What is Evolution?

In science, the word “Evolution” is used to describe how living things have changed over time. From titchy tiny life that we wouldn’t have been able to see, to huge beasts such as elephants and whales and everything in between.Something that has changed permanently has evolved. We can track these changes back to the beginnings of life.

Scientists use fossils and living things found all around the world to work out where we came from and how it happened.


Imagine you are playing a game of Chinese whispers with a group of friends. You whisper “All Life started in the water” to the friend next to you. She whispers it to the friend sat next to her, who then whispers, “All Life farted in the water” to the one sat next to him.

He didn’t hear too well and turns to the friend next to him whispering, “All life farted in the world tour.” She continues and whispers, “All life farted on the world tour” to the person next to her. Each of your friends in turn whisper what they think they heard. When it gets to the final person, he says loudly, “Paul Fife farted on the world tour.”  Poor Paul blushes furiously and decides never to eat Brussels Sprouts again!

This is like evolution. Changes are accidental. Nothing changes all in one go, but in little pieces over time. All these pieces come together to create something totally different.

When did it all start?
Life started about 3.4 billion years ago. That’s over 10 billion years after the universe was formed.


Primordial Soup

How did it start?

No-one knows exactly because they weren’t there! Scientists suggest it all began in a mix of water and some of the chemicals found on earth. They call it the Primordial Soup (yum, yum!).The weather was a lot more stormy then than it is now. Water and gases were thrown together, excited by a bit of lightening every now and then. Everything in the soup bumped into each other, sometimes making something new, sometimes not. After a bit more bumping and jostling, somewhere, somehow, life was born. It was a bit simple at first. It was a bit simple second and third as well.

But very, very slowly, after a lot of practice, things started to get more complicated and a lot more exciting.

So what came first?

Living things (also known as organisms) that were made up of a single cell. They are called single-celled organisms!A cell is a thick liquid (called cytoplasm). In the liquid it has:

  • Instructions to tell it what to do
  • An energy source
  • Security to make sure the instructions are being followed

To stop everything escaping, there is a wall (known as a cell wall) around it all.

All the bacteria today are single-celled organisms. (When scientists talk about them, they often use the word “prokaryotes“).

Then what?

Without babies there wouldn’t be any more life. But there were no need for Mums and Dads with the first single-celled organisms. They just copied themselves and split into two.Those two copies could make more copies and split into two again (2+2 =4).  Four copies can double and split again (8) and again (16) and again (32) and……. you get the idea.  Eventually there were lots of single-celled organisms.

BUT – it wasn’t quite that simple.

It turns out that whilst these organisms were really good at copying, they weren’t always accurate. The cell has instructions to tell it what to do. Sometimes (although not very often) these instructions weren’t copied exactly and they would tell the cell to do something else.

The Cell made mistakes?
Did it get bad marks in its test?

What? No! And besides, red pen hadn’t been invented yet. A mistake in copying led to one of three things:

  • A slight difference that was so small it wasn’t noticed
  • A bigger difference that made the organism do something different
  • A bigger difference that killed the organism

See – instructions are important!

It meant that one copy of the instructions for a single-celled organism might be ever so slightly different to another copy for the same type of single-celled organism. These differences are called mutations.

Unfortunately, the single-celled organism had no real control over whether any mutations were made and what kind of differences they would lead to.


Mouth Cell
Orange Mouth Cell
Tail Cell


Yes, mutations. It’s just a word scientists use when they want to describe a difference between one set of instructions and another.


Why do these mutations matter?

A big difference might cause a big change quite quickly. Over time, lots of small, unimportant differences might also come together make a massive change. A change could be anything from what the cell did to what colour it was. After lots of time and several mutations, there were many types of single-celled organisms doing slightly different things.

Eventually some of these single-celled organisms got together. Because they all did different things, they could all share. Now there were multi-celled organisms.

That’s like a singer and a keyboard player meeting up for the first time. Individually they could only sing or play music, but together they could do both and form a band.

New Cell

Ooh, musical cells?
No! I said like a band – not that the cells became a band.
So what did they do?

