Pants are a relatively recent development...I'd expect that we'd find that pants evolved from leggings in areas of extremely cold weather...Inuit cultures, Native Lapps, Siberians. I'd also postulate that pants caught on more quickly in cultures that were dependent on horseback riding like among the Mongols. Unbifurcated would seem to actually be more like the historical norm with bifurcated being the johnny-come-lately.

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AA