Wind the clock back to roughly 541–520 million years ago. In the long quiet just before, Earth’s seas held mostly soft-bodied creatures—simple grazers and filter feeders.
Then, in what the fossil record registers as a geological blink, bodies diversified and behaviors multiplied: eyes sharpened, carapaces stiffened, limbs articulated, burrows deepened, and entire food webs bootstrapped themselves into existence. We call this the Cambrian explosion - wikipedia 
Two forces likely amplified each other. First, predator–prey escalation: once true predators appeared, staying a soft, slow blob was no longer a stable strategy. Prey that could detect, decide, and defend survived more often; predators that sensed, pursued, and grasped more effectively were rewarded, driving an arms race that favored better vision, segmentation, coordination, and novel materials.
Second, sexual selection: once bodies and behaviors varied, mate choice rewarded credible signals of fitness—pushing traits toward expressivity and recognizability. These forces acted atop enabling conditions (e.g. oxygenation, developmental toolkits such as Hox genes), explaining why complexity didn’t just creep; it cascaded. wikipedia wikipedia wikipedia
What actually changed was systematic. Sensing: photoreception became image-forming eyes, turning gradients into objects and enabling pursuit and evasion. Decision-making: reflex arcs gave way to neural programs that integrated signals and coordinated multi-limb actions—primitive loops of option generation, selection, and action. Materials: soft gels gained shells, plates, and spicules; tissues specialized; bodies became architectures.
Niches: sediments became 3-D labyrinths; the water column a hunting ground; reef-like communities emerged. In short, complexity emerged where selection pressures rewarded recognizable, transmissible, and composable innovations that made sense to other organisms—prey, predators, and mates - wikipedia
# See - Design Stance