Final answer:
Bilateral symmetry is indicative of an organism that moved actively within its environment, allowing for directed mobility and the development of a head and tail region, which is a contrast to the stationary nature associated with radial symmetry.
Step-by-step explanation:
Bilateral symmetry is a key indicator of an organism that moved around within its environment. This type of symmetry, which can be seen in animals like butterflies, crabs, and humans, includes a division through a midsagittal plane, resulting in two superficially mirror-image halves with distinct anterior (head) and posterior (tail) ends, as well as dorsal (back) and ventral (front) surfaces.
This evolutionary trait promotes active and directed mobility, enhancing the organism's ability to seek resources and engage in predator-prey interactions. In contrast, radial symmetry is typically associated with a more stationary or limited-motion lifestyle and is seen in organisms such as sea anemones and jellyfish, which can be divided into identical halves along any plane cut through their longitudinal axis.