It used to be as easy as animal, vegetable, and mineral. My fifth-grade daughter was assigned some homework for the weekend. She had to illustrate the Five Kingdoms. The Five Kingdoms? Well, there's Munchkin Land, the Emerald City, Winki Country... "No, Mom, you know, Animals, Plants, Fungi, Monderans..." What? Monderans?
When I was her age, all life was divided into the plant kingdom and the animal kingdom. Since then, discoveries have strained the definition of a plant or an animal. There are things that just don't fit. As soon as we think we've got it figured out and create new definitions, along comes another lifeform that doesn't fit the new taxonomy, either. Since when is fungi not a plant? I looked up taxonomy at Wikipedia, and found this image by Peter Halasz. Things have changed a bit since I was in school. I thought kingdoms were at the top of the taxonomy tree. I understand "life", to separate it from non-living things, but "domain" is a new one to me, and for good reason -it was introduced in 1990. The resulting six kingdoms have yet to make it to my child's elementary school.
My daughter was having a hard time finding a picture of "Monderans" (which sounded like some kind of art to me), so I enlisted the help of Google. Thanks to the "Did you mean..." feature, we determined she meant Monerans, which I then found out was a fancy name for bacteria. Bacteria were once listed in the Protista kingdom, which started as a catch-all for unicellular organisms that were just too hard to classify as plant or animal. Bacteria were thrown out when it was determined they don't have nuclei. Every kingdom must have its standards.
Fungi was separated into its own kingdom (Eumycota) in 1968, but didn't make it into high school texts until years later. Fungi diverged from plants about a billion years ago, since they didn't want to get with the program and biosynthesize using chloroplasts, which were all the rage at the time. These renegade mushrooms still don't use chloroplasts, but they are green with bioluminescence.
Archaea, or archaeobacteria, are single-celled organisms with no nuclei. That sounds like bacteria, but biologists have determined that chemically, they have more in common with plants and animals they do with bacteria. So... in 1977 they got their own kingdom! Or, domain, if you go by the three-domain system.
The three domains, therefore, are Bacteria (or Monera), Archaea, and Eucaryota (everything else). Bacteria and Archaea are their own kingdoms (for now), and the Eucaryota domain is divided into Plants, Animals, Protists, and Fungi.
In the five-kingdom system taught in fifth grade this week, Bacteria and Archaea are still classified as Monera. I can well understand why. You try deciphering the difference between the two, and imagine translating that into a fifth-grade reading level textbook! I found some nice examples of Monera which my child dutifully rendered on posterboard. There'll be no markdown over the spelling, since the teacher spelled Monerans with a "d" in the instructions.
The six-kingdom system isn't the end of the taxonomy struggle. In high school, I got in trouble for something I don't recall now, but the teacher assigned me to write a ten-page essay on "the meaning of life" as a punishment. To make things easier, I interpreted that as the definition of life, and handed in twenty-five pages. Along the way, I became fascinated with viruses. Scientists were debating whether or not viruses should be classified as a kingdom, or whether they should be classified as living at all. To be a living thing, a species must be able to reproduce. Viruses can reproduce, as anyone who has suffered the flu can attest. But they cannot do so on their own; a virus must invade a host cell, inject its own DNA, and depend upon the host cell to reproduce, as described here.
The virus conundrum still exists, but the very edge of the definition of life gets even weirder when you consider prions, which are nothing more than proteins, but can replicate themselves. Mad Cow Disease is caused by those baffling prions, which is why they haven't found a cure yet.
It took me about two hours to get caught up on what's new in taxonomy since I finished school ...a few years ago. Once again I am humbled to find out how much there is still to learn about the earth we live on. So here's a nice picture of the two kingdoms we are most familiar with.
Update: My child got an A on her poster, since she could demonstrate an understanding of the material better than the other students!
Update 5/26/11: The child featured in this article scored high enough to get into an advanced placement science class when she starts high school this fall!