A gateway substance is something that may be menial but could lead to something bigger, an example is weed in the drugs world, weed is not hard to get and is usually a gateway drug to stronger types of drugs and potentially more dangerous
Multicellular organisms do this by assigning specific roles to specific cells. This enables various cells within a larger organism to collaborate in order to preserve homeostasis. Since they are made up of just ONE type of cell, unicellular organisms do not need cell specialization to maintain homeostasis.
