Without the rigid dorsal nail, ground reaction force causes the soft pulp tissue of the hallux (big toe) to displace upward and outward. The toenail provides a rigid opposing wall, dramatically increasing pulp hydrostatic pressure and preventing phalanx bending stress.
The nail bed is dense with mechanoreceptors. When the toe compresses against objects or terrain, the keratin matrix creates a high-gradient shear zone against the underlying dermis, multiplying tactile sensitivity required for balance and micro-adjustments.
Do humans still need them? Yes. While claws evolved for climbing and defense, flat nails co-evolved with bipedal stride dynamics. Toenails optimize energy transfer during late stance push-off and shield the vulnerable distal phalanx from high-energy micro-fractures.