They could share things like finding food and moving about. Maybe not as cool – but much more important to keep them alive. Once living things could get together, life got a lot more exciting. Over time, more and more of them joined together, making the large, complex animals and plants we see today. Each single cell had a particular job to do but the whole organism shared in the success.

Life had evolved from simple cells to more complicated organisms.


Did the cells decide what they wanted to be?

No. Mutations happen by accident. There is no definite control over where a mutation is made or whether it will make a big change, or a small one. Most mutations take place as a cell copies itself ready to split into two. The two new copies will then have any mutations in their instructions. These mutations might make a difference, or not, to the new cells.

Instructions that are really important to the organism tend to be more protected. If changes in these important instructions are made, an organism may be more likely to die before it makes copies or has babies. (Making copies or babies is known as reproduction).  If the organism dies and doesn’t reproduce, the mutation that caused it will disappear entirely. Really useful mutations may mean that the organism reproduces much more quickly and becomes very common (a process known as natural selection).

In small organisms such as bacteria and viruses, the mutations that cause change only have to occur in one cell. Because of this, they evolve more quickly.

In larger organisms, there may have to be mutations in lots of different places before change occurs. These organisms evolve more slowly.


Wow – there must be gazillions of living things!

We have no idea how many there are – but gazillions of actual living things is probably a good guess. Most of these are bacteria. (All living things are split up into groups based on how alike they are. There are millions of these groups.)

There are lots more of the smaller animals and plants on Earth. Smaller things tend to reproduce faster than larger organisms. Also, the larger organisms need lots of smaller things to eat.

Not every type of animal or plant that ever existed is living today. Some, like the Dinosaurs, were wiped out by natural disasters. Others, like the Dodo, were eaten so there were none left. When these animals disappear for good they have become extinct.


How long does all this evolution take?

A very long time. Planet Earth was formed for over 1 billion years before single-celled organisms appeared. Single-celled organisms were around for about 2.5 billion years before they started joining up to make larger, multi-celled organisms. Even then the idea didn’t take off immediately. Everything else has come along a bit faster. It has only taken another billion years to get from the first multiple cell to us humans.

We are part of a group of animals called mammals. Mammals as we know them today can control their own body temperature (so they don’t have to wait for the sun to warm them up, like lizards, for example), are covered in hair or fur and generally give birth to their babies instead of laying eggs. They also feed them milk from their mammary glands. Mammals have been around for about 200 million years. That means they appeared about 3.3 billion years after the first single-celled organisms.

Mammals can be split into several groups. Humans are part of one of these groups, known as primates.  Primates evolved about 80 million years ago.

Names are given to all groups of living things so we don’t mix them up. Scientists use a language called Latin (it was spoken a long time ago) to name these groups. Human primates have the Latin name Homo sapiens (say “hoemow sappyens“). The earliest Homo sapiens appeared about 200-500 thousand years ago.

Put another way, if the earth were created 580 years ago and life began about 170 years ago, Homo sapiens have been on the planet for about 3 hours. Not really very long at all.


Why didn’t we appear earlier?

Humans are very complicated animals. Our bodies are very complicated and we need oxygen to breath.When the Earth first formed, there was little or no oxygen. Over time mutations created organisms that could make oxygen. At first only animals needing little bits of oxygen could breath well.


So not us,then?

No. Think really small – more bacteria maybe. I know they are not animals, but this really is a slow process.We need the air we breath to contain about 21% oxygen (so one part oxygen to four parts something else  – nitrogen mostly and a bit of carbon dioxide and argon). Plants are really good at using carbon dioxide and turning it into oxygen. Lots and lots of plants were needed over a long time to create enough oxygen for us to survive


But loads of animals need lots of oxygen…

They do and eventually large animals came along. Primates have been around about 400 times longer than we have. Not only do we need lots of oxygen, we also have really complicated brains.  Such brains use up a lot of energy – which means a lot of food. If there had been nothing to eat, we wouldn’t be here.



Yes it is. From titchy tiny life to us – and most of it by accident!