>Thackeray's use of both forms demonstrates. "The alternative to the masculine generic with the longest and most distinguished history in English is the third-person plural pronoun. Recognized writers have used they, them, themselves, and their to refer to singular nouns such as one, a person, an individual, and each since the 1300s."

I don't really think it's an evolution, as much as the mid1800's tendency to over classify and striate things going away.