When Adam and Eve sinned, they broke fellowship with God and lost the Kingdom.  God had a plan of reconciliation which involved His Spirit.  In order to accomplish this plan, God needed a human body to house His Spirit and correct the problem.  Since sin came into the world through a man, Adam,  it needed to be removed by a man.  Therefore, God used a human (his Son) to break the power of the adversary over mankind and regain the authority and dominion Adam once held.