Just goes to show how far French has fallen: “je suis, tu es, il est, nous sommes, vous êtes, ils sont” comes from “sum, es, est, sumus, estis, sunt”, and “je suis, tu suis, il suit, nous suivons, vous suivez, ils suivent” comes from popular (not Classical) Latin “sequo, sequis, sequit, sequimus, sequitis, sequunt”. But now “sum” and “sequo” are basically the same….
Note that the classical Latin form is “sequor, sequeris, sequitur, sequimur, sequimini, sequuntur” (e.g. “non sequitur”), which is a deponent verb, meaning it looks just like the passive form but in fact it’s active (so, “amor, amaris, amatur” means “I am loved, you are loved, he is loved”). “Loquitur” is also an example of a deponent verb meaning “he speaks”, e.g. “res ipsa loquitur”. Just saying, because some commenters might claim that there’s no such form as “sequo, sequis, sequit” (from the infinitive “sequere”). It’s true that there’s no such form in classical Latin, but there was such a form in popular Latin, and a source I consulted tells me that it derives from the popular Latin form.