Re: What is the difference between hominin and hominid when classify humans?

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.