Carrying enormous brains, bipedal humans colonized the Earth. As usual, acquiring our specific traits was largely the result of maximizing quality of nutrition and efficiency of foraging by natural selection. It seems like changes in food availability influenced our ancestors.

Searching food, consuming it, and using it for biological processes are critical for organism ecology. The shift in relation between energy acquired with diet and its expansion might be the cause of many health problems of modern humans. In the energy dynamic between an organism and its environment, the maintenance energy needed to stay alive and the productive energy for offspring are impacted by the type of habitat and should be balanced. The long-term success of the species can be achieved if enough funds are distributed towards reproduction.

Bipedalism not only frees the arms to carry children and reach for foods, but is also more energy efficient than quadrupedalism and might help regulate the body temperature by lower exposure to heat and cooling cranial blood, freeing our brain development from temperature constraint. For example, modern foragers, the best available model for the patterns for sustaining life by early humans, walk several miles a day.

The energy saving therefore must have been a significant enough factor for natural selection, and the calories can be freed for breeding. This morphological revolution was a result of climate change that resulted in patchy distribution of food sources. Then, australopithecines increased their brain volume by one fifth, followed by Homo habilis by another third of that, and Homo erectus by another two thirds.

Brains consume about sixteen times more energy than muscles. A diet richer in nutrients and calories is required to sustain  a bigger brain. Compared to fruit, meat can provide two to four times more calories, and more than ten times compared to leaves. Humans do not have large teeth with thick enamel of australopithecines who tried to survive on tough parts of plants like roots. Most probably, there was synergistic interaction between improvement in diet and brain expansion, supported by multiple other factors.

Homo erectus seem to have established the first hunting-and-gathering economies. There are fewer animal food calories per unit area than total plant food calories, so carnivores need much larger territories, especially in colder and drier climates. The meat quality is strongly impacted by the animal habitat. Human beings developed an extraordinary variety of dietary strategies for one species.

Optimizing caloric intake seems to be the main challenge also for modern humans.

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