R E V I E W Open Access
The evolutionary roles of nutrition selection
and dietary quality in the human brain size
Roberto Carlos Burini
and William R. Leonard
Background: Humans and other primates have evolved particular morphological and biological traits (e.g., larger
brains, slower growth, longer-lived offspring) that distinguish them from most other mammals. The evolution of
many distinctive human characteristics, such as our large brain sizes, reduced gut sizes, and high activity budgets,
suggest major energetic and dietary shifts.
Main body: Over the course of the last three million years, hominin brain sizes tripled. It is often taken for granted
that the benefit of a larger brain is an increase in “intelligence”that makes us stand out among other mammals,
including our nearest relatives, the primates. In the case of humans, brain expansion was associated with changes
in diet, foraging, and energy metabolism. The first marked expansion occurred with the appearance of the genus
Homo. Improved diet quality, allomaternal subsidies, cognitive buffering [by earlier weaning and longer juvenile
periods], reduced costs for locomotion and by cooperative behavior, and reduced allocation to production, all
operated simultaneously, thus enabling the extraordinary brain enlargement in our lineage.
Conclusion: It appears that major expansion of brain size in the human lineage is the product of synergistically
interacting dietary/nutritional and social forces. Although dietary change was not being the sole force responsible
for the evolution of large brain size, the exploitation of high-quality foods likely fueled the energetic costs of larger
brains and necessitated more complex behaviors that would have selected for greater brain size.
Keywords: Human brain evolution, Human encephalization, Evolutionary foraging, High-quality diet
Humans share a common ancestor with the chimpanzee
and bonobo that likely existed in East Africa some 6 to 7
million years ago (mya) [1,2]. The earliest unambiguous
fossil hominins, such as Australopithecus anamensis and
A. afarensis, were bipedal, but still retained some aspects
of arboreal lifestyles, as demonstrated by a combination
of long forelimbs, curving phalanges, and barrel-shaped
thorax . With the emergence of the genus Homo at
2.0 mya (H. ergaster/erectus), there was a marked in-
crease in body size, mainly in females, who almost
double in size compared with Australopithecine . The
oldest fossil evidence for Homo sapiens is from Southern
and Eastern Africa, dating to about 160–250 kya [6,7].
With the emergence of H. erectus, the human lineage
has experienced remarkable morphological and physio-
logical changes, including (a) marked increases in both
brain and body size, (b) the evolution of human-like
body proportions, (c) major reductions of posterior
tooth size and craniofacial robusticity, and (d) reduction
of the gut [8,9].
The most important of these has been our high levels of
encephalization (large brain to body mass). Since the
emergence of early hominins, brain volume tripled over
the course of human evolution. It is often taken for
granted that the benefit of a larger brain is an increase in
“intelligence”that makes us stand out among other mam-
mals, including our nearest relatives, the primates .
The neocortex volume has grown out of all proportion
to the rest of the brain during the course of primate evo-
lution. In small animals, the volume of the neocortex in-
creased from only 16% of the volume of the whole brain,
* Correspondence: firstname.lastname@example.org
Department of Public Health, UNESP Medical School, Botucatu, SP 18.618-970, Brazil
Full list of author information is available at the end of the article
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International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and
reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to
the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver
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Burini and Leonard Nutrire (2018) 43:19
to 74% in Hominoidea. In contrast, the relative volume of
the cerebellum remained constant at 13% of whole brain
volume, regardless of the absolute size of the brain [11,12].
When the volume of white matter is plotted against the
volume of gray matter, there is a remarkably tight relation-
ship between these two variables in all animals from the
pygmy shrew to the elephant. The volume of the white
matter approximately increases as the 4/3 power of the
volume of the gray matter. As the size of the neocortex
progressively evolved, so also did the volume occupied by
myelinated nerve fibers. Therefore, the data supported the
view that the neocortex increased in size and complexity
and there was necessarily a disproportionate increase in
the volume of brain devoted to “wiring”.
