Man has long been on the verge of overpopulating the Earth, if you believe the warnings.
Around 200 A.D., the Roman writer Tertullian lamented that "we are burdensome to the world, the resources are scarcely adequate to us." The population at the time is believed to have been 200 million, barely 3 percent of today's 5.8 billion.
Demographer Joel E. Cohen, head of the Laboratory of Populations at New York's Rockefeller University, cites Tertullian's warning in his 1995 book "How Many People Can the Earth Support?"
Cohen also reports on the effort in 1679 by Antoni van Leeuwenhoek, the Dutch inventor of the microscope, to calculate the maximum number of people the Earth can support. At the time, the population was probably 600 million.
Leeuwenhoek projected the population density of 17th-century Holland over what he supposed was Earth's total land area. He came up with a maximum of 13.8 billion people.
Demographers point out the flaws in his simplistic approach. But, in fact, authoritative projections today suggest world population may top out between 11 billion and 20 billion in the next century.