Putting it colloquially, “Starfish ain’t got no head.” Despite their having only 5 or so arms that look alike and no head, starfish are more closely related to humans in evolution than are the great majority of all other animals that have eyes, a brain, and symmetric left and right sides.
How did starfish get their very odd shape? They developed that shape a few hundred million years ago, and did so from ancestors that had so-called bilateral (two-sided) symmetry. This is an interesting story in the large. Our really remote animal ancestors of about half a billion years ago also had radial symmetry, ‘arms’ alike, like the jellyfish. Many evolved into nearly a million species with two-sided symmetry, such as modern butterflies, and us.
Now, a fossil ‘starfish’ (an echinoderm, or tube-foot) shows the two-sided symmetry. It was found in the Atlas Mountains of Morocco. It shows a mix of two-sided symmetry, evident near its mouth, and also the paired rows of tube feet and sensors found on all five or so arms of modern starfish.
A new interpretation of modern starfish is not that the arms are like limbs of other animals, but its “face-down” surface on the sea floor is more like a head. Genetic studies of starfish and relatives might inform us about how we “kept our head” while starfish just about lost it.
This has been an outreach activity of the Las Cruces Academy, viewable at GreatSchools.org
Source: Nature, 25 Sept. 2025, pp. 864-5.
Image: Same
