I don’t have a time for when be showed up, but I’m sure it’s because it’s always the first to appear in a “new” language. In Old English, be is beon, beom, or bion, meaning be, exist, become, or happen. It comes from the Proto Germanic biju, “I am, I will be”, and can be traced to the Proto Indo European bheue, be, exist, or grow.
Is, the most common word for those of us who write in present tense, comes from the Old English is and can be traced to the Proto Indo European es, to be. This word is still apart of many languages, from the German ist to the French est to the Latin esse. It can also be found in English words like essence. So originally, it was another word for be, or bheue, and now they’re both the same word.
Was comes from the Old English wesan, waes, and waeron (the origin word for the plural were), which are actually forms of the word wesan, to remain. Wesan comes from the Proto Germanic wesanan and the Proto Indo European wes, remain or dwell. Although it used to be its own word, sometime between Proto Germanic and Old English it turned into the past tense of be.
Am comes from the Old English eom, to be or remain, and Proto Indo European esmi. Back in Old English, it only appeared in the present tense and until the thirteenth century meant something like “come to be” while existence was expressed with wes up there. There’s also are, which was earun/aron in Old English and probably came from the Proto Germanic ar, a possible variant of es. I guess people started combining the various definitions of be until they found forms that worked.
You can bet I’ll be doing more of these so-called simple words.
Tony Jebson’s page on the Origins of Old English