-Cubone is a younger Kangaskhan and evolves to Marrowak if male, and if female it evolves to Kangaskhan. Missingno evolves into Kangaskhan when leveled up.
-Placeholder value of M' evolves into a Kangaskhan when leveled up
-Game Freak may not have wanted kids to play a game where a Pokemon evolves from the mother Pokemon's death
-Male animals tend to be more attractive to get mates, whereas females were larger so they could care for many young. Kangaskhan was larger and likely the female version of Marrowak.
-The child in Kangaskhan looks like Cubone
-Game Freak may have intended to have an evolution of 4 stages, or have splits like in later games. Programming issues may have led to them scrapping the idea.
This tree makes sense. I found it on another site.