It is the frontal lobe volume that has increased out of all
proportion in humans. As the total brain volume increases,
more neural capacity becomes available in the frontal cor-
tex, especially in the frontal lobe. This seems to be associ-
ated with markedly improved basic cognitive abilities .
Selection for large brain size
Only with the emergence of the H. ergaster/erectus
lineage, i.e., an approximate of 4 to 5 mya after the split
from the chimpanzee lineage [14,15], did hominins take
the place of the largest brained animal. Hence, this pos-
ition was obtained only recently, in the last 1.8 mya. The
hominin brainy prominence pales in comparison to the
approximately 20 to 30 mya that some marine mammals
have been maintaining their large brains. Until the be-
ginning of the Pleistocene, dolphins were the largest
brained creatures on the planet, exceeding all primates,
including the hominins .
Because of their high costs, large brains are rare and are
achieved only when animals are under strong selection.
The trigger for brain expansion in marine mammals was
probably environmental, specifically the cooling of the
southern ocean . In the case of humans, there have
been two environmental periods associated with their two
major periods of brain expansion. The first marked in-
crease in hominid brain size occurred with bipedal run-
ning and correlates with the appearance of the genus
Homo (2.0 mya), when absolute brain size increased to an
average of 30% from H. habilis to the earliest African H.
In general sense, both episodes of human brain expan-
sion were triggered by environmental factors, specifically
foraging, therefore suggesting that diet may have been an
important factor in providing the nutritional basis for the
selection of larger brains .
The evolution of bipedalism has traditionally been related
to changes in the environment, including increasingly dry
conditions and the expansion of open habitats, a more
wooded and humid context . These changes in the en-
vironment presented new challenges to arboreal apes,
some of which started to forage the more scattered re-
sources of the mosaic landscape at ground level .
Trunk morphology of the Australopithecines and
modern great apes contrasts with that of modern
humans in having a conically shaped rib cage, flaring at
the waist. The inference is that Australopithecines likely
retained large ape-like guts, in contrast to reduced gut
sizes of H. sapiens. .
Gut size is associated with both the bulk and the di-
gestibility of food. Diets characterized by large quantities
of food of low digestibility require relatively large guts
characterized by voluminous and elaborated fermenting
chambers (the stomach and/or large intestine [colon]).
Conversely, higher quality, more easily digestible diets
require relatively smaller guts and are characterized by
simple stomachs, reduced colons, and proportionately
long small intestines (emphasizing absorption). An ex-
treme example of folivores is the cow whereas carnivores
typify the other pattern .
Differences in both diet composition and food process-
ing likely shaped the reductions in jaw and tooth size with
the advent of Homo . The Australopithecines were
characterized by large jaws, faces, and teeth (megadonty),
a morphology consistent with the consumption of lower
quality, fibrous foods. Recent evidence from dental micro-
wear and stable isotope analyses have confirmed the rela-
tively lower-quality diet for the Australopithecines,
particularly for the east African robust Australopithecines,
who appear to have consumed large amounts of grasses
and sedges [21,22]. Moreover, fruits are more ephemeral
and patchy in their distribution and often contained in a
protective casing, which may make the foraging niche of
frugivores relatively complex [20,23–25]. Therefore, as
folivores have a relatively easy foraging strategy that re-
quires little in the way of learning, primates eating leaves
could potentially start foraging fairly early on in life. It has
been shown that folivores tend to wean at an earlier age
compared with frugivores .
Tool use and foraging strategies
Changing environmental conditions in Plio-Pleistocene
Africa, involving a decline in plant productivity and an
increase in secondary biomass, would favor a shift in
diet in hominin species, either to specialization on lower
quality plant foods or routine access to meat [27,28].
Members of the genus Homo would be expected to have
had a higher-quality diet than those of the Australopith-
ecines. Their diet probably differed by the incorporation
of more underground storage organs (soft bulbs, tubers,
etc.) or the preferential consumption of animals. Meat
consumption by early Homo might also be inferred from
Burini and Leonard Nutrire (2018) 43:19 Page 2 of 9
polish on tools and by cutmarks on bones. The stone
tools appear before Homo, at 2.58 mya suggesting “...that
hunting and/or aggressive scavenging of large ungulate
carcasses may have been part of the behavioral reper-
toire of hominins”.
