Journey of Life - Seas of Life 6
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.
The first shells probably developed by accident. Early snails began to store mineral waste from their diet on their backs. This hardened into shells. And as these shells became thicker or more spiny, they also became predator-proof.
This scallop can clam up if it sees danger coming,but its simple eyes haven't detected this slow-moving scallop-eating starfish. Once it feels the danger, though, the scallop has a surprising reaction. It's a great escape, but with its heavy shell, the scallop soon runs out of steam.
Their armor has always weighed snails down. But half a billion years ago, some escaped the dangers of the seabed altogether. So how did they do it? The solution – a buoyancy aid. Snails filled their shells with gas. It was the rise of the super snails-the nautiloids. Later their cousins rolled up their shells, and became the ammonites.
Ammonites were so successful that they would become the most numerous fossil for over three hundred million years. They ranged from tiny little critters like this one to monsters, like this. This shell would have contained a giant two-metre-long slug with tentacles. These ridges worked as stabilizers, keeping it on an even keel as it bobbed along looking for food. It was the age of the ammonites. This fossil graveyard near Lyme Regis was formed by just one shoal. It's part of a huge fossil bed that stretches for several miles inland, gives you an idea of just how prolific these creatures were.
But a built-in float wasn't the only answer to a bulky shell, because evolving down another branch on the tree of life was a flexible lightweight suit of armor, as worn by the very first bugs.
And so the arthropods evolved. Animals with external jointed skeleton. Their hard bodies fossilized well, and this is the most famous of them, the trilobite. This is just one of the thousands of species of trilobites that existed over hundreds of millions of years. |
