Humans went through a major development 50,000 years ago, when the species, which first developed 150,000 years earlier, started to develop tools and art started to flourish.

Testosterone levels in humans was beginning to moderate down to modern concentrations around the same time, according to a new study.

Study was undertaken of 1,400 modern and ancient skulls, which led to the understanding of decreasing testosterone levels. Among these were 13 skulls more than 80,000 years old, along with 41 specimens aged between 10,000 and 38,000 years, and 1,367 modern skulls, representing 30 ethnic backgrounds.

Researchers believe that lower levels of the male hormone may have led to a greater degree of understanding between people, reducing violence, allowing arts and tool making to become more advanced.

"The modern human behaviors of technological innovation, making art and rapid cultural exchange probably came at the same time that we developed a more cooperative temperament," Robert Cieri, from the University of Utah and lead author of the study, said.

Decreasing testosterone levels were noticeable through the changes to the shape of human skulls from the period. Thick eyebrow ridges receded, as heads became rounder.

Brian Hare and Jingzhi Tan, a pair of animal cognition researchers from Duke University, believe this theory is in line with behavior observed in animal studies. Siberian foxes were selectively bred in a previous study, which developed additional juvenile physical characteristics and became friendlier with each other over several generations.

Investigators are still uncertain whether concentrations of the hormone decreased around 500 centuries ago, or if fewer testosterone receptors were available in developing humans.

Testosterone levels can also affect social interactions of our primate relatives, according to researchers. Male chimpanzees experience a large increase in testosterone levels during puberty, while concentrations among bonobos is small. When chimps become stressed, their bodies release additional testosterone, while cortisol, a hormone related to stress, floods the bloodstream of bonobos. Social interactions between chimpanzees are much more prone to violence than similar incidents between bonobos. Brow ridges are also much more pronounced in chimps than they are in the mellower species.

"If prehistoric people began living closer together and passing down new technologies, they'd have to be tolerant of each other. The key to our success is the ability to cooperate and get along and learn from one another," Cieri told reporters.

Investigation of the human skulls and what they can tell us about the role of testosterone in fostering human cultural development will be profiled in the journal Current Anthropology.

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