June 4 (Bloomberg) -- An explosion of art, music, jewelry and hunting technology appeared 45,000 years ago because of increased population density, rather than the evolution of the human brain, a study said.

Researchers used genetic estimates of ancient population sizes, archaeological artifacts and computer simulations of social learning. They found complex skills involving abstract thinking would be passed down through generations and across groups only when populations reach a critical level, according to the study in tomorrow’s edition of the journal Science.

Increased interaction between groups, the sharing of ideas and the exchange of raw materials that led to the flowering of human culture may explain why concentrated centers of industry, such as California’s Silicon Valley, produce technological innovations, said Mark Thomas, 44, a senior author of the study and a senior lecturer at University College London in England.

“People learn from their parents or teachers in their group, and this model demonstrates you have to have a critical number of people learning to develop complexity,” Adam Powell, 28, a co-author of the study and a doctoral student at the London university. “The actual invention of all these technologies was probably very common, but was only passed on as density increased.”

45,000 Years Ago

Anatomically, modern humans have been around for an estimated 200,000 years, yet scientists have found the first widespread evidence of sustained symbolic behavior and abstract thinking emerged about 45,000 years ago. The findings include musical instruments, body decoration with shell beads and tattoos, bows and arrows and microlithic stone blades, according to the study.

An ancient example of figurative art was discovered recently in a German cave, depicting a woman with enlarged breasts and genitals, Powell said. One of the oldest of its kind, it dates back 35,000 years ago, according to a May 14 study published in the journal Nature.

Powell said his study may explain why modern human behavior appeared to emerge in different regions of the world at different times. Evidence was seen sporadically as far back as 90,000 years ago in sub-Saharan Africa, with a more sustained pattern 40,000 years ago, and in Europe and western Asia 45,000 years ago. Archaeological samples indicating similar skills were found in eastern and southern Asia and Australia 30,000 years ago. Population densities would have reached a critical point in sub-Saharan Africa and Europe at about the same time periods, according to the study.

Popular Illustrations

The impetus for human development has been pondered throughout history, Thomas said in an interview. The 1968 film, movie “2001: A Space Odyssey,” illustrated the mystery with a scene of a symbolic monolith slab on an ancient landscape that enabled humanlike apes to make tools and think critically.

“In reality, what I suspect is these technologies were being invented all the time, but they never hung on for more than a few generations. You need sustained evidence for it to be visible in archaeological record.”

Not everyone is convinced the demographic model caused the behavioral change. Richard Klein, an anthropology and biology professor at Stanford University, said the study is flawed because the examples it cites of human behavior prior to 50,000 years ago are either misdated artifacts or are open to interpretation as to their level of advancement.

Competing Theory

“They have it wrong,” Klein said in a telephone interview. “This paper does not belong in print.”

Klein is a proponent of a competing theory that attributes the development of modern human behavior to a genetic change to human brains 50,000 years ago.

“These behaviors appear to have been part of a package that significantly enhanced human fitness -- the ability to survive and reproduce,” Klein wrote in a study that was published last year in the journal Evolutionary Anthropology. “It is in this sense that they signal true evolutionary change as opposed to mere historical change.”

To contact the reporter on this story: Ryan Flinn in San Francisco at rflinn@bloomberg.net.

Last Updated: June 4, 2009 14:00 EDT