Is the baptism (but not John) interpolated in Mark?

[Secret Alias]

So let's grant Ben's argument that Against Praxean is essentially about a 'different sort' of adoptionism than what scholars suppose was associated with the Gospel of Mark. But how was the Gospel of Mark read in antiquity? I don't think we have any actual evidence outside of assumptions developed from the current text. In fact, all we have left really - in terms of 'ancient interpretation' of Mark - is Irenaeus's - unexpected - connection between 'the gospel of Mark' and a scenario where Jesus is crucified and Christ stands by impassably. Indeed even the assumption developed around the baptism of John narrative viz. an adoptionism which assumes Jesus to have been a mortal man immersed by John the Baptist with the announcement regarding 'the Son of God' has absolutely no ancient attestation whatsoever. Which is very odd. Let me explain what I mean.

While it is true that the surviving canonical gospel of Mark reads like a certain type of adoptionism, it has to be noted that no one from antiquity actually witnesses that kind of adoptionism associated with the Gospel of Mark. I am curious. Where do scholars see explicit examples of (1) Jesus is a fully human man who (2) was baptized in the Jordan by John (3) dove came down, voice from heaven (3) Jesus became Christ at that moment Christianity? Where is this understanding made explicit and explicit with respect to how the gospel was used to support this position. I for one can't think of any heretical group besides a 'hunch' it was a certain 'Jewish Christian' community.