Tools allowed hunting and/or aggressive scavenging of
large ungulate carcasses would have improved the en-
ergy intake and the overall quality of the diet. Conse-
quently, there was gut reduction and cranium-dental
anatomy changes. It became possible to achieve the food
adequacy by eating less frequently and with reduced ali-
mentary bolus. By requiring less time to feed and pro-
vided with language, our ancestors have had more time
for social familiarity.
The other shift that likely occurred with greater hunting
and wider exploitation of animal resources among early
members of the genus Homo was an increase in ranging
behavior, activity budgets and total daily energy expend-
iture. Contemporary human hunter-gatherers move over
much larger areas than modern apes. A typical day ranges
for modern foragers average over 13 km/day, significantly
greater than those of chimps and gorillas whose ranges
are less than 2 km/day . Similarly, adjusting for body
mass, home range sizes for human foragers are some five
to six times the size of those of the apes [26,29]. These
larger territorial needs for humans appear to be associated
with larger daily energy budgets. Recent work by Pontzer
and colleagues [30,31] has shown that humans are indeed
the “high energy”apes. Using the doubly labeled water
(DLW) technique to quantify total daily energy expend-
iture (TEE; kcal/day), they have shown that humans have
systematically and significantly greater energy budgets
than chimpanzees, gorillas, or orangs. It appears likely that
the elevation in human total energy demands initially
emerged with the evolution of the Homo, in association
with a foraging regime that was exploiting more animal
material and required movement over larger areas than
was typical of earlier hominin species [29,32,33].
Factors driving brain size evolution
A series of important hypotheses has been advanced to
explain the origins of unusual human features and the
interrelationships between such variables as brain size
and diet, longevity and foraging strategy, social
organization, and the evolution of intelligence. The
search for plausible explanations for the factors driving
brain size evolution raised “Ecological and Social Brain
Hypothesis”which highlight ecological and social con-
texts . Both hypotheses are not exclusive . In
both hypotheses, dietary acquirements are included as
major environmental factors for selective brain enlarge-
ment. However, it is suggested that human ecological
and social intelligence evolved in the context of a dis-
tinctive foraging strategy, with strong benefits to
cooperation, selecting for greater social intelligence and
subsequently language skills . Specifically, the eco-
logical hypotheses theorize that brain size increased sim-
ply as a by-product of increasing body size.
The social brain hypothesis
The social brain hypothesis assumes that large brains
have been selected for specifically to provide the cogni-
tive basis for maintaining social cohesiveness through
time. Indices of social complexity (including group size
and the frequency of social play) correlate with relative
neocortex volume, in primates and a number of other
mammalian orders as well. There is a clear relationship
between social group size and neocortex volume across
primates [including modern humans], and cooperative
acquisition, sharing food and information, and educating
young people are all key elements of foraging .
In general sense, big neocortices are a primate special-
ity and, broadly speaking, the neocortex is a mammalian
evolutionary invention . Additionally, the fact that
the neocortex volume has grown out of all proportion to
the rest of the brain during the course of primate evolu-
tion constitutes the principal evidence in support of the
social brain hypothesis .
The ecological hypothesis
Across mammals, brain size scales to body mass in so that
as body size increases, so does the brain. As so, early H.
erectus marked an increase in both brain and body size
towards the evolution of human-like body proportions [8,
9]. Hence, ecological hypothesis has an assumption that
brain size increased simply as a by-product of increasing
body size, since the two do seem to be locked together in
a close allometric relationship .
This theory has been mainly criticized based on the fact
that brains are energetically much more expensive than
muscle or skeletal material [18,35]. Indeed, it can legitim-
ately be argued that rather than brain size being a conse-
quence of body size, larger body size might actually be a
consequence of the demand for a larger brain. This fol-
lows from the fact that the energy costs of living are not a
direct linear function of body mass, but rather increase
only as the 0.75 power of body mass (Kleiber’sLaw).
