Blue eyes are typically associated with beauty, or perhaps Frank Sinatra. But to University of Wisconsin anthropologist John Hawks, they represent an evolutionary mystery.
For nearly all of human history, everyone in the world had brown eyes. Then, between 6,000 and 10,000 years ago, the first blue-eyed baby was born somewhere near the Black Sea.
For some reason, that baby's descendants gained a 5% evolutionary advantage over their brown-eyed competitors, and today the number of people with blue eyes tops half a billion.
"What does it mean?" asked Hawks, who studies the forces that have shaped the human species for the last 6 million years.
Nobody knows. It is one of the unanswered questions about evolution that persist 200 years after the birth of Charles Darwin, whose birthday will be celebrated worldwide Thursday.
Darwin amassed a lifetime of observations on plants and animals to famously conclude that all life on Earth evolved from simple organisms through a painstakingly slow process of tiny random changes and a continuous contest for survival of the fittest.
Though Darwin published his masterwork "On the Origin of Species" 150 years ago and died in 1882, studies on evolution continue apace. Much of that effort focuses on the species Darwin considered the pinnacle of the evolutionary process: Homo sapiens.
Until recently, conventional wisdom held that human beings had mastered their environment so thoroughly that the imperative to evolve in many ways diminished about 10,000 years ago, when agriculture gave rise to more-stable societies.
"People thought that with technology and culture, there'd be no reason for physical things to make any difference," said Milford Wolpoff, a paleoanthropologist at the University of Michigan. "If you can ride a horse, it doesn't matter if you can run fast."
That turned out to be wrong. As it happens, the pace of evolution has been speeding up -- not slowing down -- in the 40,000 years since our ancestors fanned out from Ethiopia to populate the globe.
And in the 5,000 to 10,000 years since agriculture triggered the growth of large societies, the pace has accelerated to 100 times historical levels.
"When there's more people, there are more mutations," Wolpoff said. "And when there are more mutations, there's more selection."
