At the time Stupa I was built, Buddha himself was not represented in human form, but symbolized by the horse on which he rode away from his palace, by the wheel of law, by his footprints, and by the pipal tree under which he found enlightenment.
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When Stupa I was constructed, Buddha was represented as a horse, a wheel, his footprints, and the pipal tree where he became enlightened.
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Buddha has always been represented as a human.