There are a number of factors involved in weight experienced by a particular mass on the surface of the earth. There is of course the Mass of the earth itself, the variable distribution of heavier elements in the earth itself, the motion of the ocean mass due to tides etc, the motion of the moon and other components but for the sake of this argument I would like to consider these factors idealised as fairly constant at any particular point in time. One other major component to an object's "weight" on the surface of the earth however is the fact that all object on the Surface of the earth are rotating around the earth at the same speed as the surface. This means that we are in a decaying orbit around the earth's center of mass, with the downward portion of that vector constantly arrested by our constant contact with the surface of that mass. This is a component of our measured weight as well. ~600 million years ago the earth's rotational period was around 21.9 hours instead of 24 hours. This logically means that if you existed 600 million years ago because of your increased "sub-orbital" velocity on the surface of the earth, your same mass would in fact "weigh" less than it does today.