Hierarchy is everywhere in biology. For example, as mentioned many times in my postings, we know why the pecking order is so important in human societies. It is because women tend to marry upwards in their own group or class. This tends to leave inept or genetically handicapped males without offspring. The result of this is that deleterious genes tend to become extinguished in due course.
But neuroscientists from the University of Wyoming and INRIA (France) led by Henok S. Mengistu have realised that there is a much deeper — and simpler — reason why hierarchy is so rampant. This is the cost, in energy terms, of making the connections.
In the human brain when a message reaches the end of its neuron it then has to jump a gap — technically called a synapse — by chemical means — before it can continue being transmitted to the next neuron. The energy-cost of this jump is far greater than the energy required by a message simply running along a neuron.
We have billions of neurons in our brains. Every neuron is capable of influencing any other neuron. However, if all neurons had actual synaptic connections with every other neuron then the energy costs of the trillions of possible connections would be enormous.
If, however, they can be connected in a hierarchical way, rather like our road systems — between major roads and progressively smaller and smaller ones branching off — then there need to be far fewer connections. The energy costs of the whole brain — already substantial in any case — is now far lower than if every neuron had a direct, physical connection with every other.
Like all great ideas it is very simple to grasp once the penny drops. In fact, the hierarchical principle is paramount everywhere in life but the neurological example can serve as the easiest one to demonstrate it.