Squeak originally made use of the Smalltalk-80 image from Smalltalk, that is how it descends from real Smalltalk, versus the other Smalltalk vendors that are still around (not sure how they created their versions).
This also applies to Pharo, at least for the initial versions as they forked out of Squeak.
Clascal came to be, because Smalltalk was too demanding for Lisa's hardware.
jecel in the comment thread i linked implies that there was more of a relationship than that, although of course none of the code in the object memory and bytecode interpreter could literally be the same because it was in a different programming language
This also applies to Pharo, at least for the initial versions as they forked out of Squeak.
Clascal came to be, because Smalltalk was too demanding for Lisa's hardware.