Hard to say. The earliest conception of God could've been unlike anything we see today. A lot of modern hunter/gatherer groups are animists- they believe that spirits are in everything: people, rocks, trees, everything. Australian natives have the Dreaming, which is very interesting and different. There is some evidence that spirituality predates homo sapiens, although that's heavily disputed. Humans, though, have undoubtedly held spiritual beliefs from the very beginning, although they probably didn't look much like the Judeo-Christian concepts we're familiar with.
