Here's an interesting thought I just had, while chatting about parenting in the Riker thread. When we think of Data procreating, we always assume another android, like Lal, but if Data ever formed a coupling with another biological being, like he did with Jenna D'Sora, what's to say he couldn't use Soong's DNA and create a new Soong offspring, so his mate could reproduce naturally?
Data looks like Soong. He was created by him. By all rights, he could conceivably father a new Soong line. Who's to say that "Fully functional" didn't also include a built in reproductive factor that would do that very thing? The intention was to make him as human as possible. That would seem to be a prominent part of being one.
We never hear about Soong having had any biological offspring. Perhaps he couldn't or preferred not to, and opted for this option instead. It's ultimately just as valid. What a huge change to human evolution that would be. My granddad was an android built by my great granddad