"The Tree Shack" - Charles E. Kupchella

Late in the otherwise placid 1950's a young Pennsylvanian has a surprising and increasingly jolting encounter with a group of charismatic scientists from the planet, Casparia. The scientists tell him they are in the final stages of an effort started by their people millions-of-years earlier to shape and refine humankind on Earth. Chuck, only a semi-willing study-subject at first, is recruited to help pave the way for a future collaboration between the people of Casparia and the people of Earth.