tl;dr Originally Proto-Indo-European had no genders. The concept was added piecemeal. Subjects were marked with ·s. Results of an action or grammatical objects were marked with ·m, this was generalised and transferred onto actual objects/things. This was the birth of the first gender distinction, neutrum, and the ·s words assumed the role of the default gender. Collective words (before the invention of plural) and abstract words ended in ·a. The animate/inanimate distinction mentioned by thread neighbours is a red herring, disregard that theory.
The conflation of genus and sexus and hence the unfortunate misnaming of default gender as masculine and abstract gender as feminine which is still in use today is due to bad Roman grammarians; the HN submission article mentions that in contrast Varro understood it better.
See <https://www.belleslettres.eu/content/deklination/genus-gende...> around the 40 to 60% content region.
tl;dr Originally Proto-Indo-European had no genders. The concept was added piecemeal. Subjects were marked with ·s. Results of an action or grammatical objects were marked with ·m, this was generalised and transferred onto actual objects/things. This was the birth of the first gender distinction, neutrum, and the ·s words assumed the role of the default gender. Collective words (before the invention of plural) and abstract words ended in ·a. The animate/inanimate distinction mentioned by thread neighbours is a red herring, disregard that theory.
The conflation of genus and sexus and hence the unfortunate misnaming of default gender as masculine and abstract gender as feminine which is still in use today is due to bad Roman grammarians; the HN submission article mentions that in contrast Varro understood it better.