Anyhow, I found this by John Palmer, who if you don't know is like the Elvis of brewing. Here's a quote from an article in BYO entitled "BIAB: Tips from the Pros" :
Brew in a bag (or basket) might actually be the original home brewing method from thousands of years ago, and traditional mash and lauter tuns may actually be new-fangled contraptions to enable large scale brewing in the relatively recent past centuries. Therefore, embrace this new-old method and don’t be afraid to adapt it to modern brewing.
He's probably right.
Obviously brewing was a process that developed over a long period of time with unique methods prevailing regionally. This method, or straining with baskets and linens, is just one possibility.