Why your thumb has one bone fewer than your fingers, and whales have many more
Count the bones in your fingers and toes and you reach 56, fourteen per hand and foot. Each finger has three, the thumb only two. That same pattern holds for most land mammals, yet a pilot whale's flipper hides a digit with ten of these bones, and early reptiles carried up to five in a toe.
These digit bones are called phalanges, after the ancient Greek battle formation of soldiers standing shoulder to shoulder in rows, a bit like fingers laid side by side. Anatomists label each by position: proximal ones meet the palm or sole, intermediate ones sit in the middle and are usually middling in size too, and distal ones form the tips. Thumbs and big toes lack the middle bone. In the fourth and fifth toes the middle and tip bones are often fused.
The fingertip bones carry intriguing details. Each ends in a flat, wide expansion called an apical tuft that supports the fleshy pad and the nail. The thumb's tip bone is subtly lopsided, with uneven spines and a strong attachment for the long flexor muscle, an asymmetry that keeps the thumb pad turned towards the other fingers for the widest possible grip. Of all the hand's bones, the tip bones are the first to harden in development, and they do so starting from the far end rather than the middle. Toe bones tend to be shorter and more squashed than their counterparts in the hand.
Biologists summarise the pattern as a phalangeal formula, counting bones digit by digit from the innermost. Humans, like most land mammals, score 2/3/3/3/3 in both hands and feet. Primitive reptiles typically had 2/3/4/4/5, a layout that persisted in modified form in later reptiles and in the ancestors of mammals.
Whales broke the rule. Their flippers show hyperphalangy, extra bones added to the digits. A humpback's flipper reads 0/2/7/7/3, and a pilot whale's reaches 1/10/7/2/1. In hoofed animals, the tip bones instead support and shape hooves and claws, and in primates the equivalent bones are called ungual phalanges.
Source: Phalanx bone