This means the bigger the body is, the less energy it needs
per kilogram of weight to sustain life, so larger animals
can afford less to have relatively larger brains than smaller
bodied animals. Thus, body size is acting as a develop-
mental constraint on brain growth, not as a factor select-
ing for brain growth. The high cost of both growing and
running brains means that we still need to provide an ex-
planation as to why a species should want to increase its
brain size above the bare minimum necessary to ensure
survival and successful reproduction. Unless these two
had become energy costless!
Burini and Leonard Nutrire (2018) 43:19 Page 3 of 9
Energy constraints for the enlarging brain
The brain is one of the most energetically expensive or-
gans in the vertebrate body; therefore, the large amount of
energy required to maintain brain tissue should impose
serious constraints on brain size evolution . In fact,
humans expend a much larger share of their resting en-
ergy budget on brain metabolism than other primates or
nonprimate mammals. Human brain sizes are some 2.5–3
times those of other primates, and, in caloric terms, this
means that brain metabolism accounts for ~ 20–25% of
resting metabolic rate (RMR) in an adult human body, as
compared to about 8–10% in other primate species, and
roughly 3–5% for nonprimate mammals. In humans, costs
are even greater in infancy, with our brains consuming
about 50% of the resting energy as newborns, 66% of
RMR at age 4 years, as compared with 20% for adults .
The high energetic costs of the human brain highlight
the importance of strong selective benefits to having
a larger brain and enhanced cognitive abilities in
human evolution [39,40].
The energy cost of evolutionary encephalization could
be explained by three proposed hypotheses: the direct
metabolic constraints, the expensive tissue trade-off, and
the energy trade-off .
The direct metabolic constraints hypothesis
The direct metabolic constraints hypothesis suggests
that due to the energetic cost of maintaining brain tis-
sue, overall RMR should be positively associated with
brain size, as seen in a large sample of placental and
marsupial mammals. Hence, larger brains are paid for by
a permanent increase in the net energy intake of an
The mass-specific metabolic rate of the brain is approxi-
mately nine times higher than the average of the
mass-specific metabolic rate of the human body as a
whole and approximately 16 times that of skeletal muscle
. Surprisingly, despite their disproportionately large
CNS metabolic rate, humans follow the same relationship
for nearly all animals, as the rate of basal energy
expenditure increases as the 3/4 power of body weight
(Kleiber’sLaw). Consequently, RMR does not appear
to be significantly elevated in humans and other more
encephalized mammals. That is, there is no evidence of an
increase in RMR sufficient to account for the additional
metabolic expenditure of the enlarged brain.
The expensive tissue trade-off hypothesis
The lack of correlation between RMR and relative brain
size in encephalized mammals raises the question of
how the increased energetic demands of larger brains
are compensated. The “expensive tissue hypothesis”
posits that the metabolic accommodation of large brains
is achieved by a reduction in the mass/sizes of other tis-
sues with high energy demands .
In addition to the brain, the heart, kidneys, and splanch-
nic organs [liver and gastrointestinal tract], all make sub-
stantial contribution to overall RMR. Together with the
brain, they account for 60–70% of RMR despite making
up less than 7% of the total body mass. The tissues which
make up the remaining 93% of body mass display corres-
pondingly low rates of energy turnover .
Specifically, the heart and the kidneys have mass-specific
metabolic rates considerably higher than the brain, the en-
ergetic demands of which are comparable to those of the
splanchnic tissues .
The extent to which the liver could be reduced with
encephalization is probably constrained by the energy
requirements of the brain, which uses glucose exclu-
sively as its fuel. Since the brain effectively contains no
energy reserves, it is critically dependent on the contin-
ual supply of glucose from the blood, maintained in fast-
ing states, by the liver. Similarly, since most of the mass
of the heart consists of the rhythmically contracting car-
diac muscle, it is difficult to envisage how any significant
reduction in the size of this organ could take place with-
out compromising its ability to maintain an adequate
circulation of blood around the body. The maintenance
of high tissue perfusion rates will be particularly import-
ant to the brain, which requires a continuous supply of
high levels of glucose and oxygen .
