If we look a bit deeper, we must observe that the ground and rock has no need for the other three levels, it can exist alone. The plant life needs the ground. It takes the organic mineral and restructures it into the plant as a organic being. The plant however has no need for that level above it. After this, there is the animal life.
Most animals eat plants.
The animals assimilate the plant into their bodies as they grow. The animal can not use the organic level to grow because it can not convert the organic mineral into body cells. Until now, each level needed the level below it in order to grow. Man is no exception, in order to benefit from the mineral life below him he must eat that which is below him. The mineral can be gotten in two ways.