That’s because H. erectus probably lived in complex, multilevel societies similar to those of modern hamadryas baboons. At least, that’s the case anthropologists Larissa Swedell and Thomas Plummer, both at Queens College, City University of New York, make in the International Journal of Primatology. Swedell and Plummer argue that a dry environment led both species to evolve intricate social structures. 


Hamadryas baboons live in the semidesert lowlands of the Horn of Africa and the southwestern portion of the Arabian Peninsula. Their social lives are organized in a four-tiered system. A few hundred of the monkeys aggregate in a large, loose group called a troop. Troops huddle together on their sleeping cliffs at night to deter predators. During the day, troops splinter into smaller groups because it’s a more efficient way to forage in a desert, where food tends to be sparse and spread out, especially during certain seasons. The smallest group is the one-male unit: one adult male, one or more adult females and their young offspring. Several of these units form a clan. When it’s time for a young male to find his own unit, he typically stays within his clan. Because males stay close to home, neighboring males tend to be relatives and therefore cooperate with each other—even tolerating the “kidnapping” of their females by their brethren. Finally, several clans make up a band, which travels over a common home range.

H. erectus evolved 1.9 million years ago. Swedell and Plummer note that climatic changes that occurred 2.8 million years ago, 1.7 million years ago and 1 million years ago created a drier and more variable environment for the species than what any previous hominid had experienced. H. erectus lived in more open habitats and had to travel greater distances to find food. Like hamadryas baboons, this probably favored smaller foraging groups during the day and larger communities at night for safety.

As H. erectus traveled more and dealt with new habitats, it added new food to its diet: meat and underground tubers. Getting both required new technologies. The greater cognitive demands of such procurement may in part explain why the species evolved larger brains. This created some challenges for females, however. Big brains require a lot of energy. As a consequence, Swedell and Plummer suggest, feeding and raising bigger-brained babies may have been too big a task for a female to accomplish on her own.

To help one another raise offspring, females may have started to live in small groups with their female kin. (Postmenopausal grandmothers may have been especially useful in helping to rear their grandchildren.) Selection would have favored males who could monopolize such groups. In exchange for exclusive breeding rights, males could help females protect and perhaps even feed their children. The result: a group analogous to the hamadryas baboon’s one-male units. The benefits of male cooperation in defending groups from outside males or predators may have led to the formation of larger groups akin to the baboon’s clans and bands. Clan (or band)  males may have also worked together to hunt large game.