Saturday 23 February 2013

Let's Call the Whole Thing Off! You Say Hominidae and I Say Homininae.

[UPDATE UTC/GMT 15:49 on Monday, February 25, 2013
Between the stars is the original introduction to this piece. I started and stopped so many times and drafted so many versions that I was despairing of ever saying anything of relevance. I left this out more by accident than by aim.]

Jonathan Marks alerted his facebook pals to this, and I'm gonna use it as a springboard to argue for a revision of the most-recent revision of the Primate Superfamily, Hominoidea, to which you and I belong.


By Cadell Last | February 13, 2013

The author asks why it is that humans are lumped with the Old World hominoids---PanGorilla, and Pongo---also known as the great apes. The author cleaves to a point of view to which I and a rare few others have propounded for quite some time. To wit, modern humans are unique among the other apes, notwithstanding all of the marvelous behaviours of which we know our ape cousins are capable. Cadell posits two reasons for the relatively recent reclassification.

a) a scientific tradition that emphasizes ancestry over emergence and
b) an evolutionary past that has left us with no extant sister taxa.
The author cites our Jon Marks and Robin Dunbar in support of a) and the vicissitudes of evolution for b). Last ends by concluding that
splitting humans from great apes taxonomically would be beneficial. Conceptually it would allow researchers to better understand the hominid/ape divergence and the key differences between humans and great apes today. But perhaps more importantly, splitting humans from the great apes allows us to reconceptualize our own humanity.  We are not the great apes; we are humans.
You don't need to have spent too much time reading the Subversive Archaeologist to be able to predict my feelings on the matter---we modern humans are unique, even within the genus Homo. Thanks to Last's musings, I have another excuse to play around with the question of hominoid taxonomy [Yes, again!]

Heeeeeee's Baaaaaaaaack. [Apologies to MGM.]

But he feels a little like he's been dragged by rope through death and back to life in the context of a cult horror classic film. [Why is he writing in the third person? Why am I writing as if I'm not here?]

I should start by saying that I've passed my 60th birthday, and I cut my human-palaeontological teeth 40-plus years ago at a time before molecular comparisons included individual base pairs. In those days it was quite all right to classify all apes in one exclusive taxonomic group, the Superfamily Hominoidea, and subdivide them as follows.

Order Primata
   Suborder Haplorhini
      Infraorder Simiiformes (or Anthropoidea)
         Parvorder Catarrhini
            Superfamily Hominoidea, broken down as shown below.

Clean and simple. No? We're different from the other apes. In retrospect this classification appears somewhat naïve. From molecular biology we now know that the oldest extant lineage among the tailless Catarrhini is the one from which the four extant gibbon genera have evolved. That hasn't changed---the gibbons are still classified as Hylobatidae.

[Off topic aside. Any idea which American country and western song is Biruté Galdikas's favourite? Scroll down for the answer.]

During my absence from the pixels before you I've many times thought I should retitle this blog the Submersive Archaeologist, so deeply this time has it seemed to me that I've descended into the rabbit-hole. Which rabbit-hole was that? The one out of which emerged the present-day view of Hominoid taxonomy, against which I've been fighting a losing battle with cladistics on the matter of what to call you and me and the other members of the genus Homo in terms of which the International Commission on Zoological Nomenclature would approve. Wood and Richmond pretty much carved in stone this new view of the Superfamily Hominoidea a few years back. ["Human evolution: taxonomy and paleobiology." Journal of Anatomy 196:19--60, 2000.] Their scholarship is not in question here. I'm quibbling about names, not anatomy; natural groups, not genes.

I lost the battle I waged with windmills over the past couple of weeks because the molecular biology can't be refuted. As we construct the hierarchical relationships between us and progressively less closely related organisms, it's simply irrefutable that we share common ancestry with ever-more-inclusive pairings of hominoid groups.

According to the consensus view,  the most recent of these common-ancestor diads to which we belong is the tribe HOMININI---comprising the Subtribe HOMININA [Genus Homo],  several subtribes of fossil bipedal apes, the Subtribe PANINA [Genus Pan (chimpanzees and bonobos)], and any subtribe or subtribes of Pan's fossil precursors.

The next most inclusive grouping---Subfamily HOMININAE---includes the tribes HOMININI and GORILLINI [Genus Gorilla and the tribe or tribes comprising Gorilla's fossil precursors].

