Tree of Life Domains: Bacteria, Archaea & Eukarya

Instructor: Angela Hartsock

Angela has taught college Microbiology and has a doctoral degree in Microbiology.

In this lesson we will consider how all the organisms on Earth are related to one another and the favored hypothesis for how all these organisms fit into three broad categories called domains.

Tree of Life

Most of us know who we're related to. Those few weird people we see every morning at the breakfast table, and the more numerous (and usually even weirder) people we see every holiday. But what if we think more broadly? What if we thought about how we're related to all the other organisms on the planet?

Now we're not just thinking about humans; we're thinking about birds, reptiles, plants, fungi, bacteria, algae, and the list goes on. Now things get a lot more complicated. When we think about all these relationships we are thinking about the tree of life. The tree of life is a visual metaphor scientists use to organize all the organisms on the planet.

You can imagine each branch as a group of related organisms
bare oak tree

Organizing Organisms

Imagine I asked you to predict which organisms are related to one another. You would probably put on your scientist hat and start analyzing the different organisms. You might start to make some observations. Maybe you notice that some organisms have wings, some have split hooves, some give birth to live young, some have scales, some are aquatic, and some have feathers. Your list of observations and categories would continue to grow based on these structural, (or morphological) differences.

This is exactly how scientists first started putting organisms into related groups. We will call that the 'old way'. Scientists like Carl Linnaeus and Charles Darwin used this kind of system. It was great at first because it kicked off an entire field of biology called taxonomy, which is devoted to classifying how living organisms are related to one another.

At the most broad level, scientists created three big groups. They put things like mushrooms in one group that was separate from plants, and plants in one group that was separate from animals. But remember, these kinds of groupings are only ever a prediction or a hypothesis. Predictions can always be updated if we learn something new.

Using Genes to Form Domains

This brings us to what we will call the 'current way'. The current way relies on inherited genes to predict relationships between organisms. You might think you don't have any idea how this would work but you probably do!

When you say that you have your mom's eyes or your dad's chin, you are really saying that you inherited that trait from your mom or your dad. The way that we inherit information is through our DNA, or our genes. So you got genes from Mom that influenced your eye traits.

The same is true at a much grander scale. Scientists had the idea to look for the genes that organisms on the planet share. A scientist you have probably never heard of, Carl Woese, keyed in on a gene for ribosomal RNA (rRNA) and started to look at how similar this gene was between different organisms. The more similar, the more related. He put forth a hypothesis that placed organisms into three really broad categories, called domains, based on similarity and dissimilarity in that rRNA gene.

What was so interesting was that this method collapsed the old groups, the plants, mushrooms, and animals, into one domain and added two domains representing some of the smallest life forms on the planet.

Example organisms to represent some of the diversity in the three domains of life.
Images of example organisms from the three domains of life.

The Three Domains

According to the current system, all the living organisms on the planet can be placed into one of three domains: the Bacteria, the Archaea, and the Eukarya.

  • The Bacteria include thousands of species of microscopic, single-celled organisms. While they don't have a lot of morphological differences (like feathers, legs, hair, organs, etc.) there are many metabolic (energy processing) differences between bacterial species.
  • The Archaea were discovered most recently and at first glance they seem very similar to the bacteria. They are also microscopic, single-celled organisms that are morphologically simple but metabolically diverse. However, based on their genes, they are actually more closely related to things like animals than they are to bacteria!
  • The final domain, Eukarya, is the one you are probably most familiar with. It includes what we think of as the morphologically complex organisms, like animals, plants, fungi, and of course, humans. But it also includes some microscopic organisms like amoebas and algae. A defining feature of the Eukarya is that they have a nucleus in their cells.

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