How has marine life changed throughout earth's history?

Similar to land-based organisms, marine life on Earth has evolved through similar processes. In fact, all life on Earth began as simple prokaryotic bacteria that lived in the oceans. Bacteria that discovered photosynthesis altered the course of biological evolution and led to the first extinction event of aerobic bacteria. Following this, eukaryotic cells that were able to use oxygen evolved, and eventually more complex multicellular eukaryotic organisms such as molluscs and trilobites.

The course of evolution also changed dramatically at the end of the Permian (250 million years ago) when a major extinction event occurred, and again at the end of the Cretaceous when another extinction event (a giant asteroid) occurred. Both events, along with numerous smaller extinction events, changed the course of marine and terrestrial evolution. Invertebrate molluscs evolved tough calcium carbonate shells in response to predation. Vertebrates evolved from earlier lamprey like eels (our lineage).

Marine life has evolved over millions of years, experiencing changes in species composition, size, and ecological roles. Major events like mass extinctions and environmental shifts have influenced marine biodiversity. The rise of complex organisms, such as fish and marine mammals, marked significant shifts in marine ecosystems. Anthropogenic activities, including overfishing and climate change, currently pose threats to marine life.