It was another 123 years before it reached two billion in 1927, but it took only 33 years to reach three billion in 1960. Thereafter, the global population reached four billion in 1974, five billion in 1987, six billion in 1999 and, according to the United States Census Bureau, seven billion in March 2012.
World population milestones were unnoticed until the 20th century, since there were no reliable data on global population dynamics. It is estimated that the population of the world reached one billion for the first ... billions. Population, 1, 2, 3, 4, 5, 6, 7, 8, 9 ... Demographers do not universally accept this date as being exact.
So, if 1 billion people took the human race 200,000 years to reach in 1804 and it took another 123 years to 1927 to make 2 billion people and then 3 billion people were reached in 1960 and so on, why is 1 billion people a usefully sustainable level of people on earth?
I'd say that it's that when other species begin dying en masse like they are now on earth in the 6th Great extinction which started the last 500 years or so, you either reduce human population back to about 1 to 2 billion people or all species go extinct eventually.
However, there might be ways to engineer a different outcome too. But, that likely would mean all or mostly all other species would be dead including all cattle, all sheep, all deer and all wild animals that aren't inside of zoos.
So, 1 billion is sustainable for thousands of years, 2 billion people might be sustainable for about 200 to 500 years.
However, the present level of population is not sustainable at all and likely we will be reducing drastically populations by the end of this century or face extinction along with all other species here on earth within a few hundred years.
However, Global Warming and global Climate change likely will reduce populations in extreme ways of both humans and animals throughout the next few centuries too, and this way big wars like World War I and II might not be necessary to control world populations.
For example, nuclear weapons have been the biggest single thing stopping world war III because everyone knows that would end the planet itself pretty quickly into an asteroid belt like the one out past Mars.