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You are here: BBC > Science & Nature > Space > Life? > How Life Starts
The Ingredients   The Recipe for Life   Did Life Arrive on a Comet?  
DNA - The building block of life on Earth THE INGREDIENTS FOR LIFE

1) Liquid water

2) Chemical building blocks like carbon, oxygen, hydrogen and nitrogen

3) An energy source
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In one corner of the Solar System, life abounds. From the frozen lakes in Antarctica to the boiling ocean-floor springs of the Pacific, planet Earth is teeming with life. But these diverse locations all share some common ground. They all contain the essential ingredients needed to create life.

  The Earth - image courtesy of NASA
Earth's seas contain the key ingredients for life

1) Liquid water

Biologists studying primitive organisms all agree on one thing. Liquid water is absolutely essential for life to evolve and survive. The search for life on other worlds is a search for places where water can exist in liquid form. But why is water so precious?

  • For life to evolve, simple chemicals must combine to form more complex ones. Many chemicals dissolve in water allowing them to mix together and react

  • Liquid water is the right temperature for chemical reactions to happen

  • Many chemicals have parts which are attracted to water and parts which are repelled by it. These forces also help reactions happen

2) Chemical building blocks

Carbon   Hydrogen and oxygen
Carbon is important because of its ability to form long chain-like molecules. Carbon chains form the backbone of organic molecules.   Hydrogen and oxygen can both bond with carbon in lots of different ways. These two elements make up water molecules. So if water is present, hydrogen and oxygen will already be there.
Nitrogen   Other elements
Like hydrogen and oxygen, nitrogen can also combine with carbon in lots of different ways. Large molecules made from carbon, hydrogen, oxygen and nitrogen also tend to be very stable.   Sulphur, phosphorus, sodium, potassium, magnesium, calcium, manganese, iron, cobalt, copper and zinc are all needed for life as we know it on Earth.

A leaf 
All living things get their energy from the Sun

3) An energy source

All chemical reactions need an energy source to drive them.

On Earth, most primitive animals and plants get their energy by absorbing ultra-violet light from the Sun. This is called 'photosynthesis'.

Humans and other animals get their energy by eating plants, or other animals. So all animals ultimately rely on energy from the Sun to live.

Stage 2: combining these ingredients to make life

Life finds a way

Until recently, it was thought that life couldn't exist anywhere that was shaded from the Sun's light. However, scientists have recently discovered organisms living deep beneath the ocean. These organisms absorb energy directly from chemicals in the water around them.

Three moons in our Solar System (Europa, Callisto and Titan) may all harbour deep underground oceans. Could they also harbour extraterrestrial life?

Is there life in our Solar System?

Go further

Paleobiology: The Precambrian
More about the chemistry behind the Urey and Miller experiment

What Came First - Protein or DNA?
The great mystery behind the beginning of life

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