The energetic significance of cooking

493

372

Neuroscientists have become used to a number of “facts” about the human brain: It has 100 billion neurons and 10- to 50-fold more glial cells; it is the largest-than-expected for its body among primates and mammals in general, and therefore the most cognitively able; it consumes an outstanding 20% of the total body energy budget despite representing only 2% of body mass because of an increased metabolic need of its neurons; and it is endowed with an overdeveloped cerebral cortex, the largest compared with brain size. These facts led to the widespread notion that the human brain is literally extraordinary: an outlier among mammalian brains, defying evolutionary rules that apply to other species, with a uniqueness seemingly necessary to justify the superior cognitive abilities of humans over mammals with even larger brains. These facts, with deep implications for neurophysiology and evolutionary biology, are not grounded on solid evidence or sound assumptions, however. Our recent development of a method that allows rapid and reliable quantification of the numbers of cells that compose the whole brain has provided a means to verify these facts. Here, I review this recent evidence and argue that, with 86 billion neurons and just as many nonneuronal cells, the human brain is a scaled-up primate brain in its cellular composition and metabolic cost, with a relatively enlarged cerebral cortex that does not have a relatively larger number of brain neurons yet is remarkable in its cognitive abilities and metabolism simply because of its extremely large number of neurons.

Section: Cost Of Being Humanmentioning

confidence: 99%

The remarkable, yet not extraordinary, human brain as a scaled-up primate brain and its associated cost

Herculano‐Houzel

2012

493

372

“…Although stone tool use and manufacture were regular activities from at least 2.6 million y ago (1), the timing of the human control of fire is a controversial issue (2), with some claims for regular fire use by early hominins in Africa at ∼1.6 million y ago (3)(4)(5). Longer chronologies for the use of fire include Wrangham's recent hypothesis that fire was a central evolutionary force toward larger human brains (6)(7)(8)(9): eating cooked foods made early hominin digestion easier, and the energy formerly spent on digestion was freed up, enabling their energy-expensive brains to grow. Using fire to prepare food made early humans move away from the former feed-as-you-go-and-eat-raw-food strategy and toward the sharing of cooked foods around fires, which became attractive locations for increased social interaction between individuals.…”

mentioning

confidence: 99%

On the earliest evidence for habitual use of fire in Europe

Roebroeks

Villa

2011

569

317

The timing of the human control of fire is a hotly debated issue, with claims for regular fire use by early hominins in Africa at ∼1.6 million y ago. These claims are not uncontested, but most archaeologists would agree that the colonization of areas outside Africa, especially of regions such as Europe where temperatures at time dropped below freezing, was indeed tied to the use of fire. Our review of the European evidence suggests that early hominins moved into northern latitudes without the habitual use of fire. It was only much later, from ∼300,000 to 400,000 y ago onward, that fire became a significant part of the hominin technological repertoire. It is also from the second half of the Middle Pleistocene onward that we can observe spectacular cases of Neandertal pyrotechnological knowledge in the production of hafting materials. The increase in the number of sites with good evidence of fire throughout the Late Pleistocene shows that European Neandertals had fire management not unlike that documented for Upper Paleolithic groups.human evolution | Paleolithic archeology | fireplaces | hafting adhesives T he emergence of stone tool manufacture and the control of fire are undoubtedly the two most significant events in the technological evolution of early humans. Although stone tool use and manufacture were regular activities from at least 2.6 million y ago (1), the timing of the human control of fire is a controversial issue (2), with some claims for regular fire use by early hominins in Africa at ∼1.6 million y ago (3-5). Longer chronologies for the use of fire include Wrangham's recent hypothesis that fire was a central evolutionary force toward larger human brains (6-9): eating cooked foods made early hominin digestion easier, and the energy formerly spent on digestion was freed up, enabling their energy-expensive brains to grow. Using fire to prepare food made early humans move away from the former feed-as-you-go-and-eat-raw-food strategy and toward the sharing of cooked foods around fires, which became attractive locations for increased social interaction between individuals. Wrangham situates these developments around the time of the emergence of Homo erectus, approximately two million y ago. Most archeologists would agree that the colonization of areas outside of Africa, especially of regions such as Europe where temperatures at time dropped below freezing (10), was tied to the use of fire to bridge gaps in the energy budget and in resource availability during winter (11). For much later periods, a greater control and more extensive use of fire is seen by some (12, 13) as one of the behavioral innovations that emerged in Africa among modern humans, favoring their spread throughout the world and their eventual evolutionary success.How strong are these hypotheses? To what extent are they backed up by archeological data on hominin use of fire? Our focus in this review is on Europe, an area of the Old World for which we have a rich record with a large number of sampling points distributed in time and space, w...

