It’s been a while since we’ve done zero and I figured since we’ve just had a month of words, why not a number?
One is an ancient number, perhaps the oldest recorded one. A bone found in the Congo from twenty thousand years ago has notches that indicate counting by ones. One itself has had some interesting forms over the centuries. The Greeks actually used α'. Seriously, an alpha with a quote mark. It was also a curvy Arabic line, the Roman Numeral I, a single dot for the Mayans, a — in Chinese, and a kind of squished N in Hebrew. Most of those make sense as they’re just single lines or dots, and apparently Greek numbers used to be just the letters of their alphabet with a quote mark at the end (their 2 for example is β').
The 1 that we use however is first recorded India in the fourth century CE, but had actually been in use for quite a while before that. It moved through the Arabic world, which changed and modified all of the Indian number symbols. One was a — in ancient India, but a 1 in Arabic, and believe me, it’s the least changed. You’ll see when we eventually get to the other numbers. Anyway, from there the numeral system spread across northern Africa, entering Europe by way of Spain.
Now for the word. One showed up in the early thirteenth century from the Old English an, one or single, and is also the origin for an. And a for that matter. The article, not the letter (yes, they’re really separate things). Seriously. Anyway! Before Old English, it was the Proto Germanic ainaz and earlier, the Proto Indo European oi-no-, one or unique. So because one is single, it is one. Okay. Sure.
Tony Jebson’s page on the Origins of Old English