A new hypothesis proposes that the Cambrian Explosion — the sudden burst of animal diversity 500 million years ago — was not driven by shells or limbs, but by the early evolution of complex nervous systems.
Schematic representation of the main points of the Brain First Hypothesis. Image credit: Ariel D. Chipman, doi: 10.1002/bies.70136.
“The period between the Late Ediacaran and the Early Cambrian (roughly 550-520 million years ago) is the most dramatic period in the evolution of animal life on Earth,” said Professor Ariel Chipman from the Hebrew University of Jerusalem.
“This period represents a sequence of increases in animal complexity and diversity, during which the biosphere transitioned from including a low diversity of mostly sessile suspension or bottom feeders to a world with numerous animal body plans occupying a dynamic tiered ecosystem with diverse feeding modes, comprising a range of motile animals moving using different modes of locomotion in different spaces.”
“This transition is usually referred to as the Cambrian Explosion.”
Instead of looking for a single trigger behind the rise in animal diversity, Professor Chipman reframes the Cambrian period as a cascade of interconnected developments, where increasing ecological complexity drove the evolution of more sophisticated nervous systems, particularly the brain.
As marine environments became more dynamic and competitive, with growing interactions between predators and prey, organisms faced new pressures to sense, process, and respond to their surroundings.
This ecological shift favored the development of more complex neural systems capable of handling increasing amounts of sensory information.
At the center of this framework is what Professor Chipman terms the Brain-First Hypothesis.
Rather than viewing complex nervous systems as a byproduct of advanced body structures, the model suggests that the expansion and regionalization of the brain came early, and played a key role in enabling further anatomical innovation.
Crucially, the researchers proposes that the genetic mechanisms underlying brain development did not remain limited to the nervous system.
Through a process known as co-option, these same genetic toolkits were reused to pattern and build other organ systems.
This reuse of existing developmental pathways helped drive the emergence of more complex body plans, including specialized digestive systems, advanced sensory organs, and segmented structures.
The increase in overall biological complexity allowed certain groups of animals to adapt to a wider range of ecological niches, contributing to their evolutionary success.
The effect was not uniform across all life forms. Instead, it was particularly pronounced in groups such as arthropods, mollusks, annelids, and chordates, lineages that today exhibit both high structural complexity and exceptional species diversity.
“Rather than thinking about a single ‘explosion,’ we should think in terms of a series of linked stages,” Professor Chipman said.
“As environments became more complex, animals needed better ways to process information.”
“The evolution of the brain enabled that, and in turn opened the door to greater diversity in body forms and lifestyles.”
“Importantly, increased complexity is not inherently advantageous. Many organisms have thrived with relatively simple body plans, highlighting that evolutionary success depends on the specific demands of an organism’s environment.”
“By shifting the focus from a single dramatic event to a sequence of gradual changes, this research offers a new way of understanding the origins of animal diversity.”
“Future work, particularly in genetics and developmental biology, may help test this hypothesis and further clarify the role of the brain in shaping the trajectory of life on Earth.”
Professor Chipman’s paper was published in April 2026 in the journal BioEssays.
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Ariel D. Chipman. 2026. An Increase in Animal Diversity was Facilitated by Ecologically-Driven Brain Complexity Throughout the Cambrian. BioEssays 48 (4): e70136; doi: 10.1002/bies.70136
