Life on Earth
Life on Earth has taken billions of years to evolve. Life is characterised as anything which has the following traits:
- Move: all living things can move in some way. Plants may look stationary, but they are able to move some parts of their structure.
- Respire: a lot of people think respiration is breathing. Really, it is a chemical reaction which converts glucose, a type of carbohydrate, to energy. All living things respire. To do this most use oxygen, but others use elements like nitrogen.
- Sense: this is when a living thing responds to its environment. If it gets cold we shiver, or if it gets hot we sweat. Seeds will sprout when spring arrives and the soil warms.
- Grow: all living things grow with time. This generally means increasing in mass.
- Reproduce: this is how new living things are created. It can be as simple as one cell splitting into two. Or, it can be more complex, with parts of cells from male and female parents fusing.
- Excrete: this is how waste products are removed from living things, like urine. All living things produce some waste. If this waste stays inside it becomes toxic. All living things have a way of removing waste products.
- Take in nutrients: all living things need to take in food. The energy from food allows them to complete the functions we have mentioned. They need to complete all these functions to survive.
Planet Earth was formed around 4.5 billion years ago. The first signs of life began about 3.6 billion years ago, in the ocean. The water in the ocean protected organisms living there from the dangerous rays of the sun. These x-rays and gamma-rays are part of the electromagnetic spectrum, which damage living things. Later, Earth developed an atmosphere. This protected living things on the land, and life continued to evolve.
Humans, as the species we are now, homo sapiens, have only been around on this planet for around one million years.