Why are Religions so Similar?

Everyone knows that there are strong and often strange similarities between different world religions, but what, exactly, does that mean? Take a common chain of religious similarities as the Christian to Buddhism to Jainism link to consider.

The very famous Ten Commandments of the Christian religion contain the supposed natural human laws: Thou Shall Not Kill, Thou Shall Not Steal, Thou Shall Not Lie, and Thou Shall Not Even-Be-Eyeballing-My-Spouse among others more unique to Christianity. Buddhism also contains a list of ten commandments in some of its most popular forms, this list also includes identical or similar laws as those to Christianity- Do Not Destroy Life (at all- not even bugs), Do Not Take What is Not Given You, Do Not Commit Adultery, and Tell No Lies and Decieve No One, also among others unique to Buddhism.

Though less noted, both Christianity and Buddhism contain a story in their mythology about a disciple of the spiritual figure (Buddha or Christ) walking on water, sinking slightly, but then being raised by the "power" of the figure. It also might be suitable to note that Buddhism arose long before Christianity.

Then Buddhism and Jainism have a VAST amount of similarities. First of all, the founders of the two religions were both Hindu princes and were given the names Prince Siddhartha (Buddha) and Prince Vardhamana (Mahavira) and they were actually born in neighboring kingdoms. Both Princes studied the Vedas, took up the Sacred Thread of Hinduism, and both married a beautiful princess. A sudden shock came to each of them, for Buddha it was observing the truth of human suffering and for Mahavira it was the death of his parents. Both princes reacted similarly to these shocks: they abandoned their kingdoms and wives to become monks attempting to understand human suffering. The result of these extended periods of meditation is the creation of the two religions, Buddhism and Jainism.

So one can see that these religions share similarites that, when compared to the vast random possibilities of mythological and ethical details, is undeniably odd. But both religious and skeptical minds can use this evidence to their own arguments. Does a commonality between the religions of the world prove that there is a fundamental spiritual essence inherit in us all? Or does it simply mean that some crazy toad-licker came up with one whacky idea and other people are no more intelligent? Fligu-Kigu if I know.