The sequencing of 36 complete Y chromosomes revealed this population explosion occurred 40,000 to 50,000 years ago, between the first expansion of modern humans out of Africa 60,000 to 70,000 ago and the Neolithic expansions of people in several parts of the world starting 10,000 years ago, Britain's Wellcome Trust Sangster Institute reported.
"We have always considered the expansion of humans out of Africa as being the largest population expansion of modern humans, but our research questions this theory," Wei Wei of the Sanger Institute and the West China University of Medical Sciences said.
"Now we've found a second wave of expansion that is much larger in terms of human population growth and occurred over a very short period, somewhere between 40,000 to 50,000 years ago."
One possible theory is that during the original out-of-Africa expansion, humans moved along the coastlines of the world, settling as they went.
Their origins and genetic makeup would make them suited to coastal life, but not to the demands of living inland.
"We think this second, previously unknown population boom, may have occurred as humans adapted to their new environment after the first out-of-Africa expansion," institute researcher Qasim Ayub said.
"It took them tens of thousands of years to adapt to the mountainous, forested surroundings on the inner continents. "However, once their genetic makeup was suited to these new environments, the population increased extremely rapidly as the groups traveled inland and took advantage of the abundance of space and food," Ayub said.