the ancient who built Reese would have looked like a human to any other human in the galaxy, we are their second evolution after all.
as for the specific, I can't recall the exact episodes, but I'm sure McKay makes reference to Reese, but the story goes that when the ancients left Pegasus, they went to milky way, a scientist tried to re-create a missing link possibly, an android that can control the aggressive nanites. it failed, killed the planet and the bugs left to destroy other worlds.
The theory is proven by several factors.
1. The human form replicators evolved into the exact same way as Pegasus replicators.
2. The Pegasus replicators (asurans) are over 10,000 years old.
3. When O'neill made the replicator disruptor using the ancient database, it makes sense, since the ancients would be familiar with replicators already and know how to disrupt them.
http://www.gateworld.net/omnipedia/races/r/replicators.shtml