Next most inclusive grouping---Family HOMINIDAE---consists of the subfamilies HOMININAE and PONGINAE [which includes all of the fossil hominoids not belonging to either the subfamily HOMININAE or the family HYLOBATIDAE].

This use of the taxon HOMINIDAE (Gray 1825) is, I believe, pre-emptive and arbitrary. By rights it should be the Family PONGIDAE that makes up of the group of hominoids that remain once you exclude the gibbons and fossil forms belonging to the family HYLOBATIDAE.

Instead, the wizards that renamed the entire superfamily chose to use HOMINIDAE (Gray 1825), most likely because according to the International Commission on Zoological Nomenclature the family name HOMINIDAE has 'priority' over PONGIDAE (Elliot 1913) in the long history of naming organisms.

While I can understand why one taxon should have priority over another when one is deciding what to call a previously recognized species or genus, the choices aren't as clear-cut at more inclusive nomenclatural ranks [I've ALWAYS wanted to use the word nomenclatural in a publication. This is as close to real publication as I'm ever gonna get again. So, I'm really stoked!]

I've made a slide that I think presents the choices entailed in classifying the hominoids. It has made it easier for me to get my head around the issues. Keep in mind that the first two chips off the old block---the gibbons and orangs---arose we know not where, at about 18 Ma and 14 Ma, respectively. All we can be certain of is that their descendants are today found only in east and southeast Asia. [Yes, I said east Asia. The gibbons are extirpated throughout most of what's China today. However they were widespread historically and figure prominently in iconography and legend in that part of the world.
In presenting the above, I'm hoping that it'll become clear that it's not a straightforward matter what to call the smaller and smaller remainders of the great geographic swathe of hominoids when all that's being considered are the molecular data from extant lineages. What, for example, do we decide to do about the inevitable evolutionary dead ends at each successive rank? As it is, the nomenclature is being stretched. A few more spurs on the family tree and we'd run out of nomina in a real hurry.

The upshot: no one's ever made such an inclusive classification as the one depicted above. Thus, I think that nomenclatural priority shouldn't apply in this case, and that the group that sits opposite HYLOBATIDAE by rights ought to be PONGIDAE (Elliot 1913), if only because PONGIDAE has priority when naming the group containing Pongo, Pan, and Gorilla and all of their fossil precursors. That had been the case up until molecular data put the kibosh on lumping Pongo with Gorilla and Pan. Adding the bipedal apes shouldn't, to my mind, imply substituting PONGIDAE with HOMINIDAE. In addition, using HOMINIDAE in this way, after so many, many years of using it in the old way, seems to me to be more confusing than it needs to be. And, alternatives are available, which wouldn't require as all-encompassing a revision as the one seen above. I say, dump HOMINIDAE as a taxon!
Nothing, for example, precludes use of the zoological rank of Epifamily as the rank immediately following Superfamily. The following provisional revised revision more closely follows what we know of the pace of hominoid evolution. The Epifamilial split occurred about 18 Ma. That of the Family happened about 14 Ma. Thus, the African great apes, more closely spaced in time---a common ancestor some time in the last 7 or so Myr---comprise a family, within which the gorillas on the one hand, and chimps and humans, on the other, inhabit different subfamilies.
Superfamily Hominoidea
   Epifamily Hylobatoidae
   Epifamily Hominoidae
      Family Pongidae
      Family Hominidae
         Subfamily Homininae
            Tribe Panini
            Tribe Hominini
               Genus Australopithecus
               Genus Ardipithecus
               Genus Sahelanthropus (?)
               Genus Orrorin
               Genus Paranthropus
               Genus Homo
         Subfamily Gorillinae
               Genus Sahelanthropus (?)
               Genus Gorilla
This suggested hierarchy violates none of the molecular data, while still providing plenty of leeway for the addition of taxa as our knowledge of hominoids expands.

I welcome your comments.

That's all I'm a gonna say about that! 

Biruté Galdikas's favourite American country music song is "Take the gibbon from my hair."

[I really hope you haven't been waiting with 'bated breath for the return of SA. Somehow the above is less satisfying than it ought to be. Never mind.]

SA announces new posts on the Subversive Archaeologist's facebook page (mirrored on Rob Gargett's news feed), on Robert H. Gargett's page, Rob Gargett's twitter account, and his Google+ page. A few of you have already signed up to receive email when I post. Others have subscribed to the blog's RSS feeds. You can also become a 'member' of the blog through Google Friend Connect. Thank you for your continued patronage. You're the reason I do this.

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