I have been reading The Dragon Seekers, a fascinating book about the early fossil hunters of western Europe. One of the things that really interests me in this book is the use of certain pieces of terminology that make sense in the context of the early 19th century understanding.
One such term is "transitional fossil" AKA "the missing link", something that young-Earth creationists (YECs) often claim doesn't exist. A transitional fossil would be a fossil that clearly stands midway between two species, say between a Homo erectus and a Homo sapien sapien (yes, our species is Homo sapien sapien, which differentiates us from Homo sapien neandertalis, sometimes simply called Homo neandertalis, or the Neanderthals). The problem with this notion of transitional fossils is twofold.
First off, it's prone to "moving goalposts." A YEC may say "there is not transitional fossils linking humans to other apes." Then you show them Australopithecus Afarensis, which is a fossil descended from other apes and ancestral to humans. Once you have provided them with this "transitional fossil", they then claim that it is invalid because you can't show a connection between A. afarensis and humans. So, you show them Homo habilis, which is, again, a link between modern humans and Australopithecines, and they will then claim that you can't show a transition between H. habilis and modern humans...and on and on it goes, every time that you show them what they ask for, they find a way to worm their way out of accepting reality. I have even had conversations with YECs where I have shown them every homonid fossil I could think of, clearly linking humans and other orders of apes*, only to have the YEC then shift the goal posts even further and demand that the only suitable evidence for evolution would be blood tests proving that these creatures were ancestral to the humans - and I can only imagine what their excuse would have been for denying that if I had been able to produce such a thing.
So, shifting goal posts is one problem with the use of the term "transitional fossil", but there is another problem, one that dogs scientific spokesmen and the public alike. That is that species are not fixed types, but are constantly changing due to the forces of evolution (genetic drift, mutation, natural selection, etc. etc.). So, any given species is constantly undergoing subtle modifications which are likely to cause new species to arise, and those that we identify either in the world around us or in the fossil record are merely "snapshots in time" of the process of change. In other words, depending on how you look at it, either there are no transitional fossils or EVERY fossil is a transitional fossil.
Now, given our modern understanding of the mechanisms of reproduction and genetic change, the term "transitional fossil" as a marker of change from one species to another becomes, really, meaningless except as a rhetorical tool, and as a rhetorical tool it's prone to mis-use by those who deny the reality of evolution (such as the YECs). However, as I have been reminded of by The Dragon Seekers, there was a point in time when this term did make sense, within the context of the knowledge of the day.
The early fossilists worked out that the animals that populated the earth had changed over time. However, most of them believed that species were fixed, and did not "transmutate", to use the terminology of the day. The reason for this was partially religious, most of these early fossilists were Christians and a solid foundation for geology and biology had not yet been worked out, making the overall creation story as good an explanation as any for the world they observed**. The reason was also partially cultural but non-religious, these people were part of a movement that sought to explain the world through categorization, and these categories tended to be thought of as both objective and fixed, an error that still creates problems.
At any rate, the thought was that species were fixed (or at least very stable), and therefore a transitional fossil representing a volatile intermediate stage would be expected if one stable species changed into another stable species. Such a fossil would be a "missing link" or transition between two stable classifications.
But, we now understand biology well enough to recognize that terms such as "missing link" and "transitional fossil" aren't very useful, but they still continue to be used, nearly two centuries after the last nails should have been hammered into their coffins. I think that there are two reasons for this. The first is that those who have a stake in viewing species as fixed, primarily the YECs mentioned above, keep trotting the terms out, claiming that they mean something in order to try to persuade others that evolution is not true. As these folks are vocal and have a fairly strong political and media presence***, they have been able to keep these terms, which serve their cause and create grief for scientists, active.
The second, and probably larger, reason is that these terms are, let's face it, catchy. Especially "the missing link." As such, the media likes to use them, and they continue to be in circulation.
As a result, these terms remain in popular culture, despite the fact that we would all be better off if they were just dropped.
And then today, I see this article, thanks to Kay. And, again, the term "missing link" is used to describe the find. However, in this case, it is a bit more appropriate. This fossil provides information about the point when primates deviated from other types of mammals, and so the term isn't complete nonsense.
*"Ape" is a taxonomic designation - in other words, it's a label used to group together animals that share certain common traits. Humans have the ape traits, and as such, we didn't "come from apes" - we are a type of ape, just as a Chihuahua and a German Shepperd, though very different animals, are both dogs.
**Contrary to popular belief, Darwin was not the first person to propose evolution, it was an idea that had been floating about, he was just the first to comprehend the mechanism and to explain it in a way that accounted for the evidence better than any other explanation that had been proposed up to that time. So, prior to Darwin, there were people who saw the evidence for evolution, but they lacked the necessary clues to put it together into a viable scientific theory.
***Think I'm wrong? Think back to the debates between the Republican candidates, and remember how several were willing to deny reality - evolution, that is - when asked? This is one of the major political parties, expected to get a huge chunk of the popular vote, and they represent the views of alot of people. Reality denial and opposition to good science isn't as fringe as many people like to think it is.