Life on Earth - Theory of Evolution


Evolution has taken place over millions of years and scientists believe this is the reason why all living things on Earth exist today. There are different views and theories about the origin of life and the evolutionary process.

How did life start?

It is not possible to be certain how life on earth began. We do know that: the Earth is about 4,500 million years old, there is evidence that living things existed on Earth at least 3,500 million years ago, no one was there to record how life began, early Earth was hotter and the atmosphere consisted mostly of carbon dioxide (with other gases such as ammonia and methane).

Scientific Theories

Through the study of simple organisms and the chemistry of living things, we can develop scientific theories about how life on Earth began. The main theory is that living things developed from molecules that could replicate, or copy, themselves, rather like DNA does. There are two possible origins for these replicating molecules: they were produced by the conditions on Earth at the time, they came from somewhere else, such as another planet in our Solar System, or further out in space. Over many millions of years these molecules joined with other molecules, becoming gradually more complex and dependent on each other. The process of evolution by natural selection eventually led to all of the different living things that we see on earth today.

Darwin's theory of Evolution

Charles Darwin was an English naturalist who studied variation in plants and animals during a five-year voyage around the world in the 19th century. He explained his ideas on evolution in a book called, 'On the Origin of Species', published in 1859. Darwin's ideas caused a lot of controversy, and this continues today, because they can be seen as conflicting with religious views about the creation of the world and the creatures in it. The basic idea behind the theory of evolution is that all the different species have evolved from simple life forms. These simple life forms first developed more than 3 billion years ago (the Earth is about 4.5 billion years old). The timeline below shows some of the key events in the evolution of life on Earth, from the first bacteria to the first modern humans. 

Natural Selection

The theory of evolution states that evolution happens by natural selection. The key points are that: individuals in a species show a wide range of variation, this variation is because of differences in their genes, individuals with characteristics most suited to the environment are more likely to survive and reproduce and the genes that allow these individuals to be successful are passed to their offspring. Individuals that are poorly adapted to their environment are less likely to survive and reproduce. This means that their genes are less likely to be passed to the next generation. Given enough time, a species will gradually evolve. You need to remember that variation can be caused by both genes and the environment. But it is


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