How has marine life changed throughout earth's history?
Through Darwinian biological evolution, often driven by extinction events.
Marine life on Earth has changed through evolutionary processes, much that same as land-based organisms. In fact, all life on the planet first evolved in the oceans as simple prokaryotic bacteria. Bacteria who learned the trick of photosynthesis changed the course of biological evolution forever and also caused the first extinction event of aerobic bacteria. From this, eukaryotic cells evolved that could utilize oxygen and then more complex multi-cellular eukaryotic organisms evolved, like trilobites and molluscs.
Invertebrate molluscs evolved tough calcium carbonate shells in response to predation. Vertebrates evolved from earlier lamprey like eels (our lineage). The course of evolution also changed dramatically at the end of the Permian (250 million years ago) when a major extinction event took place and again at the end of the Cretaceous when another extinction event took place (a giant asteroid). Both events and many smaller extinction events changed the course of marine and terrestrial evolution.