Life on Earth
Life on Earth has taken billions of years to evolve. Life includes anything which can do the following things:
- Move - all living things can move in some way, even things which look stationary, like plants, can move at least part of there structure themselves.
- Respire - we commonly think of this as breathing in animals, but it really means converting energy from carbohydrates like sugars, into something which can give the life form energy. In order to do this most living things need to use oxygen, but some can use other elements like nitrogen.
- Sense - this involves being able to respond to the environment around us, like if it gets cold we might get goosebumps.
- Grow - this is self explanatory, all living things grow with time. This generally means increasing in mass.
- Reproduce - where another organism is made from a 'parent', this can be as simple as one cell splitting into two, or as complicated as parts of cells being contributed from two parents.
- Excrete - or removing waste products, such as urine. All living things produce some waste, and if this waste stays inside the organism it becomes toxic, so all living things have a way of removing these waste products.
- Take in nutrients - all living things need to take in food of some sort to provide them with enough energy to survive and carry out the other functions of a living organism.
The first life evolved on Earth not long after the planet was first created, and oceans formed on the surface. The water of the ocean provided protection against the dangerous parts of the electromagnetic spectrum, like x-rays and gamma rays, which damage living things. Later, our Earth developed an atmosphere which also 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.