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Beefcakes such as Hugh Jackman and Brad Pitt may be the biggest draws at the box office, but in human evolution, alpha males have lost ground, a study suggests.

Back in the days when alpha males had their pick of fertile females, low-ranked "beta males" led a "sexual revolution" by caring for their offspring rather than fighting their way to the top, according to research published in the Proceedings of the National Academy of Sciences.

Pair-bonding replaced promiscuity as females began to choose good providers over chest-pounding males.

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It's the female equivalent of "gentlemen prefer blondes – but they marry brunettes."

Humanity's early social structure was "pretty much what chimpanzees have," says Sergey Gavrilets, a theoretical evolutionary biologist at the University of Tennessee in Knoxville. "Alpha males completely dominated everything," he told Cosmos magazine.

Humanity evolved into a largely monogamous society because subordinate males began to offer food and paternal care to gain the affection of females, who actively chose their mates.

"Pair-bonding provided a foundation for the later emergence of the institution of modern family," says Dr. Gavrilets, who created a theoretical model to simulate the evolution of early hominid mating systems.

The social contract went two ways, of course. Males would only provide for a mate who remained faithful, Dr. Gavrilets says. "That creates a co-evolutionary process where both provisioning and faithfulness increase in parallel."

According to his model, the quid pro quo predates human language and culture.

"This model deals with what animal biologists call social instincts and shows that some of these behaviours can be coded in our genes," Dr. Gavrilets says. "Culture came much later and only augmented things that were already in place."

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But not everyone is convinced that ancestral mothers needed a helping hand. Bernard Chapais, a paleoanthropologist at the University of Montreal, calls the study's thesis "extremely unlikely," adding that it's an assumption based on the present-day structure of human families.

He suggests that the transition toward pair-bonding was gradual and probably involved a harem-like phase in which a male bonded with several females – the systems used by gorillas.

Who said mating was romantic?

Do you find that "beta males" win out in the end?

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