The human population will top seven billion this year—more than double what it was just 50 years ago, experts say.

And these next 39 years are expected to bring about some big shifts among the biggest countries. By 2050, India will have surpassed China as the world's largest population, growing from 1.24 billion to 1.69 billion as China shrinks from 1.35 billion to 1.3 billion. And Africa's population will likely have more than doubled by then, with Nigeria slated to catch up to the U.S. numbers. The figures are described in a new report published in the July 29 issue of Science.

Along with sheer numbers, global life expectancy is projected to rise as well: from age 69 worldwide this year to 76 in 2050. By then, nearly a quarter of the world's population is expected to be over 60—which is about double the proportion that it is today.

"The demographic picture is indeed complex and poses some formidable challenges," including contraception, child mortality and retirement policies, David Bloom, a professor of economics and demography at the Harvard School of Public Health, said in a prepared statement. "Those challenges are not insurmountable, but we cannot deal with them by sticking our heads in the sand."

He and others also assert that we will continue our trend of leaving rural areas for the city. Currently just over half of the world's population lives in urban environments, but by 2050, that figure is expected to be some 69 percent of the world's 9.3 billion people.

Some researchers think our tendency for explosive population growth and concentration helped us seize the future from the Neandertals. New analysis, described in another paper published in the same issue of Science, suggests that it was early human's sheer numbers that pushed aside our predecessors, who had been around for some 300,000 years.

"Faced with this kind of competition, the Neandertals seem to have retreated initially into more marginal and less attractive regions of the continent and eventually—within a space of at most a few thousand years—for their populations to have declined to extinction," Sir Paul Mellars, a professor emeritus of prehistory and human evolution at Cambridge University, said in a prepared statement. He and his co-authors suggest that tools, culture and a diverse gene pool helped our early ancestors be fruitful and multiply. A changing climate might have been the final blow to our close cousins.

As unclear as these ancient dynamics might still be, the future remains even more unsure, noted Bloom and his co-authors on the worldwide population estimates. "The global outlook is greatly complicated by a slew of uncertainties involving, for example, infectious disease, war, scientific advance, political change, and our capacity for global cooperation," they wrote.

Image courtesy of iStockphoto/thehague