Wednesday, July 11, 2007

The Tree of Life: Archaea, Bacteria, and Eukaryota

If we are to talk about evolution, then we first have to talk about what kind of life exists on earth. The tree of life is made up of three distinct branches. The Eukaryota, Bacteria, and Archaea.

The eukaryotes include animals (humans), plants and fungi and a rich variety of micro-organisms also known as protists. The protists include parasites which can be biologically speaking very successful and they can compromise the environment of entire countries. The Eukaryotes are identified and distinguished from other forms of life by the presence of nuclei and the presence of a cytoskeleton.

Bacteria are microscopic organisms whose single cells do not have a membrane-bounded nucleus nor other membrane-bounded organelles like mitochondria and chloroplasts. Another group of microbes, the archaea, meet these criteria but are so different from the bacteria in other ways that they must have had a long, independent evolutionary history since close to the dawn of life.

Bacteria are maligned because of the human and animal disease they cause. However, some, like the actinomycetes, produce antibiotics such as streptomycin and nocardicin. Others have different functions. They live symbiotically in the guts of animals (including humans) or elsewhere in their bodies, or on the roots of certain plants, converting nitrogen into a usable form. They are seen everywhere. For instance, bacteria give the tangy taste in yogurt and the sour in sourdough bread. Bacteria are responsible for the break down of dead organic matter. The are also an important base of the food web in many environments. Life on earth may be complex but it appears that this was the basis of all early life. The oldest fossils known, nearly 3.5 billion years old, are fossils of bacteria-like organisms.

Archaea are microbes and most live in extreme environments. Those that do are called extremophyles. Other Archaea species are not extremophiles and live in ordinary temperatures and salinities.

When these microscopic organisms were first discovered in 1977, they were considered bacteria. It became apparent however, that they were not when their ribosomal RNA was sequenced. The sequencing showed that they were not closely relationed to the bacteria but were instead more closely related to the eukaryotes.

Archaea are a much different and simpler form of life. They may also be the oldest form of life on Earth. Because they requires neither sunlight for photosynthesis as do plants, nor oxygen as to animals. Archaea absorbs CO2, N2, or H2S and gives off methane gas as a waste product the same way humans breathe in oxygen and breathe out carbon dioxide.

Archaeans may be the only organisms that can live in extreme habitats such as extremely hot thermal vents or hypersaline water. They appear to be extremely abundant in environments that are hostile to all other life forms.

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