The Cambrian period, which occurred roughly 541 to 485 million years ago, is often called the "Cambrian Explosion" because it saw a rapid diversification of life on Earth. The most important evolutionary step brought by this period was the development of complex, multicellular organisms with hard body parts.

Here's a breakdown:

* Before the Cambrian: Life on Earth was mostly limited to single-celled organisms and soft-bodied, simple multicellular organisms.

* During the Cambrian: We saw the appearance of a wide range of complex animals with distinct body plans, including:

* Hard skeletons: These structures provided support, protection from predators, and allowed for more complex movements. Examples include shells of trilobites, the spiny armor of anomalocarids, and the external skeletons of early arthropods.

* Specialized tissues and organs: This allowed for more efficient functions, including feeding, movement, and reproduction.

* Bilateral symmetry: This allowed for directed movement and the development of specialized body regions, like heads and tails.

* Predation: The development of predators led to an arms race with prey, driving further evolution of body plans and defensive strategies.

The Cambrian explosion had a profound impact on the evolution of life:

* It set the stage for the development of all the major animal groups we see today.

* It led to the ecological diversification of life on Earth, with the establishment of complex food webs and ecosystems.

* It provided the foundation for the further evolution of life, including the development of vertebrates and eventually, humans.

While the exact reasons for the Cambrian explosion are still debated, it's clear that the development of complex, multicellular organisms with hard body parts was a critical step in the history of life on Earth.