of subsistence-level societies. Together, these results support prior evidence that the adoption of farming could have been motivated by greater gains per time spent working, and refute the notion that farming lifestyles are necessarily associated with increased labor time.
CONCLUSION
The authors concluded by revising our understanding of human energetics and evolution, indicating that humans afford expanded energy budgets primarily by increasing rates of energy acquisition, and not through energy-saving adaptations (such as economical bipedalism or sophisticated tool use) that decrease overall costs. Relative to other great apes, human subsistence strategies are characterized by high-intensity, high-cost extractive activities and expanded day ranges that provide more calories in less time.
These results suggest that energy gained from improvements in efficiency throughout human evolution were primarily channeled toward further increasing foraging intensity rather than reducing the energetic costs of subsistence. Greater energetic gains per unit time are the reward for humans’ intense and behaviorally sophisticated subsistence strategies. Humans’ high-cost but high-return strategy is ecologically risky, and we argue that it was only possible in the context of increased cooperation, intergenerational food sharing, and a division of labor.
We contend that the time saved by human subsistence strategies provided more leisure time for social interaction and social learning in central-place locations, which is critical for cumulative cultural evolution.
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