Because I wanted to discuss choanoflagellates in their capacity to tell us about the origins of animal multicellularity, I first found it necessary to define words like "animal" and "multicellular". I found myself delving into just how living things are organized in the first place. By the end of this entry, I probably won't even have gotten around to defining animals, much less choanoflagellates, so let's save that for next time. For now, our subject is the classification of Life.
Most of us learned early on that mushrooms are not actually plants, and we are thus able to put nearly every organism we can see into one of three groups: Animals, Plants, and Fungi.
But there are a lot of organisms we can't see, although we experience them regularly, like the good bugs that help us digest and the bad bugs that make us sick. So we have a fourth category, containing mostly* bacteria: Monera. (Of course viruses are very good at making us sick too, but I've already discussed them.)
There is one more category of things that are single-celled, like bacteria, but whose cellular structure is like that of plants and animals--think of all the stuff you saw the first time you got to put a drop of water under a microscope in school. We can call them the Protista.
This all may sound terribly simplistic, but for many years this "five-kingdom" model was the most reasonable way anyone could think of to organize Life. From a macroscopic viewpoint, it makes quite a bit of sense, and it even seems a little indulgent to separate Monera and Protista--after all, they're just little squiggly things.
However, advances in molecular and biochemical techniques allowed us to explore the realm of little squiggliness in great depth. And it turns out that if we stop letting ourselves be swayed by what things look like, and instead concern ourselves with what things are like--that is, the molecules that make them up--the organization of Life falls out rather differently. Instead of five Kingdoms, we find three Domains: Eukarya, Bacteria, and Archaea**.
To fit the Kingdoms into the Domains requires simultaneous compression and division. Monera are divided into the Bacteria and the Archaea, while everybody else gets crammed into Eukarya. As you might be able to guess from the fact that they were split up, Bacteria and Archaea (despite both being small and squiggly) are wildly different. In fact, the current speculation is that Archaea are more closely related to Eukarya than to Bacteria!
But we had no idea that Archaea even existed for the longest time, until we developed sufficient technology to start exploring extreme environments--deep-sea vents, hot springs, brine pools, and the like, where people found "extremophilic bacteria" that were as different genetically and biochemically from other bacteria as you or I. (Here is where we give major props to Carl Woese for standing up and re-organizing Life as we know it.) The best part, though, is that now we know they're out there, we're finding Archaea in all kinds of normal environments, too.
All this reorganization into domains was done primarily on the basis of genetics and biochemistry. All well and good . . . but what's the most noticeable difference between Bacteria/Archaea and Eukarya? Say the first stupid thing that comes to mind. That's right, Eukaryotes get BIGGER. We can SEE them.
Largest Bacterium: 0.75 mm
Largest Archaeum***: <.01 mm
Largest Eukaryote: 8.9 sq km, or, if you want to argue that isn't really a single individual, 83 m
This is not biologically trivial! The reason Eukaryotes can get so much larger than either Bacteria or Archaea is that they've figured out the trick of multicellularity.
If you've actually been following the links, you've seen that the largest archaeum, M. acetivorans, can apparently "form multicellular structures". However, glomming together into a colony isn't quite the same as the sort of division of labor that occurs in true multicellularity. But it is a very interesting behavior nonetheless, because multicellularity probably began as mere glomming.
So, next time, we'll look at animal multicellularity specifically, and its possible origins in the glomming together of things very like choanoflagellates (sneak preview).
* Why "mostly" bacteria? Well, the cyanobacteria are also included in this kingdom, and although they have "bacteria" in their name, they are also called blue-green algae, and all the other algae are in Protista.
** When I first learned the kingdoms, they were being taught as Eubacteria and Archaebacteria, but I think now most people have gone to Bacteria and Archaea--after all, these two are so different there seems to be no reason to imply that they are different flavors of the same thing.
*** Archaea can also singularize to archaeon or archaean.