Some verbs, in their first person singular form, have an unpronounced '-s' at the end. For example, je suis, je finis, etc.
In Latin, the suffix for first person singular was '-o' or '-m': thus sum and finio. To my knowledge, Spanish and Italian also lacks 's' at the end of first person singular verbs.
My question is how French developed '-s' in their first person singular form. Why do we have an additional 's'? Was it pronounced some time in the history of the language, or is it just an artifact of the orthography?