Starfish possess only a substantial, compressed head and lack a body.

Scientists trying to figure out where the starfish’s head is located have come to the surprising conclusion that the starfish is practically the entire body of the animal. The discovery not only solves this long-standing mystery, but also helps us understand how evolution created the dramatic diversity of animal forms on Earth.

Starfish, also known as sea stars, belong to a group of animals called echinoderms, which includes sea urchins and sea cucumbers. Their strange body design has long puzzled biologists. Most animals, including humans, have distinct cranial and caudal ends, and a line of symmetry runs down the middle of the body, dividing it into two halves of its mirror image. Animals with this bilateral symmetry are called bilateral animals.

Echinoderms, on the other hand, have five lines of symmetry radiating from a central point and no physically obvious heads or tails. However, they are closely related to animals like us, having evolved from bilaterally symmetrical ancestors. Even larvae are bilaterally symmetrical and then radically reorganize their bodies as they metamorphose into adults.

These large differences make it difficult for scientists to find and compare equivalent body parts in bilateral animals to understand how echinoderms evolved. “Morphology tells us very little,” he says. Laurent Formery at Stanford University in California. “That’s too strange.”

Formalie and his colleagues decided to examine a set of genes known to direct head-to-tail control. All bilateralist organizations. In these animals, these genes are turned on and expressed in stripes in the outer layers of the developing embryo. The genes expressed in each stripe define which point it is on the cranio-caudal axis.

The aim was to see if gene expression patterns could reveal the hidden “molecular anatomy” of echinoderms. “This particular gene suite is ideal for investigating the diversity of the most extreme forms of animals,” says the team leader. chris lowe, also at Stanford University. “I think echinoderms are a very extreme experiment in how to use that bidirectional network to produce very, very different body plans.”

To the team’s surprise, the gene that determines the head edge of bilateral animals was expressed in a line running down the center of each star star’s lower arm. The next leading gene is expressed on both sides of this line, and so on.

Even more bizarrely, genes normally expressed in the trunk of bilateral animals were missing from the animals’ outer layers. This suggests that the starfish abandoned its trunk region and released its outer layer to evolve in a new direction, Formery said.

The findings show that “the bodies of echinoderms, at least with respect to their external surfaces, are essentially lip-walking heads.” Thurston Lacari from the University of Victoria, Canada, was not involved in the study. Animals like us may have swam away to escape predation. “Echinoderms didn’t need trunks because they were hunched over and armored,” Lacari says.

The idea that echinoderms are “head-like” animals is “interesting and powerful,” he says. Andreas Heyland at the University of Guelph in Ontario, Canada. This raises some very important and fundamental questions about how ecological factors shape the evolution of anatomy, he says. “Finding the underlying conserved patterns provides important insights into how development evolves.”