Now that you know all about the characteristics of the domains, we would like to take you back to when this all started, the history of the classification system. That’s right kids, were taking it old school with a little throwback. Ever since the beginning of time it was obvious that every organism was different. As humans advanced, it became clear that there needed to be some way to classify organisms and to separate them based on characteristics. However, it was very difficult for people to agree on how to divide the organisms. In the early 300’s BC, Aristotle, the famous philosopher, tried to classify organisms into two distinct groups: plants or animals [3]. After realizing that this was too broad, he came up with the idea to separate the organisms into three groups: walking, flying, or swimming. However, this system was still not perfect. In the 1700’s, Figure 44: "Old School" a man by the name of Carolus Linnaeus came up with a more complicated, detailed system. His idea was to divide the organisms into 2 kingdoms, plant and animal. Then, these kingdoms would then be divided further into groups called genera [3]. These genera would then be divided into even smaller groups called species. When you take a close look, this is very similar to the system in place today. Then, in 1969, a five kingdom system was made by Thomas Whittaker and was used for a few years. On top of the kingdoms plant and animal, he added Fungi, Protista, and Monera [4]. Finally, Carl Woese created the 3 domain system in the 1980’s, and that system is still in place today. By simplifying the 5 kingdoms into 3 domains, the classification system became much clearer and much more widely accepted.
Although there are many differences between the organisms, there are also many similarities. And yes, even the domain Eukarya shares similarities with the other two domains. First of all, both the domain Archaea and the domain Bacteria are unicellular, while the domain Eukarya is sometimes, though not usually, unicellular. In both Archaea and Bacteria the organisms can be motile, and so can the organisms in domain Eukarya. Interestingly, all the domains have different forms of motility. The domain Archaea only uses flagella, while the domain Bacteria uses flagella, bacterial gliding, and twitching motility. See F
igure 1 to see an image of pseudomonas aeruginosa, a Bacterial organism that uses twitching motility in order to move from place to place. Lastly, the domain Eukarya has many systems that make them motile, and every eukaryotic organism has a different system. While both the organisms in the domain Archaea and Bacteria do not have a nervous system, the domain Eukarya does not always have it as well. On the other hand, there are many ways that these domains are all different. While the domains Bacteria and Eukarya contain organisms that can gain nutrients through photosynthesis, the domain Archaea cannot. Figure 2 shows a picture of a microscopic Methanobrevibacter smithii, an organism in the domain Archaea. The domain Archaea is the only domain that does not perform ph
otosynthesis, so the Methanobrevibacter smithii does not do photosynthesis. The domain Eukarya is also the only domain that can have multicellular and eukaryotic organisms. See Figure 3 to see a Protozoa, a unicellular, Eukaryotic organism. Lastly, the domain Eukarya can reproduce sexually while the other two domains cannot. It is clear that while all these domain have many similarities, it is also obvious that they have many differences.
