This argument is flawed. If there's a gift, there must have been a giver, so why don't we ask the giver? Well the Christians already have, and they like the answer they got. Why don't you like it?
Other than, 'some other group of humans told me this' what evidence do you have that the earth should be the birthright of humans more than that of any other thing on earth?
A book I read earlier this year really bent my brain when it included characters working against humanity and for an invading alien intelligence. Seeing it in print was enough to convince me that it would totally happen, but I hadn't considered the possibility before.