Through the application of the Integration Hypothesis, it can be seen that the interaction between the E and L components enables language structure (E component) and lexical items (L component) to operate simultaneously within one form of complex communication: human language. However, these two components are thought to have emerged from two pre-existing, separate, communication systems in the animal world. The communication systems of birds and monkeys have been found to be antecedents to human language. The bird song communication system is made up entirely of the E component while the alarm call system used by monkeys is made up of the L component. Human language is thought to be the byproduct of these two separate systems found in birds and monkeys, due to parallels between human communication and these two animal communication systems.