Deep in coastal mud and shallow lagoons, a tiny, brainless sea anemone is quietly using the same molecular playbook that helps human embryos decide where to put a back and a belly.
A new study from the University of Vienna shows that the starlet sea anemone Nematostella vectensis builds its body using a signaling system long associated with bilaterally-symmetric animals such as humans, insects and worms.
The work suggests that this body blueprint dates back at least about 600 million years.
Two very different animals that are not so different
Biologists usually divide complex animals into two huge groups. One contains bilaterians, which have a left and right side and a clear head to tail direction. The other includes cnidarians, such as jellyfish, corals and sea anemones, which are often described as radially symmetric.
Humans sit firmly in the first group, while sea anemones, with no brain and a simple tube-like body, belong to the second. Yet when researchers look at gene activity in some anemones, a subtle bilateral pattern appears during development and even later in adult anatomy.
It raises a natural question. How deep do those similarities really go?
How a chemical gradient lays out a body
The new research focuses on bone morphogenetic proteins, usually shortened to BMPs, and on a partner molecule called Chordin. In vertebrate embryos, BMP signals create a gradient from low to high across the forming body.
Cells read their position in that gradient and choose their future role. Regions with very weak BMP activity give rise to the central nervous system, intermediate levels help form kidneys, and high levels contribute to tissues such as belly skin.
In practical terms, this chemical slope tells the embryo which side will be back and which will be belly.
In many bilaterian animals, Chordin does more than simply block BMP signals. It can grab BMP molecules, carry them through the embryo and release them elsewhere, a process known as BMP shuttling.
That shuttling creates a sharp and stable gradient that lasts long enough to pattern the whole axis.
Sea urchins, flies and frogs all use some version of this trick, which made it a prime suspect for an ancient mechanism inherited from a very early animal ancestor.
Testing a sea anemone’s hidden symmetry
To see whether sea anemones also use this strategy, the team worked with embryos of Nematostella vectensis.
First they switched off the gene for Chordin. Without it, BMP signaling disappeared and the developing anemones failed to set up their secondary body axis, a sign that the whole system had collapsed.
Then the researchers brought Chordin back, but in two engineered versions. One form was fixed to cell membranes.
The other could move freely through the embryo. Only the mobile version restored strong BMP activity on the opposite side of the embryo and rescued axis formation, clear evidence that Chordin must act as a shuttle in this species.
An ancient mechanism with modern implications
Finding the same shuttling system in sea anemones and in distant bilaterian groups strongly hints that it arose before those lineages split, roughly 600 to 700 million years ago. As first author David Mörsdorf puts it, the fact that both groups rely on shuttling shows that “this mechanism is incredibly ancient”.
Senior author Grigory Genikhovich notes that if the last common ancestor of cnidarians and bilaterians already had a bilateral body, “chances are that it used Chordin to shuttle BMPs” while building its back-to-belly axis.
At the same time, the researchers stress that biology rarely gives completely tidy stories. Some modern bilaterians pattern their bodies with different tools, so independent evolution of symmetry in a few lineages cannot be fully ruled out.
Why this matters for today’s oceans
For environmental science, the study is a reminder that modest-looking coastal creatures carry an outsized share of Earth’s evolutionary memory. Nematostella vectensis lives in brackish salt marshes and lagoons, often in narrow ditches or shallow estuaries that are easy to overlook and easy to damage.
Because it is hardy and simple to keep in the lab, this anemone has become a key model for understanding how marine invertebrates respond to heat waves, low oxygen and pollution, as well as how their genomes interact with changing environments.
At the end of the day, if habitats like these are lost to coastal development or unchecked warming, we do not just lose another small invertebrate. We also erase living archives that still carry instructions written near the dawn of animal life.
The next time you see a photo of a thin, translucent sea anemone, it might be worth remembering that the same basic logic that shaped its body helps shape ours too.
The study was published in Science Advances.
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