7 Population Milestones for 7 Billion People

A crowded street in Delhi, India. — No one knows where the world's 7 billionth baby will be born, but some international groups say that India is a good candidate, because the population is 1.2 billion strong and birthrates are still high in some areas.

(Image credit: JeremyRichards / Shutterstock.com)

This year marks the seventh "billion-person" milestone in the planet's history. On or around Oct. 31, 2011, the world's 7 billionth person will be born, the United Nation estimates.

Even more staggering is that of the 7 billion people on Earth, about 1.4 billion of them will be old enough to have observed the arrivals of the 6 billionth, 5 billionth, 4 billionth and 3 billionth people in the world. About 42.5 million people could have blown the party horn for the birth of the 2 billionth baby.

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The world's first billion-person milestone was a long time coming. Estimates of historical populations can be rough, but the U.S. Census Bureau pegs the global population at a paltry 5 million people in 8000 B.C. Certainly, humans remained scarce until the development of agriculture some 10,000 years ago. Even after our kind began farming, it was a slow climb to the 100-million-person mark around 500 B.C.

The birth year of the world's billionth baby will never be certain, but it's likely he or she came into the world around 1805. Beethoven was big that year, and already going deaf. Lewis and Clark made it to the Pacific Ocean. Napoleon was on a roll in Europe, 10 years from his defeat in the Battle of Waterloo. With the exception of a few coastal outposts, most of Africa was a complete mystery to Europeans. In China, the Qing Dynasty had just put down the White Lotus Rebellion, a tax protest that ultimately killed about 16 million people — a reminder that mass death and population growth don't necessarily cancel one another out.

This sudden takeoff of population makes sense in the context of a phenomenon called the demographic transition. In the transition, a population goes from one with high birth rates and high death rates (imagine farming families having seven or eight kids in hopes of a few reaching adulthood) to one with low birth rates and low death rates (where parents usually have one or two kids and expect them to grow up).

The agricultural and industrial revolutions led to gradually declining death rates in the Western world starting in the 1700s, while birth rates remained fairly high for generations. As developed countries began to see falling birth rates, developing countries entered the transition period of declining death rates and high birth rates. Some countries in sub-Saharan Africa are just entering the demographic transition today.

But back to 1927. If the 2 billionth baby was indeed born in this year, he or she came into being the same year that the first trans-Atlantic telephone call was made (from New York City to London). The world was on the cusp of the Great Depression. Mao Zedong battled the Kuomintang in Hunan, China, and lost — for a time. An enormous flood along the Mississippi River inundated 27,000 square miles (70,000 square kilometers), the greatest natural disaster in U.S. history at that point.

Worldwide, the population growth rate hit a peak of more than 2 percent per year in the early 1960s, just after our 3 billionth baby's birth. The growth rate had been increasing since 1950, according to the U.S. Census Bureau, with one rapid downward plunge between 1959 and 1960. This hiccup was because of China's Great Leap Forward, a disastrous industrialization and collectivization push that killed millions and caused China's fertility rates to plummet.

Those predictions didn't come to pass, though the book did catapult serious issues of overconsumption and resource scarcity into the limelight. In 2009, the Ehrlichs wrote in the Electronic Journal of Sustainable Development that many of their dire concerns persisted, including fishery collapse, major extinctions and fears of an epidemic. The researchers admitted they had underestimated agricultural advances in feeding a hungry world, but argued that they were partially right: Between 1968 and 2009, they wrote, "perhaps 300 million people overall have died of hunger and hunger-related diseases." [7 Devastating Infectious Diseases]

So baby No. 4 billion was not entering the world at an optimistic time, population-wise. But the world had changed in many ways in the 15 years since baby No. 3 billion took his or her first breath. Forget space monkeys: The human crew of Skylab 4 returned to Earth alive and well after 84 days in orbit. Second-wave feminism had hit a stride in the U.S., ushering in a number of legal victories aimed at workplace and educational equality for women. And just as the first Australopithecus skull was found the year the world's population rose to 3 billion, "Lucy," the famed 40-percent-complete Australopithecus afarensis skeleton, was found in 1974, the year the population hit 4 billion.

Which isn't to say the Day of 5 Billion was a celebration, exactly. The then-president of the Population Institute in Washington, D.C., told the New York Times in 1986 that the 5 billionth baby was "a sobering symbol of the shocking rapidity at which the world's population is multiplying." (In a sign of how difficult exact population counts are, the Population Institute pegged the 5-billion milestone to July 1986 instead of 1987.)

But people in the 1980s were no longer as panicked over population as they had been in the 1970s. A 1986 National Academy of Sciences report argued that the time of peak population growth was past; birth rates in developing countries were down sharply, as was fertility in the developing world, though population was still expected to climb. The Carter administration had been concerned about overpopulation, the New York Times reported in 1986, while President Ronald Reagan's advisers argued that market forces and scientific progress would solve any problems caused by population growth.

In fact, it had taken about 13 years to add another billion people to the planet, and it would take 12 the next time around. In the meantime, Baby 5 Billion actually got a name: Matej Gaspar, a 7-pound, 9-ounce Croatian boy, was declared by the United Nations to be the official 5 billionth person on the planet. Matej was born at 8:35 a.m. local time in the city of Zagreb, according to Associated Press reports from the time. The secret of being the 5 billionth baby was location, location, location: Officials looked to Zagreb for the birth because the 14th World University Games were being played there at the time.

What's next for a growing world? The U.N. estimates that baby No. 8 Billion will take its first breath around 2025. According to the organization's 2010 population report, Baby 9 Billion will show up shortly before 2050, and the world will welcome its 10 billionth person by 2100. Most of the growth will be in high-fertility countries such as those in sub-Saharan Africa: The population in these areas is likely to triple. Meanwhile, intermediate-fertility countries such as the U.S., Mexico and India will grow by 26 percent, and low-fertility regions such as Europe will actually shrink by 20 percent.

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Stephanie Pappas is a contributing writer for Live Science, covering topics ranging from geoscience to archaeology to the human brain and behavior. She was previously a senior writer for Live Science but is now a freelancer based in Denver, Colorado, and regularly contributes to Scientific American and The Monitor, the monthly magazine of the American Psychological Association. Stephanie received a bachelor's degree in psychology from the University of South Carolina and a graduate certificate in science communication from the University of California, Santa Cruz.