Babies weaned before the emergence of their molars cannot eat the same solid foods as adults. It is therefore necessary for their parents or other older individuals to provide them with soft food,* in other words, “baby food.”

In this way, as early weaning and high fecundity evolved in early humans, the new life cycle stage of “childhood” was born.

■References (Books, papers, Web articles, etc.)
『サルの生涯、ヒトの生涯——人生設計の生物学』　　デビッド・Ｓ・スプレイグ　（京都大学学術出版会、2004）
『家族進化論』 第4章・第9節　人類の生活史と進化　　山極寿一 （東京大学出版会、2012）

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*For example, sweet fruits rich in sugar, or animal protein sources such as termites.

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■The References list of “Evolution of Human Sociality”