The Genetic Silence
When Laurent Formery and his colleagues at Stanford University and UC Berkeley mapped the genes active throughout a starfish's body, they found something extraordinary in what wasn't there. The genetic instructions that build the torso and tail in nearly every other animal, the molecular blueprints that pattern the trunk from ribcage to hips, were simply absent, according to the study published in Nature. Where most creatures carry elaborate genetic programs for constructing a midsection, starfish had only silence.
What they found instead rewrites the textbook definition of one of the ocean's most recognizable animals. Genes associated with head development were active throughout the starfish's entire body, according to the research. Not just in one region, but everywhere: in the bumpy skin covering the creature, radiating outward through each of its five identical arms. The starfish hadn't lost its head, as generations of biology students had learned. It had lost everything else.
A Head That Learned to Walk
Starfish belong to echinoderms, a group that also includes sea urchins and sea cucumbers, all sharing a distinctive fivefold radial symmetry that allows the animal to be divided into five identical segments, according to the study. This body plan represents a radical departure from the bilateral symmetry of most animals, including humans, which run from head to tail in a single axis. But the new research reveals that this departure was even more extreme than scientists understood.
To map the starfish body plan at the molecular level, Formery and his team used HiFi sequencing, a method developed by PacBio that can read long stretches of DNA quickly and accurately, according to the study. The technology has advanced dramatically: what once took months can now be completed in hours, at a cost hundreds of times cheaper than five years ago. This speed allowed the researchers to create a three-dimensional map of genes expressed in thinly sliced samples of starfish arms, revealing patterns invisible to previous generations of scientists.
The mapping showed a precise gradient of head genes radiating through each arm. Genes associated with the front part of the head in other animals were active in the middle of each starfish arm, according to the findings. As the researchers tracked gene expression toward the edges of the arms, genes associated with more rear parts of the head became active. The entire creature was organized like five heads fused at their frontal points, each one extending outward with its "back of the head" at the tip.
What Evolution Deleted
To understand what starfish abandoned, the research team compared genetic markers in Patiria miniata sea stars with Saccoglossus kowalevskii, an acorn worm species closely related to starfish, according to the study. Acorn worms, with their well-studied genome, retain the full bilateral body plan: a clear head at one end, a trunk in the middle, and a tail at the other. They represent what echinoderm ancestors looked like before the radical redesign.
The comparison made the deletion visible. Starfish lack genetic patterning for a trunk or torso entirely, according to the findings. Evolution hadn't just modified their midsection or reduced it. The genetic programs that build those body regions had been stripped away, leaving only the head architecture to be repurposed into a new form. Chris Lowe, an evolutionary biologist at Stanford University and study co-author, worked with Daniel Rokhsar, an expert on the molecular evolution of animal species at UC Berkeley, to interpret what this genetic absence meant for understanding echinoderm origins.
This wasn't a gradual modification. At some point in echinoderm evolutionary history, the entire trunk-building program was lost, and the head-building program was freed to organize the body in a completely new way. The result was an animal that could be divided into five identical segments, each one capable of independent movement, each one fundamentally a head structure repurposed for a radial lifestyle.
The Architecture of a Walking Head
The genetic mapping revealed that head-coding genes in starfish were active in the bumpy skin covering the entire body, according to the study. This skin, which in bilateral animals would be limited to the head region, now wrapped the entire creature. The anterior genes, those marking the very front of the head in other species, were especially active at the center of each arm, the point where all five segments met.
This architecture explains the starfish's unusual anatomy at a molecular level. Without trunk genes to pattern a midsection, and without tail genes to create a posterior end, the creature is organized entirely by head-patterning systems. These systems, which in bilateral animals create the concentration of sensory organs and neural tissue we recognize as a head, instead radiate outward through five axes, creating a body plan unlike anything else in the animal kingdom.
The discovery contradicts textbook descriptions of echinoderms as animals that have lost their head, according to the study. Imran Rahman, a principal researcher who commented on the findings, noted the significance of overturning this long-held understanding. The textbooks had it backwards: echinoderms didn't lose their heads when they evolved radial symmetry. They lost their bodies and kept only their heads, which then multiplied into five.
Evolution's Radical Simplification
The starfish body plan reveals something fundamental about how evolution innovates. The path to the echinoderm form wasn't addition or gradual modification. It was deletion on a massive scale. Entire genetic programs that build the trunk and tail, systems conserved across hundreds of millions of years of animal evolution, were eliminated. What remained was a stripped-down toolkit: just the head-building genes, freed from their usual context and deployed in a radial pattern.
This deletion created possibilities. Without the constraints of bilateral symmetry, without the need to pattern a distinct trunk and tail, the head-building program could be expressed in five directions simultaneously. Each arm became an independent unit, capable of moving, sensing, and acting without coordination from the others. The fivefold radial symmetry that defines echinoderms emerged not from adding new complexity, but from removing old constraints.
The implications extend beyond starfish to the entire echinoderm group. Sea urchins and sea cucumbers, despite their different appearances, share this same fivefold radial symmetry, according to the study. All of them likely made the same evolutionary trade: they gave up their trunks and tails, kept only their heads, and reorganized those heads into radial forms. Each group then modified that basic five-headed architecture in different directions, creating the diversity of forms we see today.
Rewriting the Blueprint
The research changes how we understand one of the ocean's most abundant and recognizable animal groups. For decades, echinoderms were described as creatures that had lost their heads during their evolution to radial symmetry. The genetic evidence shows the opposite: they lost everything except their heads. What we call a starfish arm isn't an arm at all in the conventional sense. It's a section of head, extended outward, carrying head genes in a gradient from front to back.
This finding matters beyond correcting textbooks. It demonstrates that evolution can make radical deletions, not just modifications, when a new body plan offers advantages. The transition from bilateral to radial symmetry in echinoderms wasn't a matter of reshaping existing structures. It required eliminating entire body regions at the genetic level, then repurposing what remained into something fundamentally new.
The starfish, moving across the ocean floor on its thousands of tube feet, is a creature built entirely from head architecture. Five heads fused at their frontal points, each extending outward with its posterior end at the arm tip, together forming a body plan that has persisted for over 500 million years. Evolution didn't give starfish a new body. It took away the old one and left them with nothing but head, repeated five times over, walking.