That is, someone who isn't important in terms of being a head of state, or a general of an army or whatever. Just like a carpenter or whatever, who didn't suddenly do something noteworthy later (like try to kill a head of state or something), just lead an average life. Also by "reasonably construct a biography" I mean something more than "born at this time, married x, had job y, died at this time" some details about their life, that aren't merely constructed by understanding generally what it was like to be a common person at that time and place.
My vote is for Mersu and Hekanakhte, two Egyptian men from around 2000BCE.
Hekanakhte was a ka-priest, which is...kind of hard to define in modern terms. They were like a funeral director, but they only oversaw one funeral and they did it forever. When a rich man died--in this case a man named Ipi--he left behind lands that were meant to provide an income for the maintenance of his tomb. It was Hekanakhte's job to manage those funds and make sure the rituals for the upkeep of Ipi's soul were performed.
A ka-priest was definitely an upper-class type job maybe somewhere close to a doctor in terms of social standing, so he wasn't a peasant, but also wasn't exactly Pharaoh.
Anyway, Hekanakhte's job required him to go on inspection tours of the property that supported Ipi's tomb, but Hekanakhte was a worrier and also a nag, so while he was away, he bombarded his son Mersu--remember Mersu up there?--with letters.
Hekanakhte wanted Mersu to send him grain. Hekanakhte had business advice for Mersu. Hekanakhte got himself involved in household squabbles. Hekanakhte warned Mersu that he was holding him responsible if the harvest was bad.
And so on.
The letters don't provide a complete biography, but we have some impressive details from lives lived four thousand years ago. We know the names of both men's extended family.We know that Hekanakhte was an unrelenting nag. We know that Mersu, like his father, was a priest. The letters also tell us that, though he was an old man, Hekanakhte took a concubine (or maybe a second wife) and the household hated her. Undaunted, Hekanakhte told Mersu to shut up and learn to cope ("you are five men! What can one woman do to you?").
We have the letters today because Mersu read them at a smaller tomb being built near Ipi's main tomb. After he was done reading, he apparently just threw them on the ground. When the workmen finished work on the tomb, they swept all of the debris--including the letters--into a hole and covered the hole with a stone, creating an accidental time capsule.
Some of the letters were still sealed when discovered, meaning Mersu threw them away unread.
Posted this in reply to /u/Zerim, but just so OP sees it:
[F]or the middle Babylonian period there's the merchant Dumuzi-Gamil! He was a resident of Ur who recorded his own business records (rather than trusting a scribe) and stored them under his floorboards, where they were safely preserved for almost 4000 years until Leonard Wooley dug them up. I first read about him in Paul Kriwaczek's Babylon, but there's a good account online here, in the section "The merchants of Ur".
The first recorded commoner (an illiterate cowherd) likely to have in fact existed in English history is Caedmon (late 7th century). As with all Anglo-Saxon hagiography, the particulars of his fairly short biography (via Bede) are a tad fanciful. But if all Old English hagiographies adorned with a touch of pious fancy were to be deemed fiction, virtually none of the greatest of the English clergy before the 11th century would be regarded as having existed. Needless to say, this is an unnecessary leap to skeptical absolutism. Caedmon may not have received his hymnodic talent from holy nocturnal inspiration (as Bede says). But we should likely take Bede's word for it that a tender of animals by the name of Caedmon lent his gift for hymnody to the clergy (and likely, likewise, conceived his eponymous hymn).