Evidence of evolution
We cannot watch changes in life directly. They take place over thousands or millions of years. However, scientists cannot find proof that these changes have taken place. Important evidence for evolution comes from fossils, the leftovers of ancient life. When animals or plants die they are pressed into sand or clay. Over millions of years rocks are formed.
Scientists have found out that different fossils are found in rocks of different ages. For example, the oldest rocks of our earth are about 3.8 billion years old. They contain no fossils because there was probably no life at that time. Fossils of bacteria appear in rocks that are about 3.5 billion years old. Fish, reptile and mammal fossils appear in younger rocks. Human fossils are found only in the youngest and highest rock layers.
Fossils also show that certain groups of animals have evolved from other groups. Amphibians evolved from fish that could breathe air and move on land. They had legs but also scales and a fin.
Birds probably evolved from dinosaurs. The archaeopteryx was an animal that had feathers like a bird and could fly. It also had teeth, claws on its wings and a skeleton that looked like a meat-eating dinosaur.
But even without fossils there is other proof which shows that evolution has taken place. Different species often have similar features which they probably got from a common ancestor. For example the front limbs of lizards, birds, bats and humans are very much alike. They have one bone in the upper arm, two in the forearm, wrist bones and five fingers.
Living creatures might also have structures that they have inherited from an ancestor but have become useless. They don’t need them any more. Pythons, for example, have the remains of back leg bones, but snakes do not have such legs. The appendix was used by animals that ate only plants but in our bodies these organs have become useless.
The way in which different species occur all over the world also gives us evidence for evolution. Similar species, for example, are found together in certain areas. All types of kangaroos are found in Australia. This is because the kangaroos’ ancestors also lived there.
Plants and animals do not always live in ideal places. Tropical ocean islands, for example, are ideal places for frogs to live, but no frogs are found there. This is because the frogs’ ancestors lived on the mainland and could not get to ocean islands far away.
How evolution happens
Although we are all human beings, each one of us is different. We all belong to the same species but there are never two people on earth who are exactly the same. We are like our parents because we inherit certain features from them.
Because there is not always enough food for animals and plants to eat they compete against each other in order to survive. Some individuals are better than others because they have certain advantages. On average, those that are better or stronger will survive. The advantages that they have are then passed on to their children and as time goes on these characteristics will be passed on to the whole species. We call this natural selection.
Example: In 1977 no rain fell on the Galapagos Islands. Food became very scarce and many of the island’s finches died. They normally ate small seeds that were lying on the ground. Biologists observed that finches with larger beaks were able to survive because they could eat larger and harder seeds that finches with smaller beaks couldn’t open. In the fight for food large-beaked birds had a great advantage. After the drought ended biologists found out that the next generations of finches were larger than the ones before.
Genetics and inheritance
Today, scientists know that a molecule called DNA has all the information which controls the way life will develop. This information is stored in genes and the structure of genes is called the genetic code.
When a male and female have children the male sperm and the female egg join together to a single cell with two genetic codes, one set from the mother and one set from the father. A baby then develops from this cell. This is how we get certain features from our parents.
Sometimes parts of the genetic code change by accident. We call this mutation. Some mutations in genes are dangerous, others may be an advantage. In the example of the birds, the larger beaks were a mutation that was good for the whole species.
Sometimes animals and plants fit beautifully into the world around them. The Arctic fox, for example, is adapted to the polar ice in the far north. It has a thick fur that helps it stay warm and the white colour makes it harder for enemies to see. With its hairy feet it can walk more easily in the snow.
Giraffes also got used to the world they live in. Ancient giraffes normally did not have long necks, but those that did were able to find more food because they could reach the leaves of the trees. Longer-necked giraffes had more babies than others and as a result they developed into the tallest land animals in the world.
Adaptations can cause plants or animals to look alike even if they are not closely related. The bodies of sharks and dolphins are similar, but the shark is a fish and the dolphin a mammal.
Speciation happens when one species divides itself into two or more new species. This happens, for example, when the same group of animals or plants live in different places. Sometimes species migrate to new habitats. In other cases a population may be divided by natural disasters like floods or volcanic eruptions.
When species are separated they don’t have contact with each other any more and they develop in separate ways. As time goes on the two groups become more and more different, simply because they live in different habitats maybe with more or less food or a hotter or cooler climate. If they get together again they cannot have babies any more because they are completely different.
Speed of evolution
How fast does evolution happen? Sometimes it occurs very quickly. In only a few decades insects evolved that were able to survive insecticides. Viruses also develop quickly. The AIDS virus was unknown before the 1980s.
Some animals evolve very slowly for millions of years and then change occurs very quickly.
Fossils show that many species which are now extinct belong to the same family as we humans do – Homo Sapiens. The oldest members of this family are primates that lived in Africa a few million years ago. They were able to walk upright and had a brain that was a bit bigger than that of an ape.
Charles Darwin was an English scientist who studied nature. In his famous book “On the Origin of Species” he claimed that all living plants and animals developed from earlier forms of life.
Darwin was born in England in 1809. His father was a doctor and his mother died when he was 8 years old. Although Darwin was interested in nature, he was sent to a university to study medicine, but he didn’t do well there.
In 1831 Charles Darwin was invited to sail on the HMS Beagle to study natural history. The voyage lasted for five years and took Darwin to the Galapagos Islands and other places on the western coast of South America. There he studied fossils in old rocks and noticed that there was a connection between them and plant and animal life. As time went on he developed his theory of natural selection. Those plants and animals that fit better into their environment can survive better and produce more offspring.
When his book was published it caused a lot of discussion but in a short time it was accepted by scientists around the world.