That is a question that can't be adequately or even approximately answered, but a ballpark answer would be: "When the Industrial Revolution started".
the planet earth is big and has alot of people on it.
The large red planet that starts with J is Jupiter.
People have walked on only one planet and that's our Earth. The only other place they've been is the Moon, which isn't a planet.
=== ===
Earth is the only planet that people have ever walked on. The only other body away from Earth that people ever walked on is the Moon, and that has not happened in over 40 years. The nearest planet (Venus) is always more than 100 times farther away than the Moon is. Venus is hot enough to melt spacecraft and cook people, so even if it were possible, we would not send people to land on it. The next nearest planet (Mars) is always more than 140 times farther away than the Moon is.
contaminating
Which planet to start from is the big question . .
Perhaps the Chernobyl incident in the Ukraine caused Ukrainians to move to Russia therefore contaminating Russians
Planet Earth
Pollution
the planet earth is big and has alot of people on it.
Mars
On the planet Tatooine
SATALOVE
One Planet One People was created in 1986-10.
There is at least 2.5 billion chinese people on this planet and that is a heck of a lot of people there is 6,800,000,000 humans on this planet (2011) so ya that's pretty much :)
Move the checkpoint that you start with