類人猿（注1）の四足歩行と人間の二足歩行を比べると、時速四kmくらいの速度で歩くと、二足歩行のほうがエネルギー効率がいい。しかも長く歩けば歩くほどエネルギーの節約率が高くなる。すなわち、初期の人類は長い距離をゆっくりした速度で歩く必要性に迫られて、直立二足歩行を採用したと考えられるのだ。これは初期の人類が徐々に熱帯雤林を出ようとしていたこととぴったり符合する（注2）。熱帯雤林の外では果実が散在していて、広い範囲を探し回る必要がある。これを可能にする歩行様式として、二足で歩くことが有利になった可能性がある。
しかし、長距離を歩くことになると群れの全員がまとまって移動するのは困難になる。子供や身重（注3）の女性、老人など速い速度で長距離を歩くことが難しい仲間がいる。そのため、体力のある男たちが少数のグループを組み、広く歩き回って食物を集め、それを女や子供たちのもとへ持ち帰って一緒に食べたのではないかと思われるのだ。これが食物共有仮説である。だが、サバンナへ出たサルたちは二足にならなかった。なぜ人間だけがなかったのか。それは、サバンナ（注4）へ出たパタスザルやアヌビスヒヒ、マントヒヒたちはオスがメスより格段に大きくなり、長い犬歯を発達させて群れの防御をするようになったからである。しかも胃腸の強い彼らは人類ほど広い範囲を歩き回って食物を探す必要はなかった。一方、人類の祖先は男が大きくなるところか、性差が小さく、犬歯（注5）も縮小して武器としては使えなくなっている。これは人類の男たちが捕食動物と戦うよりも、その目を避けながら食物を探し歩いていたことを物語っている。
（朝倉敏夫編『火と食』による）
（注1）類人猿：生物学上、最も人に近いサル類。
（注2）符合する：合う。
（注3）身重：妊娠中。
（注4）サバンナ：熱帯地方に見られる草原。
（注5）犬歯：ここでは、特に鋭い歯。
AI translation, cross-checked between two models.
If we compare quadrupedal walking in apes with bipedal walking in humans, bipedal walking is more energy-efficient at a speed of about 4 km/h.（注1） Moreover, the longer the distance walked, the greater the energy savings. In other words, early humans are thought to have adopted upright bipedal walking because they were compelled to travel long distances at a slow pace. This fits perfectly with the fact that early humans were gradually moving out of the tropical rainforest.（注2） Outside the tropical rainforest, fruit was scattered about, making it necessary to search over a wide area. Bipedal walking may have become advantageous as a form of locomotion that made such searching possible.
However, once long-distance travel became necessary, it would have been difficult for the entire group to move together. There would have been children, pregnant women, elderly people, and others who found it difficult to walk long distances at a fast pace.（注3） It is therefore thought that physically strong men formed small groups, ranged widely to gather food, and brought it back to the women and children so that they could eat together. This is the food-sharing hypothesis. Yet the monkeys that moved out onto the savanna did not become bipedal. Why, then, was it different only for humans? This is because among patas monkeys, Anubis baboons, and mantled baboons that moved onto the savanna, the males became much larger than the females and developed long canine teeth to defend the group.（注4） Moreover, because of their strong digestive systems, they did not need to range as widely as humans in search of food. By contrast, among human ancestors, rather than males becoming much larger, sexual differences were small, and their canine teeth had shrunk to the point where they could no longer serve as weapons.（注5） This suggests that the men among our human ancestors searched for food while avoiding predators rather than fighting them.
(From Fire and Food, edited by Asakura Toshio)
(Note 1) Apes: biologically, the group of primates most closely related to humans.
(Note 2) Correspond: to match.
(Note 3) Pregnant: being in pregnancy.
(Note 4) Savanna: grassland found in tropical regions.
(Note 5) Canine teeth: here, particularly sharp teeth.
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