Cells are the basic unit of life. Everything in biology is about cells. The first cells are thought to have emerged from underwater alkaline vents about 3.8 million years ago. Amazingly that is only about 750 million years after the Earth was formed.
These first cells were very simple consisting of little more than a cell membrane, cytoplasm, ribosomes and some genetic material. There was no nucleus, chloroplasts or mitochondria. These were the prokaryotes.
Prokaryote means “before the nucleus”. These little cells were very happy. They divided happily for millions of years. Then came the “great Oxygen War”. About 300 million years ago some of the bacteria started to photosynthesise. This was an excellent way of making their own nutrients. Unfortunately they produced a poisonous waste gas called “Oxygen”. Yes Oxygen. 300 million years ago there was very little in the atmosphere. So when bacteria started photosynthesising and making oxygen many of the other bacteria died. However another group of bacteria where able to use the oxygen for a process called aerobic respiration! These bacteria became super bacteria. They had more energy than the puny anaerobic bacteria. They grew bigger and became more complex. If they had faces they would have looked quite smug.
About two billion years ago we start to see the first eukaryote cells. Eukaryote means True nucleus. These large cells then do something weird. They start to absorb smaller prokaryotes. These prokaryotes were probably quite happy with this arrangement. They were kept safe and all they had to do was provide some energy or glucose. Over time this arrangement became permanent. and these bacteria become part of the cell. Organelles such as mitochondria and chloroplasts were thought to have arisen this way.
So we now have the amazing eukaryotic cells. What next! Multicellular is what next. If you can do great stuff with just one cell imagine what you could do with a trillion cells! Actually you do not need to imagine you can just look in a mirror and see an organism with a trillion cells - you!)
The first multicellular organisms were thought to have first appeared about 900 million years ago.
Cells do something interesting that allows life. They divide. All life starts as one cell. You started as one cell. A fertilised egg is called a zygote. Everything about you was found in a single cell - cool eh! Now there you are as one cell. So what you do is divide then divide again and again and again. This is growth. Now if you just divided and divided you would not end up as a human but rather a big blob of cells. To become a large multicellular organism yours cells need to take up a career. They need to get a job. Some cells want be red blood cells, other cells want to be nerve cells.
Before they decide on their job, cells are called stem cells (unemployed so to speak). However once they get a job they are said to be differentiated. Once a cell gets a job it cannot change its mind. So if you become a brain cell you will stay as a brain cell. You cannot change your mind and become a muscle cell.
So large multicellular organisms are possible because of the processes of cell division and cell differentiation.
Cells come in all shapes and sizes. The longest cells are nerve cells. In humans these may be a meter in length. In a blue whale they may be up to 22 meters in length. If you think that is big then some of the big sauropod dinosaurs could have had a neurone up to 50 metres in length.
Another weird cell is foraminifera. These are single cell organisms but they have a shell! some of them grow very large indeed and specimens have been measured at 20cm! Now if you go onto the internet it will tell you that the largest cell is an ostrich egg. This is why the internet is rubbish (except for my blog of course). The egg as a whole is a support system for a small cell which floats on the yolk. This is called the germinal spot. The yolk and white have no organelles and when the egg cell starts to divide, they do not divide.
So there we have cells. All weird and wonderful and the basis of life. By the way just to finish off and get you thinking. A virus is not a cell so it is not alive!