Along with the brain, the kidneys have an extremely
high metabolic rate associated with high levels of active
ion transport. It is likely that any reduction in either its
energetic expenditure or its size will reduce the max-
imum urine concentration it is capable of excreting. The
production of a more dilute urine would have been a
particular problem for hominids if they were exploiting
relatively open equatorial habitats where drinking oppor-
tunities were scarce and thermoregulatory requirements
were already placing considerable demands on their
water budgets .
A reduction in the mass of skeletal muscle would
likely not entirely balance the extra costs of large brains,
because the mass-specific RMR of muscle tissue is con-
siderably lower than those of the more “expensive or-
gans”noted above. To compensate for the increased
energy expenditure of the enlarged human brain, ap-
proximately 19 kg of muscle, about 70% of the total,
would have to be replaced by an equal amount of tissue
with no metabolic cost at all . Nonetheless, com-
parative analyses have shown that human and other pri-
mates are relatively “under-muscled”compared to other
mammalian species .
In the case of humans, reduced muscularity appears to
be, in part, a consequence of increased stores of body fat.
Indeed, humans show important adaptations in fat
Burini and Leonard Nutrire (2018) 43:19 Page 4 of 9
metabolism to accommodate the high energy demands of
the brain early in life when key aspects of human growth
and development of body composition are shaped by the
very high metabolic demands of brain metabolism. To
provide energy reserves for the high metabolic demands
of large, rapidly growing brains, human infants are born
with high body fat levels and continue to gain fat during
the first year of postnatal life. It is therefore possible that
encephalization and fat storage are complementary strat-
egies to buffer against starvation .
Lastly, the human brain and gastrointestinal tract have
a similarly high rate of organ-specific basal metabolic
energy expenditure (250 kcal/day/kg). The gut is the
only one of the expensive metabolic tissues that could
vary in size sufficiently to offset the metabolic cost of
the encephalized brain (the brain-gut trade-off hypoth-
esis). In fact, there is a close [inverse] relationship be-
tween relative gut size and relative brain size. Thus, the
brain-gut trade-off hypothesis suggests that the massive
expansion of the neocortex in humans came at the ex-
pense of the gastrointestinal tract .
Recent work by Navaretti et al.  re-examined the
possible association between the size of various visceral
organs (heart, lungs, stomach, intestines, kidneys, spleen,
and liver) and brain size in a sample of 100 mammalian
species, including 23 primate species. This study used
free-fat mass as the best proxy for body size because
body mass is highly affected by variation in the size of
adipose depots. Contrary to the predictions of the ex-
pensive tissue hypothesis, these authors found no inverse
correlation between the relative size of the brain and the
digestive tract. These results therefore indicate that the
expensive tissue hypothesis does not provide general ex-
planation for interspecific variation of relative brain size
in mammals. This finding reduces the plausibility of the
argument that human encephalization was made pos-
sible by a reduction of the digestive tract .
The energy trade-off hypothesis
The lack of support for the expensive tissue hypothesis
(investment into other costly tissues) raises questions
about the determinants of the evolution of the greatly
enlarged human brain and an increased energy alloca-
tion to the brain by savings on other expensive functions
would be a pathway to brain enlargement. The “energy
trade-off hypothesis”suggests that the cost of an in-
creased brain size can be met through a series of life his-
tory trade-offs involving changes in locomotion, growth,
and reproduction .
Another likely trade-off could be found between brain
size and the costs of growth and reproduction. The in-
creased reproductive costs associated with larger body
size in H. erectus females suggest a range of reproductive
and social solutions that could have met those costs. In
fact, having large, slow-growing babies is energetically
costly for mothers during gestation and lactation.
