On earth, the first life forms were very tiny organisms. Most of us don't think about these unicellular creatures very much, but they are all around us, and it was they who solved the basic problems that confront all living organisms.

     Although we cannot see these life forms without microscopes, they reward the patient scientist with with their complex and beautiful patterns, and their ingenious ways of moving around.  

These early life forms were also tough!  Be sure to read about extremeophiles!

     The earth formed about 4.5 billion years ago. For a long time there was nothing alive on it, but perhaps 3.8 billion years ago the first life forms appeared. They may have begun as molecules that could reproduce themselves in the early seas. However, being able to collect and hold onto the chemicals that they needed was a very useful ability. When some sort of capsule or membrane surrounded the molecules they became cells These were tiny bags of cytoplasm with a single flexible loop of DNA in them. We call these cells prokaryotes.

     These early life forms lived in the water at a time when the atmosphere had little oxygen and much more carbon dioxide than we have today. Simple and primitive as these organisms were, they had solved a number of essential life problems. They contained their essential components inside a cell wall and a membrane that allowed necessary chemicals to come in and wastes to go out. They were able to select the chemicals that they needed to live. They could reproduce themselves by simple cell division. There are still many prokaryotes on earth today.

     Some prokaryotes released oxygen into our atmosphere. At first the oxygen combined with iron and other elements that were dissolved in the water or on the surfaces of rocks, but eventually there was enough oxygen in the air to allow the next step in evolution: the appearance of eucaryotes. Eukaryotes are cells that have their DNA in a nucleus inside the cell. They are larger and more complex than procaryotes. All the multicellular life forms on earth have cells with nuclei.

     The early development of life was very slow, as can be seen from this chart. Prokaryotes were the only life forms on earth for longer than all other living organisms have existed, and they are still here. Mammals have been here only a very short time in comparison. Humans have really just arrived!

What were the prokaryotes doing here all that time?  They were making a living, staying alive in a world that was still unpredictable.  They used carbon dioxide as their important gas.  They adapted to different light intensities and water temperatures.  They learned to adapt to differences in salinity -- the saltiness of the water.  They learned to stay away from the water surface, where the ultra violet radiation of the sun would kill them.  The cyanobacteria developed chloroplasts and started to release oxygen as a waste product.  It took a long time for these tiny organisms to make the changes to the world that made the existence of more complex life forms possible

     These simple organisms also innovated in another area: reproduction. Reproduction by simple division meant that each new cell was exactly like the parent. This worked well for successful organisms, but it meant that there was little opportunity for innovation. Sexual reproduction meant that the chromosomes were shuffled in every generation, leading to more variation in the offspring. Although some of the offspring were probably less fit than the parents, others were more fit. This allowed improvement, innovation, and adaptation to marginal environments.

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