What is the oldest religion on the plant which is still being practiced? Is there an estimate as to how many religions there have been practiced since recorded history began? What were the first religions based upon?
The idea of ancestor worship is very common in surviving early religions, Aboriginal 'shamanism' is a good example of this. Also, identifying gods as natural elements is another very common theme in early religions (think of any polytheistic religion), so we can make an educated guess that the earliest religions would have been based on these ideas.
The main problem with many early religions, including the religion of the Aboriginals is the lack of written sources, and the lack of a material culture. It is true that our oldest surviving texts and buildings are Hindu, making it the longest enduring religion as a result.