One of the most iconic spots in the United States, the Grand Canyon has a storied past that began nearly 2 billion years ago. While this marvel of rock and river is magnificent for many reasons, one chapter in its natural history provided an ideal environment for accelerated evolution.
Long after its initial formation, the Grand Canyon played a crucial role in what’s known as the Cambrian explosion, which started around 530 million years ago and lasted about 40 million years. Nearly all major groups of animals alive today can trace their evolutionary origins back to the Cambrian period.
During this evolutionary turning point, the area that is now the Grand Canyon was underwater and teeming with rapidly evolving sea organisms—including a so-called 'penis worm' that featured a spiky mouth for scraping up algae.
Ancient Shoreline Offered Prime Conditions for Life
The conditions at the Grand Canyon at that time made it an ideal spot for the Cambrian explosion to boom. Back then, the area was much closer to the equator and submerged in shallow sea water. (The canyon would not be carved out by the Colorado River for another 500 million or so years.) The ecosystem’s warm waters were neither too deep nor too shallow, creating a sweet-spot environment for life to thrive and evolve.
“In the kind of environments where you have abundant food, abundant nutrients and oxygen, animals can really afford to make big physiological investments,” says Giovanni Mussini, a paleobiology doctoral candidate at the University of Cambridge.
The Grand Canyon sat on a continental shelf—a submerged part of a land mass in relatively shallow water. What is now the canyon’s eastern side was a coastline more than 500 million years ago. The coastline featured intermediate depths, which allowed sunlight to reach deeper waters. That sunlight meant photosynthesis fueled algae growth, which, in turn, oxygenated the water.
In too-deep water, sunlight isn’t able to nurture plant life, Mussini explains. In too-shallow water, there are more stressors like rough wave activity and greater exposure to ultraviolet radiation. But the site’s so-called Goldilocks zone was just right for growth.
“It looks like these kinds of highly productive, highly habitable shelf environments probably fueled evolutionary arms races,” Mussini says.
Marine Creatures Develop New Ways to Scrape for Food
Animal remnants reveal just how productive this period was. In a July 2025 paper published in Science Advances, Mussini and his co-authors describe tiny specimens they found at the Bright Angel Formation, a geological formation in a sedimentary layer from the Cambrian period. These are the Grand Canyon’s first soft-bodied fossils, coming from creatures like crustaceans, mollusks and a new species of marine worm known as the priapulid—also known as a cactus or penis worm.
These fossils date to between 507 and 502 million years ago, so the Cambrian explosion had already been in full swing for millions of years. But their adaptations were advanced, resembling anatomy that their modern-day counterparts still have.
“There is this theme of cutting-edge technology for its time,” Mussini says. “It's the kind of environment where we really see a substantial escalation in the kind of complexity of adaptations, especially the feeding strategies that we observe in the aftermath of the Cambrian explosion.”
For example, molars of two crustacean species were used to grind up plankton swept up by hair-like spines. Fossils of slug-like mollusks feature what Mussini describes as “conveyor belt”-like teeth that helped them scrape algae and bacteria from rocks. And the penis worm used a filtering mechanism in its teeth as well as what Mussini describes as a “raking and scraping apparatus.”
