If any single person is responsible for this male-centric usage, it’s Anne Fisher, an 18th-century British schoolmistress and the first woman to write an English grammar book, according to the sociohistorical linguist Ingrid Tieken-Boon van Ostade. Fisher’s popular guide, “A New Grammar” (1745), ran to more than 30 editions, making it one of the most successful grammars of its time. More important, it’s believed to be the first to say that the pronoun he should apply to both sexes.
The idea that he, him and his should go both ways caught on and was widely adopted. But how, you might ask, did people refer to an anybody before then? This will surprise a few purists, but for centuries the universal pronoun was they. Writers as far back as Chaucer used it for singular and plural, masculine and feminine. Nobody seemed to mind that they, them and their were officially plural. As Merriam-Webster’s Dictionary of English Usage explains, writers were comfortable using they with an indefinite pronoun like everybody because it suggested a sexless plural...
Meanwhile, many great writers — Byron, Austen, Thackeray, Eliot, Dickens, Trollope and more — continued to use they and company as singulars, never mind the grammarians. In fact, so many people now use they in the old singular way that dictionaries and usage guides are taking a critical look at the prohibition against it. R. W. Burchfield, editor of The New Fowler’s Modern English Usage, has written that it’s only a matter of time before this practice becomes standard English: “The process now seems irreversible.” Merriam-Webster’s Collegiate Dictionary (11th ed.) already finds the singular they acceptable “even in literary and formal contexts,” but the Usage Panel of The American Heritage Dictionary of the English Language (4th ed.) isn’t there yet.
All new to me. You learn something every day.