Created by lmg719 almost 6 years ago
Fossils provide a record of organisms that lived a long time ago. They also provide evidence that animals and plants can change over long periods of time. The fossil record is often incomplete. However, there is a good fossil record for the evolution of the horse.
ossils Fossils are the preserved remains or traces of dead organisms. They are found in rocks.Fossils can be formed from: Hard body parts, such as bones and shells, which do not decay easily or are replaced by other materials as they decay. Parts of organisms which have not decayed. For example, dead animals and plants can be preserved in amber (hardened tree resin), peat bogs, tar pits or in ice Casts or impressions, such as foot prints or burrows. These become covered by layers of sediment, which eventually become rock.
The fossil recordFossil remains have been found in rocks of all ages. Fossils of the simplest organisms are found in the oldest rocks, and fossils of more complex organisms in the newest rocks. This supports the theory of evolution, which states that simple life forms gradually evolved into more complex ones.
Evidence for early forms of life comes from fossils. By studying fossils, scientists can learn how much (or how little) organisms have changed as life developed on Earth. However, many early forms of life were soft-bodied - and have left few traces behind.
The horseOne of the few animals for which we have a fairly complete evolutionary record is the horse. All the main stages of the evolution of the horse have been preserved in fossil form.
Over 60 million years, the horse evolved from a dog-sized creature that lived in rainforests into an animal adapted to living on the plains and standing up to 2 metres high.In the process its multi-toed feet, that were adapted for walking across the forest floor, evolved into single-toed hooves more suited for running over open country.
ExtinctionIndividuals that are poorly adapted to their environment are less likely to survive and reproduce than those that are well adapted. Similarly, it is possible that aspecies that is poorly adapted to its environment will not survive and will becomeextinct.Here are some of the factors that can cause a species to become extinct: New diseases New predators New, more successful competitors Changes to the environment over geological time - such as a change in climate A single catastrophic event - such as a massive volcanic eruption or a collision between an asteroid and the Earth
A species may also become extinct through speciation (see the next page).The fossil record shows that many species have become extinct since life on Earth began. Extinction is still happening and a lot of it occurs because of human activities. We compete with other living things for space, food and water, and we are very successful predators.The dodoThe dodo was a heavily-built flightless bird, roughly the size of a swan. Engraving of the extinct dodo Dodos lived on Mauritius, an island in the Indian Ocean. The island was uninhabited and the birds had no natural predators.Then Mauritius was colonised by the Dutch in 1638. Dodos were hunted for food and easy to catch because they were not afraid of people. New competitors were brought onto the island, including pigs, cats and rats. These ate the dodos' eggs and their young. Within 80 years, the dodo was extinct.
New speciesNew species can arise as a result of isolation. This is where two populations of a species become geographically separated. For example, Charles Darwin describedspeciation of finches this way.Darwin studied the wildlife on the Galápagos Islands (a group of islands on the equator, almost 1,000 km west of Ecuador). He noticed that the finches (songbirds) on the different islands were similar to each other.However, the finches showed wide variations in their size, beaks and claws from island to island - for example, their beaks were different depending on the local food source. Darwin concluded that, because the islands are so distant from the mainland, the finches that had arrived there in the past and had changed over time.
The Fossil Record
Old and new species