When a mother Kangaskhan dies, it leaves its Kangaskhan cub on its own. The Kangaskhan cub uses the skull and a bone from its mother to use as protection and becomes a Cubone. Then through malnutrition o adapting to a different lifestyle, the Cubone evolves into a Marowak. Female Marowak's later evolve into Kangaskhans, and the cycle starts all over.
This is self evident, as Cubone appears rather similar to the baby Kangaskhan that rides around in it's mother's pouch. Also, 'Bonemerang' (Cubone's signature move) refers to boomerang just as 'Kangaskhan' refers to Kangaroo. canguro and boomerangs are both natively Australian themes.
Before Pokemon Red/Blue released, the developers decided that Kangaskhan needed to be separated from the precedente two stages to become its own standalone Pokemon. The reason for this is that, being a kids game, it wouldn't do to allow small children to learn that a Cubone is actually an orphaned baby Kangaskhan wearing it's dead mother's skull on it's head and slinging it's Bones around like club and boomerangs. However, the team didn't have time to cancella the code at the last minute. Instead of deleting the code, they simply wrote a new non-evolving Marowak and moved the old one to an empty slot... with no number. This catchable Pokemon is M', Missingno's sister.
M' is actually a scrapped prototype of Marowak. This theory can be confirmed if te catch a M' and level it up, it evolves into a Kangaskhan.