The constraints of our bipedal gait require a narrower
pelvic inlet so that the infant’s skull can pass through the
pelvic canal. Consequently, human babies are altricially
born, with skulls not fully formed and, although humans
have the largest relative brain sizes of all animals, they are
born with brains that are approximately 12% of their body
size, similar to other apes. However, in humans, dramatic
brain growth (similar to prenatal brain growth) occurs
postnatally in the first year of life, providing a uniquely
longer and more rapid phase of growth compared with all
other mammals. Following this, our brain consumes 50%
of the total energy intake as newborns, and 66% of RMR
by age 4 years, as compared with 20% for adults. To ac-
commodate for these high energetic demands of brain
early in life, human infants are born with large quantities
of body fat and show marked reductions in body weight
growth around 4 years—the time during which brain en-
ergy demands are greatest .
Moreover, it is believed that a dietary impact on human
brain size should occur during this brain fast-growth
period, i.e., through maternal milk . Indeed, recent
comparative analyses have shown that across primates.
It is expected that species with larger brains need a
longer period of postnatal brain growth, and this is gen-
erally completed at weaning. Earlier weaning is likely to
have occurred in H. erectus, in response to a major in-
crease in reproductive costs associated with increasing
female body size. Shortening the inter-birth interval, by
weaning children earlier, increases the female reproduct-
ive rate and allows some of the costs of caring for
dependent weanlings to be shared among others .
Hence, encephalization has implications for maternal en-
ergetic costs in childrearing and thus for parenting strat-
egy. Similarly, early weaning may have been important
for the continued pattern of brain growth in childhood.
In fact, providing such care could increase the fitness of
grandparents, leading to selection for longer lifespans,
and an extension of life history phases generally .
Furthermore, humans wean children early relative to
their dental development, but provision them with suit-
able foods throughout their childhood. They are there-
fore able to offer more energy and nutrients than would
be available just from maternal milk .
Among primates, the prevalence of non-maternal care
varies greatly from none at all to occasional grooming
and playing to feeding and holding infants. Data for
hunter-gatherer societies show that children are held or
carried by someone other than the mother for 20–50%
of the time. Childcare, provisioning or even education
by grandparents, may affect the survival of their
grandchildren or reproductive rate of their children, with
implications for the evolution of slower or later
Burini and Leonard Nutrire (2018) 43:19 Page 5 of 9
maturation. Humans have a unique period, childhood, in
their life cycle, in which body growth is very slow but
brain growth continues. The childhood phase is particu-
larly important for learning, because children have few
responsibilities to take up their time, because of cogni-
tive abilities that help efficient learning of particular
skills (i.e., language), and because environmental com-
plexity during childhood can have a positive and direct
effect on brain structure .
Thus, species with larger brains need a longer period
of postnatal brain growth and, since this is generally
completed at weaning, a later age at weaning, which has
implications for development, the dependency of off-
spring, birth spacing, and hence population growth.
Brain growth to adult size is almost complete by the
age of seven, when childhood ends. After this period,
body size starts to increase steadily. However, there is
another uniquely human life history phase, adolescence,
characterized by a spurt in skeletal growth, culminating
in sexual and behavioral maturity [50,51].
Overall, this model of human life history evolution
posits that large brain size, a longer juvenile period, and
a longer lifespan all co-evolved in the context of a com-
plex foraging niche. Human foraging is relatively com-
plex and skill intensive that it requires a longer juvenile
period for learning this skill set. As such, it is only later
in adulthood that an individual becomes a fully product-
ive forager, generating a surplus of energy to support the
long developmental period for the sub-adults [52,53].
Thus, we humans combine a long juvenile period and
lifespan, with caring and providing food for inefficient
weanlings, thereby increasing not only the period of
growth and time available for learning but also the re-
productive rate .
A number of contexts in human evolution could have
favored cooperative behavior relatively early, for in-
stance, group defense and hunting large mammals at
close quarters. The exploitation of large “packet”food
resources that can be divided provides incentives for
food sharing . Additionally, provisioning and food
sharing probably arose with the adoption of cooperative
breeding and substantial meat acquisition among the
earlier representatives of the genus Homo .
