A groundbreaking hypothesis suggests that the Cambrian Explosion, which marked a rapid surge in animal diversity around 500 million years ago, was primarily influenced by the early evolution of complex nervous systems rather than the development of shells and limbs.

“The phase between the late Ediacaran and early Cambrian periods (approximately 550 to 520 million years ago) represents the most significant evolutionary development of animals on Earth,” stated Ariel Chipman, a professor at the Hebrew University of Jerusalem.

“During this time, there was a substantial increase in animal complexity and diversity, transforming the biosphere from a realm characterized by low diversity of primarily sedentary and bottom-feeding organisms to a dynamic environment filled with various motile animals, showcasing diverse locomotor modes and occupying dynamic ecosystems with an array of feeding strategies.”

“This transformative phase is often referred to as the Cambrian Explosion.”

Instead of seeking a singular reason for the surge in animal diversity, Professor Chipman reconstructs the Cambrian period as a series of interconnected developments in which escalating ecological complexity spurred the evolution of sophisticated nervous systems, particularly the brain.

As interactions between predators and prey intensified and marine environments grew more competitive, organisms faced heightened pressures to detect, process, and respond to their surroundings.

This ecological shift enabled the evolution of intricate neural systems adept at processing increasing sensory information.

At the core of this framework lies what Professor Chipman terms the ‘Brain First Hypothesis’.

This model proposes that brain expansion and regionalization occurred early in the evolutionary timeline, significantly contributing to subsequent anatomical innovations rather than being a mere byproduct of advanced bodily structures.

Notably, the researchers indicate that the genetic mechanisms responsible for brain development were not confined to the nervous system alone.

Through a phenomenon known as co-option, these genetic toolkits were repurposed for the formation of other organ systems.

This reutilization of existing developmental pathways facilitated the emergence of more complex body plans, including specialized digestive systems, advanced sensory organs, and segmental structures.

The rise in overall biological complexity allowed certain animal groups to thrive in a broader range of ecological niches, enhancing their evolutionary success.

This trend was not uniform across all life forms; it was particularly pronounced in groups like arthropods, mollusks, annelids, and chordates—lineages known for their high structural complexity and remarkable species diversity today.

“Instead of conceptualizing a single ‘explosion’, we should consider a sequence of interlinked steps,” Professor Chipman asserts.

“As environments evolved to be more complex, animals required improved methods to process information.”

“The evolution of the brain has made this possible, paving the way for even greater diversity in body forms and lifestyles.”

“It’s crucial to note that increased complexity is not inherently superior; several organisms have thrived with simpler body designs. This highlights that evolutionary success hinges on the specific demands of an organism’s environment.”

“By refocusing from a singular dramatic event to a series of gradual changes, this study offers a fresh perspective on the origins of animal diversity.”

“Future investigations, especially in genetics and developmental biology, may verify this hypothesis and further clarify the role of the brain in shaping the trajectory of life on Earth.”

Professor Chipman’s research paper was published in April 2026 in the journal bio essay.

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Ariel D. Chipman. 2026. Throughout the Cambrian, increases in animal diversity were driven by ecologically driven brain complexity. bio essay 48 (4): e70136; doi: 10.1002/bies.70136