During the history of life on Earth there have been five major mass extinctions. Maybe humans will be responsible for the sixth.
First Living Organisms in the Sea
The Earth is thought to be about 4.5 billion years old (where a billion is a thousand million), and life evolved around 3.5 billion years ago – probably in the sea. The first organisms were probably rather like some of the modern bacteria, with ‘true’ cells evolving about 2 billion years ago.
The first living things were very small, and they had the planet to themselves until around 600 million years ago. Something momentous happened, when groups of cells began to work together – forming multicellular plants and animals. Many evolutionary ‘experiments’ were conducted in the early days, and most of the weird creatures that were fossilised in this period simply died out.
Mass Extinctions
The Earth has been subjected to at least five ‘Mass Extinctions’ – events that dramatically altered the planet:
- The first (443 million years ago - mya) killed off many of the early invertebrate ‘experiments’.
- The second (368 mya) wiped out over 20% of the marine families that remained.
- The third (252 mya) killed more than half the animals then living in the oceans.
- The fourth (about 200 mya) saw the end of the very successful (until then!) ammonites.
- The fifth and last, 65 million years ago saw the end of most of the marine reptiles (as well as the dinosaurs on land).
We are waiting for the sixth!
The Age of Fish
Between 418 and 354 mya the fish were the ‘highest’ life-forms in the sea. Many of these were huge (20 foot long) and heavily armoured, and a few had four, lobed, fins. (Only the coelacanths remain from this period, and they are thought to be the ancestors of the amphibians and reptiles (land animals).
Marine Reptiles
About 250 mya some of the land reptiles began to re-colonise the sea. They were very successful until the last extinction wiped them out, along with the dinosaurs. During their nearly 200 million year reign they were the top predators of the oceans, with animals such as the ichthyosaur behaving much as dolphins do today.
Marine Mammals
After the extinction of the dinosaurs and most marine reptiles the mammals evolved on land. Around 50 mya some of these went back to the sea, and the first of the whales appeared. Later (about 14 mya) the ancestors of the other marine mammals – dugongs, seals and their allies – joined them.
Modern Oceans
Sharks and coelacanths are still around from the age of fish, and the turtles remain from the age of reptiles, but the whales represent the ‘highest’ life-forms in modern seas. All five previous mass extinctions were caused by natural phenomena, but maybe the next one is already underway – caused by human activity.
Main reference: Ocean – Dorling Kindersley 2006
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