The first organisms to develop a hardened endoskeleton were the echinoderms. This group of marine invertebrates includes starfish, sea urchins, sea cucumbers, and brittle stars.

Here's why:

* Early Evolution: Echinoderms evolved around 540 million years ago during the Cambrian Explosion, a period of rapid diversification of life.

* Calcium Carbonate: Their endoskeleton is made of calcium carbonate, a mineral that provides structural support and protection.

* Unique Features: The endoskeleton of echinoderms is unique in its composition and structure, consisting of ossicles (small, interlocking plates) that are arranged in a specific pattern.

While other groups, like vertebrates, later evolved endoskeletons, echinoderms were the first to develop a hardened internal skeleton.