Board Thread:Theories and Speculation/@comment-25319479-20171101152711/@comment-6036561-20171101191743

BerserkerPhantom wrote: Not really, no. But Senju Clan is known for it, and Hashirama isn't the first to unlock it (It was mentioned in one of the databooks I believe. We have it written as if it originated from the Senju Clan, I guess it had to come from somewhere)

We assume that it was originated from the Senju clan because (i qoute from the WR page):

Despite being known as a kekkei genkai, Hashirama Senju is the only known human to have naturally possessed Wood Release.

But again, there is nothing that says (or implies) that only Senjus can unlock WR. Again, WR may be simply extremely rare to unlock, and that's all