When we look around us, one of the easiest things for us to do usually is to take note of the differences and similarities in characteristics of organisms and objects around us. For a long time, this was how living things were classified – based on their outward appearance. For instance, some animals have hooves, while others do not. These similarities in appearances between different organisms were thought to be due to evolution from common ancestors, as Darwin showed us. Using fossil records, scientists have begun to draw a family tree of living organisms (Alberts et al. 2014).
Over time, however, disparities between organisms became very great and this method of building a family tree of living organisms became obsolete. Even more so, this method proved useless for microscopic organisms. Microbiologists proposed other methods of classification, which were also deemed inaccurate due to the fact that they were based on differences, and it is difficult to ascertain whether particular differences are due to evolution or something else entirely (Alberts et al. 2014).
Nowadays, genome analysis is used. Over time, DNA is subject to random changes, and these changes accumulate. By determining the number of differences between the DNA sequences of two organisms, the evolutionary distance between these two organisms can be determined quantitatively (Alberts et al. 2014).
A result of these techniques has been the discovery that prokaryotes consist of two separate groups that diverged early on in the history of life on Earth. These two groups are bacteria and archaea. Later on, the first eukaryotic cell formed when a particular type of ancient archaeal cell engulfed an ancient bacterium. Today, when we think of the three major divisions of living organisms, we think of bacteria, archaea, and eukaryotes (Alberts et al. 2014).
Archaea are often found in environments that humans do not frequent. Some of these environments including bogs, sewage treatment plants, ocean depths, salt brines, and hot acid springs, to name a few. While it is true that archaea are often found in places that humans avoid, they are also found in some more homely environments, which may include soils, lakes, and the stomachs of cattle. Archaea are similar in outward appearance to bacteria, but differ from bacteria in their machinery for handling genetic information. In this way, they are like eukaryotes. Archaea appear to be similar to bacteria in their apparatus for metabolism and energy conversion (Alberts et al. 2014). Below you will find a diagram of a tree of life, which provides some examples from each of the three major domains of living organisms. The diagram in is no way meant to be detailed or specific, but it also summarizes some of the information discussed in this post.
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