God’s covenant with Adam in the garden was conditional.  To the obedient man the tree of life and all its blessings were his and his descendants.  By this, Adam became the representative, the federal head, of humanity.  Therefore, when Adam sinned, the effect was disastrous, a sin nature inherited by all his descendants and evident in their ongoing sin and death that resulted.   Only another federal head who was sinless could fulfill the covenant,  rescue man from his predicament, and restore him to fellowship with his covenant Father.