Pokeconspiracy Wiki

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 or 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 its mother's pouch. Also, 'Bonemerang' (Cubone's signature move) refers to boomerang just as 'Kangaskhan' refers to Kangaroo. Kangaroo and boomerangs are both natively Australian themes.
Before Pokemon Red/Blue released, the developers decided that Kangaskhan needed to be separated from the previous 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 its dead mother's skull on its head and slinging its bones around like clubs and boomerangs. However, the team didn't have time to delete 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 you catch a M' and level it up, it evolves into a Kangaskhan.