They're often used in place of the generally more appropriate simple and complex. By that usage, an amoeba is primitive and an aardvark is advanced. But where it gets confusing is that primitive and advanced are also sometimes used to mean "like something that evolved earlier" and "like something that evolved more recently," respectively -- so they use primitive to describe a stegosaurus and advanced to describe a spider monkey, when in fact both of those are about equally complex. (It gets even murkier when you throw in questions of relative intelligence.)
It bears keeping in mind that while modern organisms vary greatly on the simple/complex spectrum, they all have lineages that have been around exactly the same amount of time -- 4.3 billion years, give or take a day or two. All known lineages of terrestrial life converge on a single life form nicknamed LUCA -- the Last Universal Common Ancestor -- around four billion years ago. To our eyes, LUCA probably wouldn't have looked like much. It probably resembled species we now classify as bacteria.
But all life on Earth descends from it. And as far as the primitive/advanced bit, the only difference is in that time, some of the lineages changed a great deal more than others did.
The reason this comes up is because of a link sent to me by a friend and frequent contributor of topics for Skeptophilia, about a species of fairly modern-looking jellyfish that was found in rock strata that are 505 million years old.
The species, named Burgessomedua phasmiformis, was a free-swimming, tentacle-laden predator with a bell on the order of twenty centimeters in diameter. It, like many of the Cambrian explosion fauna, were found in the exceptionally well-preserved Burgess Shale Formation of the Canadian Rockies in British Columbia.
Artist's impression of live Burgessomedusa in the Cambrian seas [courtesy of artist Christian McCall]
Jellyfish and most of the other members of Phylum Cnidaria are generally scarce in the fossil record, because their bodies are primarily water. If you've ever seen a dried-up jellyfish on the beach, you know what I'm talking about; there's barely anything left. (Don't assume that this means they're harmless, though. Even the dried tentacles of a Portuguese man-o'-war can pack a dangerous sting.) But you can see how astonishing it is not only to have one create an impression in sedimentary rocks, but to have that impression last for 505 million years.
So the exceptional preservation of this extremely rare fossil animal is amazing enough. But what I find even more mind-boggling when I think about the life back then is the bigger picture of what the Cambrian Period was like. At that point, all life was in the water. There was (more or less) the same amount of land as there is now, albeit configured completely differently -- but on that land was not a single living thing. No plants, no fungi, no animals. Nothing. It was a vast expanse of empty rock, sand, and dust.
At this point, the first terrestrial plants wouldn't make their appearance for another fifty million years, and even then, they were highly water-dependent and very likely clustered along shorelines. The first vascular plant -- one with the internal plumbing most plants have today -- that appears in the fossil record is Cooksonia, which appeared during the mid-Silurian Period (about 430 million years ago). It was a strange, rather Dr. Seussian thing:
[Image licensed under the Creative Commons Smith609 Ground texture from Image:Mud closeup.jpg, Cooksonia pertoni, CC BY 3.0]
But when Burgessomedusa was swimming in the Cambrian oceans, all that lay millions of years in the future. This glimpse of the dawn of time gives us a picture so alien to our current mental image of the Earth it's hard to believe it's the same planet.
What this tells paleontologists, though, is that even in the early Cambrian, there were relatively modern-looking jellyfish -- and that even though today's cnidarians are advanced in the sense of "length of their lineage on Earth," they haven't changed much at all during all those hundreds of millions of years. The general reason for such stability is that the body plan works; there's little selective pressure to favor alterations in a system that does fine as is, however "primitive" it may look to us.
As a writer friend of mine posted yesterday:
The details might be off a little, but the gist is accurate enough.