MNN - Mother Nature Network

Biodiversity — the different number of life forms on planet Earth — is important to understand if we're to survive.

Scientists estimate that there are between 10 million and 110 million species on Earth. So far, we have catalogued about 1.4 million forms of life. The awesome diversity that exists on our planet today represents about one percent of all species that have ever lived here.

Life began 3.5 billion years ago. Ultraviolet radiation relentlessly bombarded the planet. The thin atmosphere was a toxic mixture of hydrogen, ammonia, methane and carbon monoxide with very little or no free oxygen.

Lightning was a major force in that world of very active volcanic upheavals. Bolts of lightning hit the ocean and from one of those reactions came the beginning of life, with the formation of DNA (deoxyribonucleic acids, the main constituent of chromosomes, which carry the genetic code).

DNA is the fundamental building block of life, with power to replicate itself as the blueprint for the formation of amino acids (the building blocks of proteins). The oldest forms of life (and first fossils), bacteria, fed on carbon compounds that had accumulated in the seas. Over time, it made its own food from free hydrogen released from volcanic activity. Miraculously, there came a time when this blue-green algae developed the ability to harvest the sun's energy and use carbon dioxide in primitive green or chloro cells, thus making its own food. One major byproduct of this activity was the release of oxygen.

So much oxygen was released that some oxygen floated to the upper atmosphere and formed the ozone layer. That remarkable oxygen-rich layer filters and prevents almost all UV radiation from reaching the Earth's surface. Once this layer was in place, life on our planet began. First in the ocean, and then from the ocean to the land, life proliferated.