The story of the woman caught in adultry does not appear in the earliest extent versions of the Gospel and appears to have been added sometime around the 4th century.
Key word: Appears
Appearances can be deceiving.
There is lots of discussion on this topic, from both sides.
Here is one interesting take, but there are many others. http://www.direct.ca/trinity/john8.html
Also, notice from this particular article:
"This passage can be found in translations that date from the second century. This can be confirmed by the comments on this passage by the Early Church teachers which range from Didascalia (third century) to Saint Augustine (430 AD). Saint Augustine gives us a little more insight into the problems of this passage by many of his time..."
However, for the curious, it goes even deeper than this. Enjoy!