Relationship between human relative brain size and diet
The energetic constraint of large brain size evolution
was not overcome solely by greater food intake, but also
by higher-quality diets. Leonard and colleagues (2007)
 have shown a positive correlation between brain
size and dietary quality for 33 different primate species,
including humans, after adjusting for differences in body
size. Thus, across all primates, greater energy allocation
to brain metabolism is associated with consumption of a
higher quality, more energy-rich diet. Humans fall at the
positive extremes for both parameters, having the largest
relative brain size and the highest quality diet .
A high-quality diet could have benefited encephalization
by directly increasing the total energy available to fuel an
increased RMR and, also by permitting a relatively smaller
gut, thereby reducing the considerable metabolic cost of
this tissue. Hence, the relationship between relative brain
size and diet is primarily a relationship between relative
brain size and relative gut size .
Humans had, in effect, conserved energy by greatly re-
ducing the size of the intestine and then shifted this basal
energy expenditure to support a much larger brain [the
brain-gut trade-off hypothesis], but the two were inde-
pendent. Therefore, a reduction in gut size would only be
compatible with high-quality, easy-to-digest food .
Dietary quality has been shown to be correlated with
brain size, in that folivorous (leaf-eating) primates have
smaller brains, while primates who eat fruit and animal
foods (insects, meat) generally have larger brains .
Similarly, members of the genus Homo would have had an
even higher-quality diet than the Australopithecines by
the incorporation of more underground storage organs
(soft bulbs, tubers, etc) or the preferential consumption of
animals. The reduced size of the face and grinding teeth
of H. erectus, coupled with its more sophisticated tool
technology, suggest that these hominids were consuming
a higher-quality and more stable diet that would have
helped to fuel the increases in brain size .
The role of the meat
Changing environmental conditions in Plio-Pleistocene
Africa, involving a decline in plant productivity and an in-
crease in secondary biomass , would have favored a
shift in diet in hominin species, either to specialization on
lower quality plant foods or routine access to meat. Meat
is a very useful source of protein, many minerals and vita-
mins, and also essential fatty acids that humans require
. Although there is considerable variation in the diets
of modern human foraging groups, studies have shown
that modern human foragers typically derive over half of
their dietary energy intake from animal foods .
Recent analyses of stabilize isotopes and dental micro-
wear of early East African hominins appear to support this
model, suggesting two distinct dietary pathway in latest
Australopithecines and early members of the genus Homo.
The robust Australopithecines (A. boisei)appeartohave
subsisted on larger amount of low-quality plant foods
(e.g., C4 plants, such as savanna grasses and sedges),
whereas early Homo appears to have had a broader, more
varied diet .
The role of fat
The evolutionary “higher quality”diet means that we
need to eat less volume of food to get the energy and
Burini and Leonard Nutrire (2018) 43:19 Page 6 of 9
nutrients we require. Comparative analyses of primate
dietary patterns indicate that the high costs of large hu-
man brains are supported, in part, by diets that are rich
in energy and fat .
The higher consumption of meat and other animal
foods among human hunter-gatherers is associated with
diets that are higher in fat and denser in energy. Relative
to other large-bodied apes, modern humans derive a
much larger share of their dietary energy from fat.
Dietary fat is our second most important energy-produ-
cing macronutrient. Contemporary foraging societies de-
rive between 28 and 58% of their daily energy intake
from dietary fat. Those groups living in more northern
climes [e.g., the Inuit] derive a larger share of their diet
from animal foods and thus have higher daily fat intakes.
Conversely, tropical foraging populations generally have
lower fat intakes because they obtain more of their diet
from plant foods .
Besides, yielding energy dietary fat also contains func-
tional fatty acids and vitamins essential for growth, devel-
opment, and maintenance of good health . Therefore,
while dietary change might not be the prime force respon-
sible for the evolution of large human brain size, improve-
ments in dietary quality and increased consumption of
dietary fat appear to have been a necessary condition for
promoting encephalization in the human lineage .
