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Theory of Evolution

You and your family and friends all live on planet Earth. There are also many other types of organisms that live here with us! Have you ever wondered how we all got here? Biologists try to answer this question with the Theory of Evolution.

An image showing the evolution of man.

The evolution of man.


Biologists are not completely sure how life began on Earth. But they have spent many years studying different living things. We have learned many facts and found a lot of evidence. This has let biologists come up with ideas to explain how life got started. We call these ideas theories.

The theory of evolution is usually used to describe how new living things form over time. But to better understand how living things formed, we can go back even further in time. In fact, we’ll go back 13.8 billion years to the start of the universe.

The Universe and Earth Form


The universe, the Earth, and all of the other planets didn’t always exist. From our view 13.8 billion years ago, we would see the whole universe smashed into a very tiny ball. Then, a big explosion of energy caused that tiny ball to expand and form the universe. The theory that describes this explosion is the Big Bang.

An image of the earth in space.

The Earth formed 4.6 billion years ago.

Even after the Big Bang, it took billions of years for the Earth to develop. When the Earth formed, it looked very different than it does today. It took over a billion years for the Earth to develop the right environment for life to start.

The First Cells

Eventually, the environment contained a “soup” of molecules that could make DNA and RNA. These building block molecules allowed the first cells to develop. The cells were very simple, but these prokaryotic cells were the very first organisms. They were the start of all life on Earth.

Evolution of Animals

After the first cells formed, it took many years for animals and humans to evolve. The presence of the first cells changed the Earth’s environment. This allowed brand new types of life to develop.

An image of the Biological evolutionary tree.

Life started as tiny, single-celled organisms, and animals evolved slowly over time.

Over time, the characteristics of organisms changed. This allowed more complex organisms to form, some with more than one eukaryotic cell. Eventually, all the animals alive on Earth today evolved.

Darwin’s Theory

Darwin’s theory of evolution is the one most scientists accept as true. Darwin was a scientist in the 1800s. He took a trip around the world to study plants and animals. Based on what he saw, he came up with a theory to explain how new species can form over time.
His theory included a concept called natural selection. Natural selection describes how groups of organisms can change over time. The changes depend on which of them survive best in their habitat.

Other Great Resources:

Our Universe – European Space Agency: https://www.esa.int/esaKIDSen/SEMBT5WJD1E_OurUniverse_0.html

How Did Life Start on Earth? – Las Cumbres Observatory: https://lco.global/spacebook/how-did-life-start-earth/

The Beginning of Life on Earth – BBC: http://www.bbc.com/earth/story/20161026-the-secret-of-how-life-on-earth-began

(Video) ‘From the Big Bang to Me:’ https://www.youtube.com/watch?v=nYkdDniQJyw

Written by: Alexa Wnorowski