Ability to store more fat shows humans depend on fat based metabolism typical of carnivores.
Humans are much fatter than other apes
Male and female hunter-gatherers have an average body fat level of 9 percent and 24 percent, respectively, which is quite lean by modern standards . But compared to primate bonobo males and females that have less than 1 percent and less than 4 percent body fat, respectively, humans are relatively fat 
Hunter‐gatherer populations are remarkable for their excellent metabolic and cardiovascular health and thus are often used as models in public health, in an effort to understand the root, evolutionary causes of non‐communicable diseases. Here, we review recent work on health, activity, energetics and diet among hunter‐gatherers and other small‐scale societies (e.g. subsistence farmers, horticulturalists and pastoralists), as well as recent fossil and archaeological discoveries, to provide a more comprehensive perspective on lifestyle and health in these populations. We supplement these analyses with new data from the Hadza, a hunter‐gatherer population in northern Tanzania. Longevity among small‐scale populations approaches that of industrialized populations, and metabolic and cardiovascular disease are rare. Obesity prevalence is very low (<5%), and mean body fat percentage is modest (women: 24–28%, men: 9–18%). Activity levels are high, exceeding 100 min d−1 of moderate and vigorous physical activity, but daily energy expenditures are similar to industrialized populations. Diets in hunter‐gatherer and other small‐scale societies tend to be less energy dense and richer in fibre and micronutrients than modern diets but are not invariably low carbohydrate as sometimes argued. A more integrative understanding of hunter‐gatherer health and lifestyle, including elements beyond diet and activity, will improve public health efforts in industrialized populations.
During human evolution, the body changed in shape, partially to accommodate bipedal locomotion. Concurrently, brain size underwent a three-fold increase recorded in evidence from fossils and from comparative anatomy of chimpanzees, Homo sapiens’ closest living relatives. Because soft tissues like muscle, skin, and fat do not fossilize, and little information is available on these components for the genus Pan, reconstructing tissue changes has primarily relied on what is known about humans. This study presents unique quantitative data on major body components of muscle, bone, skin, and fat of 13 bonobos (Pan paniscus) for interpreting evolutionary forces that have shaped the human form for survival in a savanna mosaic environment.
Keywords: body composition, bonobo, Pan paniscus, human evolution, Homo sapiens
Implications for Human Evolution.
The negligible measurable fat in all seven P. paniscus males was unexpected, overriding captivity, age, and body mass. Among wild chimpanzees, there is little indication of an ability to mobilize fat stores during times of caloric restriction, a key adaptive feature found in orangutans and possibly to a lesser degree in gorillas (24, 52, 53). Without selection pressure for storage fat, and with over half of body mass in muscle, the male P. paniscus does not easily accumulate body fat, even under optimal circumstances of captivity. Remarkably, none of the males and females manifested detrimental health as a consequence of having little fat, in stark contrast to H. sapiens.
There is evidence in female P. paniscus that fluctuation in body fat is associated with reproductive history. Individual paniscus female 1 (PF1) with the most body fat was lactating at the time of death and had considerable fat in her breasts, trunk, and limbs. Individual paniscus female 4 (PF4) died within 1 wk after giving birth to a full-term offspring, her first, and had notable fat deposits in the trunk and limbs. Individual paniscus female 3 (PF3) had no offspring or pregnancies during her life, had remarkably low levels of dissectible fat, and stood out as having the highest percentage of muscle mass among the females (44.1%, Table 1). Body fat is also sexually dimorphic in some monkeys, as well as in gorillas and orangutans (23–25), and has a demonstrated role in reproduction (54–56). The same is true for H. sapiens (19, 43–48).
In the course of human evolution from early australopithecines onward, the ability to store and mobilize body fat must have played an increasing role in successful reproduction (43, 44, 57, 58), coupled with the shift to bipedal locomotion (59). We posit that early australopithecine females, such as P. paniscus, put on more body fat than the males and had the ability to vary their adiposity with reproductive cycles. We conclude that body fat was sexually dimorphic in australopithecines, as it is for P. paniscus, variable in females but consistently low in males: as high as 8–10% in females and 2–3% in males. Although Wells (19) suggests that both female and male australopithecines had 11–13% body fat, our data suggest, to the contrary, it is unlikely that male australopithecines approached the level of body fat found in male Hazda hunter-gatherers.
It may be that in early Homo and H. erectus, with the increase in brain size (60) and body mass (61), fat began to play a more significant role in female reproduction. As the early Homo species expanded out of Africa, the ability to put on, store, and mobilize body fat provided a clear adaptive advantage for both females and males as a backup against extremes of food availability in a variety of environments, as well as mitigating adverse effects on pregnancy and lactation in females. Although body fat for males may not be as critical for reproduction as it is for females, we suspect that male H. erectus would have needed an estimated 7–8% body fat as a buffer against “seasonal hunger” (43, 45, 46). In order for females of ancestral species of Homo to nourish larger brained infants without the extended developmental period characteristic for H. sapiens (62), they probably needed additional body fat, in the range of 12–14%.
Increased body fat during human evolution reflected more than one selective pressure. Females experienced increased nutritional demands for successful reproduction and for maintenance of a high level of activity, walking several miles a day collecting and carrying food and dependent infants (cf. 44). These needs were compounded by pronounced seasonal variation in food sources characteristic of the tropical savanna mosaic (63) and the unpredictability of new environments as early Homo species expanded their home ranges regionally and geographically.
Our larger fat reserves are original to us, in the sense that they’re not a trait held by our group’s last common ancestor . Our other energy source, carbohydrate, is stored in amounts (400-500 grams) about 10 times smaller than fat.