Gravity is the potent force on Earth's surface that pulls any matter towards the earth by acting on its mass. Hence, objects pulled move with an acceleration due to gravity (g), which has a universal value of 9.8metres per second^2
You take any of the "signs of change in populations of organisms and their environments". For imagine, Australopithecus (early man) turning to Homo sapiens. In that way they could tell that you took this journey.