Requirements of LCPUFAs
In addition to the energetic benefits associated with
greater meat consumption, it appears that such a dietary
shift would have also provided increased levels of key
fatty acids necessary for supporting the rapid hominid
brain evolution .
Half of human brain composition is fat, and 20% of its
dry weight is long-chain polyunsaturated fatty acids
(LCPUFAs). Consequently, improvements in consump-
tion of dietary fat were a necessary condition for pro-
moting encephalization [61,62].
Mammalian brain growth is dependent upon sufficient
amounts of two LCPUFAs: docosahexaenoic acid (DHA)
and arachidonic acid (AA), and it appears that mammals
have a limited capacity to synthesize these fatty acids
from dietary precursors. Hence, species with higher
levels of encephalization would have greater require-
ments for DHA and AA . Consequently, dietary
sources of DHA and AA were likely limiting nutrients
that constrained the evolution of larger brain size in
many mammalian lineages .
The evolution of large human brain size was driven by a
complex web of interacting forces, social, developmental,
and nutrition. While it is clear that the high levels of
encephalization of humans and other primate species
are reflective of higher levels of sociality and cooper-
ation, and behavioral complexity, it also appears that the
metabolic fuel for evolving large brains was based on im-
portant shifts in diet, foraging, and energy metabolism.
Humans, as a group, have a higher-quality diet than
expected for our size, supporting the high energy costs
of our large brains. Additionally, while the RMRs of
humans do not appear to be significantly elevated over
what is predicted for a primate of our size, our TEE and
activity levels are considerably higher than those of other
apes. These relatively higher levels of energy expenditure
appear to have been the product of selection for foraging
regimes that necessitated movement over larger areas,
and higher activity budgets to procure energy-rich diets.
Large brain size may have facilitated also more compli-
cated extractive foraging strategies and acted as a secondary
selection pressure for encephalization . Additionally,
primates have evolved key differences that can be related to
diet and lifestyles directly, such as slower growth rate, a re-
duction in litter size, and producing larger offspring that
live longer compared with other mammals .
If the exploitation of the high-quality foods, such as
animal products, nuts, or underground tubers, required
more complex behaviors, then this also could have acted
as one of the selection pressures for the observed in-
crease in brain size. It has therefore been suggested that
early hominins evolved larger brains as a dietary shift to-
wards more meat, and other higher quality resources
that allowed for a smaller digestive tract and reduction
in craniofacial architecture .
Overall, body and brain size are intimately tied to diet
and life history patterns across mammalian species. As
such, the evolution of distinctive human life history
characteristics are reflective of processes that operate
among mammals, in general. Overall, one can propose
that during human evolution improved diet quality, allo-
maternal subsidies, cognitive buffering, reduced locomo-
tion costs, and reduced allocation to production all
operated simultaneously, thus enabling the extraordinary
brain enlargement in our lineage .
A: Australopithecus; AA: Arachidonic acid; CNS: Central nervous system;
DHA: Docosahexaenoic acid; DLW: Doubly labeled water; H: Homo; i.e.: For
example; kya: Thousand years ago; LCPUFAs: Long-chain polyunsaturated
fatty acids; mya: Million years ago; RMR: Resting metabolic rate; TEE: Total
daily energy expenditure
The authors acknowledge the CNPq-Brazilian National Research Council
The authors provided personal resources for funding this paper.
Availability of data and materials
Burini and Leonard Nutrire (2018) 43:19 Page 7 of 9
The authors contributed equally to the concept, conclusions, and writing.
Both authors read and approved the final manuscript.
Ethics approval and consent to participate
Consent for publication
The authors declare that they have no competing interests.
Springer Nature remains neutral with regard to jurisdictional claims in published
maps and institutional affiliations.
Department of Public Health, UNESP Medical School, Botucatu, SP 18.618-970,
Department of Anthropology, Northwestern University, Evanston, Il 60208,
Received: 5 April 2018 Accepted: 18 July 2018
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