Scientifc standards for new genera?

Per manahattan’s request, I’m asking this question: “What is the scientific standard for declaring new genera?”

A group of researchers, lead by Meave Leakey (of the anthropology Leakeys), discovered a skull in Kenya. Here’s a link the Washington Post article about the discovery. The group decided that the skull was different enough from the “Lucy” skeleton that they gave it a new genus and called in Kenyanthropus platyops, which means “Kenyan flat-face” (though makes me think Kenyan platypus). Lucy (Australopithecus afarensis), on the other hand, had a skull with more pronounced jutting of the jaw, somewhat between human and ape. Not all scientist agree that that a new genus is warranted on such little evidence.

So the questions are: 1) What is the criteria used to determine a new genus rather than just a variation in an existing genus? 2) How much evidence is required? 3) Who decides? What happens when one group says they found a new genus and another groups says no? Anthropologist death match?

Just to show that anthropologist can have a sense of humor, here’s a quote from the Washington Post article from Daniel Lieberman:

Which is such a great quote, I’m making it my sig.

I listened to a radio interview on NPR Friday, I think it was with Daniel Lieberman, who made the point that the small information from the new fossils was a poor basis for a new genus.

Basically, the answer to your question is “Anthropologist death match.”

All of the “higher” taxonomic categories above species, from genus on up, are largely subjective. “Species” can be defined in such a way that they have some objective reality (although there are various definitions, including the sometimes hotly debated “biological species concept” and the “phylogentic species concept”). Genera and the other categories basically serve for “bookkeeping” - pigeonholes in which to stack species.

Within taxonomic groups, there is usually a consensus among taxonomists about the degree of similarlity required to place species in the same genus. But this differs among taxonomic groups. For example, I have heard it said that each family of birds is more or less equivalent to a genus of beetles as to the degree of morphological distinctness they contain. And even within birds generic standards differ between say, seabirds and songbirds. Basically, taxonomists try to create standards that will result in genera that make manageable units; i.e. that won’t result in thousands of one-species genera or in a single genus containing thousands of species. There has been some effort recently to define genera and other categories in terms of genetic divergence, but this also a contentious topic.

Basically, for human fossils, since there are so few and we have absolutely no idea of interpopulation or interspecies variability, there is little practical consensus about generic limits. There are probably almost as many classifications as there are anthropologists. Many individual fossils have been given at least species status at one time or another, and especially in the past each new find got awarded a new genus as well.

I should also mention that the title of Jared Diamond’s book “The Third Chimpanzee” is based on his contention that an Anthropologist form Mars would no doubt include humans and the two species of chimpanzees in the same genus, not seeing a lot of difference between them. If apes (and humnas) were beetles, they would probably all be in the same genus.

Hmm, I should have studied more anthropology in school.

So, part of it is an ego thing with with each person wanting to discover a new genus.

The story I heard on NPR (probably the same one as Padeye) someone made a similar point about the new skull and Lucy. If they both jumped out from behind a bush, you couldn’t tell them apart from more than a few feet away.

I think that taking a bit more care with classifying potential ancestor of humans makes sense. Rather than jump to conclusions based on a single skull (not even a complete skeleton) can be misleading in the quest to trace human evolution.

The fact that this process is somewhat arbitrary does not mean there is sloppiness in determining evolutionary relationships. You can put together evolutionary trees with a reasonable degree of accuracy, though it can be difficult with as little to work on as we have with human relatives. The arbitrary part is where on the tree to put the dividing line. In the end, it’s really not that important. All taxonomic designations from species to kingdom are artificial and arbitrary. They just provide a standard reference frame that gives everyone an idea of what’s realted to what.

What actually defines a genus, anyway? If two animals can produce healthy offspring, they are the same species. Is there a similarly simple definition for genera?

As I tried to indicate in my first reply, the answer is simply no. As Smeghead said, while it may be possible to identify the branching pattern between species, the morphological or genetic level of the branch node you define as a “genus” is pretty much arbitrary. It is arrived at by consensus of the taxonomists working on that particular group of organisms, and can vary substantially between groups.

I should also reiterate that there are various species definitions out there. The criterion of the “ability to produce healthy offspring” is a bit oversimplified even for the prevalent “biological species concept.”

By “anthropologist death match” I mean a knock-down drag-out fight in the scientific journals. The victor will be determined by audience response - that is, by consensus of the anthropological community as to whether a new genus is justified or not. And that will evident by whether or not other scientists refer to the fossil by Leakey’s name in their own publications, or instead include it in some existing genus.

Remember that when first described Java Man (Pithecanthropus erectus) and Peking Man (Sinanthropus) each received their own genus, but are now almost universally recognized as belonging to our own genus as the species Homo erectus. Likewise other genera proposed by the Leakey dynasty such as Zinjanthropus are now virtually never recognized.