Final answer:
The Cambrian Explosion was facilitated by two key evolutionary developments: the rise in atmospheric oxygen levels and the evolution of Hox control genes. Increased oxygen levels likely came first, providing the energy needed for more complex life forms. Hox genes then allowed for greater morphological diversity and complexity.
Step-by-step explanation:
The Cambrian Explosion is noted for the rapid diversification and evolution of animal species approximately 541 million years ago. Two evolutionary developments that helped facilitate this phenomenon are primarily associated with the rise in atmospheric oxygen levels and the evolution of Hox control genes.
The increase in atmospheric oxygen levels likely came before the genetic and developmental changes. Rising oxygen levels provided the energy necessary for the development of larger and more complex organisms. Subsequently, the evolution of Hox genes provided the developmental flexibility that allowed for a wide variety of animal body plans. These two factors combined could explain the rapid increase in biodiversity during the Cambrian period.
Despite numerous mass extinctions that have occurred since the Cambrian period, most of the extant animal phyla we see today evolved during this time due to these and other complex factors that promoted a wide array of survival strategies and adaptations in the face of environmental pressures.