Fossils record the history of life on Earth in rocks. But how can we learn about evolution, origination of new species and mass extinctions from fossils?
Fossils are the remains of once living organisms that are preserved in the rocks formed from the mud or sand in which their dead bodies lay. In most fossils, only the hard parts are left, such as the spicules of sponges, the shells of molluscs and sea urchins, and the skeletons of vertebrates. Soft tissues are very occasionally preserved, for example where the animals were buried so suddenly that there was no time for them to be decomposed by microorganisms.
Fossils record the history of life on Earth
The fossil record is very incomplete, for it is estimated that only about 1% of all the species of animals and plants that ever lived have been found. But they do illustrate the broad outlines of the history of life on earth over the 3,500 million years since the first microorganisms. These were single-celled bacteria-like organisms. Some, called stromatolites, formed onion-like layers of calcium carbonate. Others were preserved in silica solution which eventually formed flint.
From about 2,000 million years ago, larger, more complex cells with a nucleus in the middle called eukaryotes are found. The next great step, around 1,000 million years ago, was the evolution of multicelled organisms. First there were fungi, algae and simple animals. Then, around 540 million years ago, a dramatic event called the Cambrian Explosion happened. This was the rapid appearance of fossils of all the main kinds of invertebrate groups: arthropods, molluscs, brachiopods, echinoderms and so on, and earliest vertebrates.
The next revolution in life was the conquest of land, first by wingless insects, spiders and centipedes. These were soon followed by vertebrates, primitive amphibians that evolved from fish about 360 million years ago. They quickly spread over the land, giving rise to many new amphibians and reptiles, and eventually to the warm-blooded mammals and birds. Pre-humans and finally humans themselves commenced their evolutionary story away from ancestral ape-like primates about 5 million years ago.
Fossils show a continual pattern of origination and extinction
Evolution is usually thought of as the change of one species into another, and of course this is how new species arise. But the great majority of species do not evolve but go extinct, and are replaced by new species that have already arisen. The nonstop pattern of extinctions and originations is called taxonomic turnover, and affects every level of classification. Not just individual species, but whole groups of species such as genera, families and orders. The diagram below is an evolutionary tree of the tetrapods, the four-legged animals, and shows how many groups long since disappeared, were replaced by others, and these finally replaced by the living ones. The lesson is that when looking at the fossil record over long periods of time, evolution is just as much a matter of extinction as of origination.
Fossils reveal mass extinction
One of the most dramatic discoveries about life on Earth revealed by the fossil record is called mass extinction. Roughly every 20-30 million years throughout life’s history, a sudden, world-wide catastrophe caused the loss of a majority of the species, in seas and lakes and on land alike.
The diagram shows what percentage of genera went extinct at different times over the last 540 million years. The high peaks are the mass extinction events. The greatest of these occurred 250 million years ago. It is called the end-Permian mass extinction, and it is calculated that over 90% of the then living species were lost. The best known was the end-Cretaceous (K-T) mass extinction, 66 million years ago, because it was the end of the dinosaurs. But the 60% loss also included a great many species from most other groups, such as the ammonites, related to squids and octopuses, that had been a major part of the sea fauna until then but never recovered.
The main cause of mass extinctions is probably huge volcanic activity, generating thousands of tonnes of carbon dioxide that heated up the world by as much as 15°C and sulphur dioxide that created acid rain killing off land plants. The end-Cretaceous catastrophe is different from the others because it also coincided with a massive meteorite impact. If not the sole cause, it undoubtedly contributed greatly to the severity of this particular extinction.
- The timeline of evolution of life on earth
- Patterns of evolution in the fossil record
- The big five mass extinctions
Palaeontologists believe that only about 1% of all the animals that ever lived have been found as fossils. How do they come up with this figure? Think about reasons why finding a fossil of a particular ancient species is so unlikely. You can also use the further reading suggestions in Dr Kemp’s article to help you develop your answer. Your answer should be 300 words or less.
Dr Tom Kemp, Emeritus Research Fellow in Biology
I spent many years at Oxford University’s famous Natural History Museum, researching fossils of vertebrate animals, and thinking and writing about evolution. At the same time, I was lucky enough to be Biology Tutor at St John’s College, where I grew to love teaching these subjects. Even though I am supposed to be retired now, I still write and teach, and it is a privilege to be involved in helping you come to understand how important evolution is to understanding much about the world we live in.