Earth + Fire = Lava
Lava + Air = Stone
Water + Stone = Sand
Water + Earth = Swamp
Swamp + Sand = Clay
Air + Fire = Energy
Swamp + Energy = Life
Clay + Life = Golem
Golem + Life = Human
You can get this relatively early on and its required to finish the game, but here's the fastest path:
Can be created in Episode One using the following combinations:
Earth + Fire = Lava
Lava + Air = Stone
Water + Stone = Sand
Water + Earth = Swamp
Swamp + Sand = Clay
Air + Fire = Energy
Swamp + Energy = Life
Clay + Life = Golem
Golem + Life = Human