* Larval development: Echinoderm larvae exhibit clear bilateral symmetry. They have distinct left and right sides, a single mouth, and an anus located at the opposite end. This is a strong indicator of their ancestry, as bilaterally symmetrical body plans are common in many other animal groups.
* Fossil evidence: Fossil records show that early echinoderms, like the extinct group known as *Carpoids*, were bilaterally symmetrical. This suggests that radial symmetry evolved later in echinoderm evolution.
* Genetic evidence: Modern research in developmental genetics has uncovered molecular evidence indicating that the genes responsible for establishing bilateral symmetry in other animals are still present in echinoderms, even though they are not expressed in their adult forms. This suggests that the genetic blueprint for bilateral symmetry was present in their ancestors and has been retained throughout their evolution.
* Evolutionary advantage: Bilateral symmetry is advantageous for locomotion and directed movement, allowing organisms to move efficiently and explore their environment. Radial symmetry is more suitable for sessile or sedentary organisms that need to sense their surroundings equally in all directions. It's likely that early echinoderms were more mobile and transitioned to a sedentary lifestyle later, resulting in the development of radial symmetry.
In conclusion, the combination of larval symmetry, fossil evidence, genetic analysis, and the evolutionary advantages of each body plan strongly suggest that echinoderms evolved from bilaterally symmetrical ancestors. Their radial symmetry is a derived trait, adapted to their unique lifestyle.
