|MadSci Network: Evolution|
What’s the difference between hominin and hominid? This one gets tricky, and more than a bit confusing. We are dealing with the sciences of systematics and taxonomy. The idea behind these sciences is that they create names that are (1) not confusing, (2) equally and well understood by all scientists that make use of the terms, and (3) provide information about the evolutionary and/or morphological relationships among animals. It sounds like a good set of goals. Unfortunately, we scientists have failed on every single objective. That’s why it’s so tricky and confusing. To start, let’s look at the basic concepts of taxonomy. The system we use today was developed by Linnaeus in the 1700’s (note that this is well before concepts of evolutionary relationships came into play, which is one of the problems). Linnaeus came up with a seven tiered system for organizing life: Kingdom, Phylum, Class, Order, Family, Genus, Species. The system sounds good, except that none of these seven groups is clearly defined. Biologists have been arguing for over 100 years about what constitutes a species. The best definition to date, know as the biological species concept as proposed by Mayr, states that a species is a group of animals that share a common gene pool and that are reproductively isolated from other groups. This concept seems to work well, and is testable – an important quality in any scientific endeavor. You merely put the two types of animals together and see if they will, or can, mate. If they can – they are the same species. If they can’t – they are different species. Sounds good, but then how do you apply this to asexual species (creatures that don’t need a partner to reproduce – and there are lots of these) or fossil species (creatures that are extinct, and so there is no way to apply the mating test)? The next problem, even if you solve the species question, how do you group species together to make a Genus? How do you group genera together to make a Family? Etc. You look for evolutionary relationships. Species that share a common ancestor would be in the same genus. Genera that share a common ancestor would be in the same Family. And so on. Now, to apply this problem to humans. Humans are Kingdom: Animal; Phylum: Chordate; Class: Mammal; Order: Primate – up to this point you don’t get much disagreement among scientists. But what about Family? Well, in old days (up until the 1980’s) humans were thought to differ from the other apes at the family level. That made humans Hominoids and apes Anthropoids. That made the word “hominid” a family level distinction that includes all the human species that ever evolved (including the extinct ones) that excludes the apes. Most specialists today use the work “hominid” to mean just that, although recent research shows that it is incorrect usage. (For you purists out there, I am for the moment ignoring the difference between Family and Super Family. The difference between those levels comes down to the same basic problem anyway.) Recent work shows that apes are not a monophyletic group (all descended from one ancestor), so that chimps and gorillas share a more recent ancestor with humans than they do with the orangutan. That means that, on the strict taxonomic level, chimps and gorillas are hominids. There are some specialists that use the term in this way – although it gets very confusing when they do. If chimps and gorillas are hominids, what then do we call the group that leads to humans but not to chimps and gorillas? For that, we come up with a new taxonomic level called Tribe, that lies between Family and Genus. The Tribe hominini describes all the human species that ever evolved (including the extinct ones) that excludes the chimps and gorillas. So, when scientists use the word hominin today, they mean pretty much the same thing as when they used the word hominid twenty years ago. When these scientists use the word hominid, they mean pretty much the same thing as when they used the word hominoid twenty years ago. Of course, there are still plenty of scientists around today that use the words exactly they way they used them twenty years ago. And, all of the papers that were published just a few years ago probably use the older terminology although their interpretations are still very current and valid. If you’re more confused now than you were before, you are just about where you should be. We scientists really need to clean up shop in this area. Paleoanthropologists get a lot of criticism over this issue, especially from scientists who study the evolution of other species. However, those scientists are no better off in their taxonomic problems. They simply benefit from the fact that only a handful of people study individual non- human species, which makes it easier to come to an agreement. But when you come to a hot topic area, dinosaurs for example, the taxonomic situation is just as confused and confusing. References: Mayr, E (1970) Populations, Species, and Evolution. Harvard University Press: Cambridge. McKenna, MC and Bell, SK (1997) Classification of Mammals above the Species Level. Columbia University Press: New York Simpson, GG (1967) Principles of Animal Taxonomy. Columbia University Press: New York Szalay, FS and Delson, E (1979) Evolutionary History of the Primates. Academic Press: New York.
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