Long ago primitive sharks had ridges running down their sides from gill to tail. Later, muscles grew into the folds, and eventually, the central portion of each ridge receded while the ends enlarged to form fins both fore and aft. All was well.
Then one day several hundred million years ago, a fish was swimming blissfully in a shallow pool. The tide went out and much to the fish’s surprise, she could use her five-rayed fins to move around a bit on the rocky bottom. The tide came in and she swam away, never to give this event another thought. The world, however, was forever changed.
For many generations thereafter, that fish’s offspring went back to the pool and progressively got better at moving across the rocky surfaces on their fins. Some even started squirming short distances overland from pool to pool. They developed pockets in their throats that could absorb oxygen directly from the air, which was far more efficient than deriving oxygen from moving water through their gills. Not only was breathing more efficient on terra firma, but also plant food was abundant and there were no predators. The fish slowly developed stronger skeletons, waterproof skin, and longer fins. They now walked and hopped and only returned to the water when they laid eggs. Amphibians were born. Birds and reptiles were next; the sequence is under some dispute, but those fins were now assuming various shapes to facilitate flying, running, and even fighting. For now, there were competitors.
Then mammals came along with warm blood, hair rather than feathers or scales, and the ability to nurse their young. The earliest mammals were tiny and entirely intimidated by the dinosaurs. They wisely kept out of sight; but when those terrible lizards bit the dust, mammals came out of their crevices. Some went to the mountains, others to the plains, some to the trees, others underground. The ones who adapted most efficiently to their new environments survived and thrived.
Mammals assumed many sizes, shapes, hair colors, and habits. Those pectoral fins from their fish ancestors became amazingly diverse and particularly suited for special purposes on land. Various habitats favored different adaptations. Long fingers in some instances favored reaching the best berries and eggs. Sharp claws in other instances could shred bark or competitors. For elephants, their noses evolved into their grasping appendages, capable of uprooting trees and picking up peanuts. Elephants, therefore, benefited more from broad, thick weight-bearing platforms for hands and feet than they would from five dexterous digits. Other large animals found no use for five digits and efficiently adapted to their environmental niches with fewer. The rhinoceros has three digits, the camel two, and the horse only one, best for running, which horses do on their fingernails–well, hoofs.
Some mammals found life on the land too dangerous or competitive and went underground. Moles, with strong tunneling tools for upper limbs, are particularly well adapted for this life. An extra bone, which at first glance looks like a sixth finger, widens the hand.
A few mammals found a special niche by taking to the air. Bats still have five digits, but they are as thin as the elephants’ are thick. The thumb retains a claw useful for crawling, and the index and middle fingers are fused together to form the primary strut. Thin webs of skin stretch from fingertip to fingertip to provide tools for flight.
Other groups of mammals decided that life in the ocean was not all that bad and rejoined their fish ancestors. Sea otters continued with paws and claws, while whales and seals developed flippers for maximally efficient swimming and for chasing their fish relatives.
Then there are human hands — not really that good for anything. We can’t bear hundreds of pounds of pressure on them while we pull down trees with our noses. We can’t walk on them for weeks across hot sand or run a mile in two minutes on our fingernails. Nor can we flap our elbows and fly, swim the oceans for our entire lives, or burrow across the neighbor’s lawn. Ah, but here is the beauty of the human hand — not terribly good at any one thing, but pretty good at a lot of things. We can bear some weight on them and we can swim a little. If we need to run we can use our feet. This frees our hands to use tools rather than to be tools. Need to dig? Pick up a shovel. Need to cut? Grab scissors. Need to fly? Build an airplane or reserve a ticket online. Even fly away for a fishing vacation. Just respect the fins on those scaly creatures you catch and know that you share some ancestors.
Roy A. Meals is an orthopedic surgeon who blogs at About Bone.
Image credit: Roy A. Meals, Shutterstock.com