Journey of Life - Seas of Life 12
Steve travels back 3.8 billion years to when life began. Journeying round the oceans, he explores life's first laboratory and discovers how the incredible variety of sea creatures arose, from the first microbes to hagfish and dolphins.
For modern cephalopods, brainpower’s overcome the need for a protective shell. They're simply smarter than the average fish.
So why didn't the seas become dominated by clever cephalopods? That's because just when it looked like brain might triumph over brawn, fish chanced upon a masterstroke, and that masterstroke was a skeleton made of a new material--bone. With it, new bony fish diversified like never before. The bony skeleton was a landmark development, providing stronger structural support, greater protection, more effective gills and improved agility.
Then for even more subtle maneuvers, fish evolved two pairs of matching fins, jointed to both sides of their body by bones. This really was a winning combination and its legacy would stick. Guess what? It’s why we humans have a pair of arms and a pair of legs.
But even bony skeletons have drawbacks.
"Have you ever tried racing into the sea? It's virtually impossible to run through, and that's because water is 800 times denser than air. And once you’re in, it's either sink or swim. Now if bony fish were ever to dominate the seas, they'd have to overcome the same two problems--how to move fast in the dense liquid, and how to stay afloat despite heavy bones."
To stop sinking fish needed the equivalent of buoyancy tanks on a submarine.
"Less gas, and we sink , more we rise."
But whereas a sub uses compressed air, a fish must carry its gas supply in its bloodstream. The answer was for fish to come up with an onboard float--the swim bladder, a large gas-filled sac that inflates and deflates as needed.
Gas diffuses back and forth between the swim bladder and the bloodstream, letting fish float effortlessly at any depth. Despite being heavy-boned, the fish is, in effect, weightless.
"Brilliant, but there is one big flaw in this ingenious system. Diffusion of gas between the swim bladder and blood takes time, so there’s a..." |
