system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How did "s" become the letter that pluralises nouns ("one apple, ten apples") in so many languages?
gpt
Around the time of the late Roman Empire, c. 400 a.d., the western Roman Empire was breaking apart. Latin broke apart with it. North and west of the Italian peninsula, we think, people started to speak their latin in a different way than people to the east and south of the Italian peninsula. In the west, they started generalising the rule for plurals in the latin accusative case (adding -s) for just, all nouns in all cases. In the east, they used the nominative case instead, leading them to make their plurals by changing the final vowel. The western latin evolved into French, Spanish, Catalan and Portuguese, while the eastern Latin evolved into Italian and Romanian. (So this is how we get Spanish hombre/hombres but Italian uomo/uomini). Sardinian, right on the dividing line, is more similar to Latin and Italian but has -s for plurals. 

In a later, totally weird coincidence, Anglo-Saxons in Britain at some point started forming lots of plurals with an -as ending, which eventually became -es and then just -s. -as (or -az) was already a normal plural pattern for nominative case words with certain endings in proto-germanic; in English it was just generalised to lots of other words over time. Why this happened in English but not in Dutch (which forms some, but not all plurals with -s) or German (which dropped the -as plural completely) is not entirely clear. Possibly it could be because of contact with Latin and Old French? But English preserved a lot of 'irregular' plurals from its Anglo-Saxon roots anyway.