How is modern earth different from earth over four billion years ago

Earth was not habitable four billion years ago but now it is the only known planet that supports life.

Explanation:

Four billion years old earth was made up of dust and gases. It is claimed by scientists that the sources from which earth was formed included  – a) Lunar rocks, b) Meteorites and c) Zircon crystal. The temperature of earth at that time was extremely high such that all the metals with in it were in molten state. Also there was no oxygen in the atmosphere. All these conditions made earth a non habitable planet.  

But the present earth is a habitable planet with life on it that is dependent on its resources like air and water. Also the temperature of earth is much supportive to the existence of life on it.  

i'm thinking (a) to keep sand in place; ////////////

explanation: a sand fence or sandbreak, like a snow fence, is used to force windblown, drifting sand to accumulate in a desired place. sand fences are employed to control erosion, sand dune stabilization, keep sand off roadways, and to recruit new material in desert areas.