“…The daily energetic cost (measured from the basal metabolic rate) of mammalian bodies scales across species as a power function of M BD with an average exponent of 0.75 (21-23). The greater the caloric need of a species, the greater the time that must be spent on feeding, which is modulated by factors such as food availability (24), time required for ingestion [which varies depending on the food composition and the structure and capacity of the oral cavity (25)], the digestive rate of the gastrointestinal system (26), and the caloric income of the diet (27,28).Despite the obvious metabolic costs entailed by increasing brain mass (M BR ) and M BD , it remains to be determined whether brain and body size are indeed metabolically limiting in a way that would be physiologically relevant and constraining for primate evolution. The energetic viability of a nonhibernating primate species depends on the balance between the energy requirements associated to its M BD and its brain size, and its daily caloric intake (E IN ) during the hours available for eating.…”

mentioning

confidence: 99%

Metabolic constraint imposes tradeoff between body size and number of brain neurons in human evolution

Fonseca-Azevedo

Herculano‐Houzel

2012

154

120

Despite a general trend for larger mammals to have larger brains, humans are the primates with the largest brain and number of neurons, but not the largest body mass. Why are great apes, the largest primates, not also those endowed with the largest brains? Recently, we showed that the energetic cost of the brain is a linear function of its numbers of neurons. Here we show that metabolic limitations that result from the number of hours available for feeding and the low caloric yield of raw foods impose a tradeoff between body size and number of brain neurons, which explains the small brain size of great apes compared with their large body size. This limitation was probably overcome in Homo erectus with the shift to a cooked diet. Absent the requirement to spend most available hours of the day feeding, the combination of newly freed time and a large number of brain neurons affordable on a cooked diet may thus have been a major positive driving force to the rapid increased in brain size in human evolution.encephalization | expensive tissue hypothesis | brain metabolism T he human brain is a linearly scaled-up primate brain in its relationship between brain size and number of neurons (1), having evolved while being subjected to the same cellular scaling rules that apply to primates as a whole (2, 3), including great apes (4). With the largest brain among primates, humans thus have the largest number of neurons among these and possibly all mammals (5), a number that we estimate to be close to three times larger than in gorillas and orangutans, owners of the next largest brains among extant primates (4). We are not outstanding primates, on the contrary, in body size: gorillas can grow to be three times larger than humans. This discrepancy between body and brain size led to the predominant view that encephalization (that is, a larger brain size than expected for body size) is the main characteristic that sets humans apart from other primates and mammals as a whole (6). Although a relatively large brain compared with body size is expected to bring cognitive advantages (6), and despite the well-documented evidence for an increase in brain size during human evolution (7), there is still no consensus on what mechanisms or reasons led to brain enlargement in the Homo lineage.Why are the largest primates not those endowed with the largest brains as well? Rather than evidence that humans are an exception among primates, we consider this disparity to be a clue that, in primate evolution, developing a very large body and a very large brain have been mutually excluding strategies, probably because of metabolic reasons (4, 8). The brain is the third most energy-expensive organ in the human body, ranking in total organ metabolic cost below only skeletal muscle and liver (9). Accordingly, several studies have suggested that the main constraints to increasing primate brain size in evolution are metabolic in nature (10-18). The human brain, in particular, has come to cost ∼20% of the total body resting metabolic rate, even though it ...