Did Gen Suwa just save paleoanthropology?

That depends on whether these teeth are really from a gorilla, I suppose.

Chororapithecus teeth compared to gorilla mandible

Chororapithecus abyssinicus teeth compared to gorilla mandible. Photo credit: Gen Suwa/University of Tokyo.

Oh yeah, sure, "saved paleoanthropology" is overdramatic. But what am I supposed to write? Over four years, we have had a series of genomic comparisons narrowing down the age of the human-chimp common ancestor to something like 2/3 the age of Sahelanthropus. I said it was a crisis, and it is: these data sources must agree. Either we have to cast out a bunch of hominids, or we have to wrench the genes by around a factor of two.

Now, Suwa and colleagues show up with a 10-million-year-old gorilla. A 10-million-year-old gorilla works just fine with 7-million-year-old hominids. It doesn't work at all with a 7-million-year-old human-gorilla common ancestor. So there's no doubt about the centrality of this particular ancient gorilla -- if it is one.

So far, all the articles I've seen have someone on the record expressing some reluctance to accept the teeth belonged to the gorilla lineage. Reuters has Peter Andrews; Nature has Jay Kelley; National Geographic has Richard Potts.

Should we be skeptical? Well, there are lots of convergences among Miocene apes. Many of the dental convergences are detailed in our paper about Sahelanthropus, available open-access from PaleoAnthropology. These convergences make it difficult to identify hominids based on the teeth alone. They also make it hard to say that any particular big-toothed, leaf-eating ape is definitely a gorilla. After all, if it eats like a gorilla, why shouldn't it have teeth like a gorilla?

Suwa and colleagues go to some pains to demonstrate that the dental similarities with gorillas are more than enamel-deep. Their strongest argument is that the tooth morphology exhibits a derived gorilla-like condition well below the surface, at the enamel-dentine junction. That is, while the tooth was forming, the initial growth surface took on a distinctive shape which was then reflected by the form that the growing enamel took.

The most distinctive features of the Chororapithecus dentition are the derived shearing structures seen in portions of its molars (Fig. 2), despite a generally low cuspal topography (the latter is apparently a primitive retention).
Examination of internal morphology by micro-computed tomography (micro-CT) demonstrates that these occlusal features were underlain by distinct enamel-dentine junction (EDJ) structure (Fig. 2). In particular, the straight to weakly concave mesial protocone crest seen in the EDJ of CHO-BT 4, -BT 5 and -BT 6 is gorilla-like, and is formed by a mesiobucally located junction of the mesial protocone crest and mesial marginal ridge. Such spatial placements are best considered to be regulated by enamel-knot-related signalling patterns during early morphogenesis [23, 24], and may be one of the underlying causes of the mesiodistally elongate upper molar shape generally characteristic of folivorous primate species. In the lower molars, the most distinctive EDJ topography occurs at the trigonid crest, the structural counterpart that occludes with the upper molar mesial protocone crest. The high trigonid EDJ crest is continuous between the metaconid and protoconid cusp tips (Fig. 2). Because recent experimental and quantitative genetic studies suggest significant degrees of morphogenetic independence between corresponding upper and lower molar structures [25, 26], the presence of a functionally integral inter-jaw pattern of morphological expression, as seen in the Chororapithecus molars, suggests adaptation by natural selection, as opposed to chance emergence of neutral morphological minutia.

Still, "minutia" is a loaded term. Why shouldn't an ape that evolves the same shear characteristics as a gorilla molar use the same developmental process to achieve them? The more that development of the teeth are constrained by these genes, the more likely it is that different lineages will evolve in parallel.

Nor is it entirely obvious that Chororapithecus is actually gorilla-like in these characters. The paper compares two ratios involving cusp dimensions measured internally beneath the enamel cap. That's high-tech, but the ratios do not sort out gorillas from chimpanzees, don't sort Chororapithecus from either of those apes or early hominids, and -- even worse -- it's not even clear how these ratios may vary with size. Does Chororapithecus look sort-of like a gorilla on these ratios because it's a sort-of gorilla? Or because it's big? The enamel is relatively thicker than gorillas, like other Miocene apes and orangutans. Clearly the specimen is much less derived than gorillas, but could that be because it isn't a gorilla?

Well, there's the problem: there's not too much to go on with these teeth. I think Suwa et al. laid out as good a case as there is. A 10-million-year-old gorilla can't be expected to look just like gorillas today. It's not like the teeth look more like something else besides a gorilla. Gorillas really are far more derived in these dental characters than the Chororapithecus dentition, which makes the comparison more difficult. And so, the conclusion of the paper is equivocal:

The similarities seen between the two genera raise the possibility that Chororapithecus is a Miocene member of the Gorilla clade. Alternatively, with its combination of thick enamel and distinct molar cresting pattern, Chororapithecus may represent a unique adaptation that is convergent with gorillas in molar structure and function. Although the evidence for phylogenetic affinity between Chororapithecus and Gorilla is inconclusive, it may be that the basal members of the gorilla clade shared large tooth size and incipiently enhanced molar shear as a part of an herbivorous diet that accompanied (presumed) larger body size. Chororapithecus may then represent one example of adaptational (and perhaps phyletic) differentiation within that clade.

I don't know about anybody else, but I don't think this helps us with our little problem very much. Here's what I think: the problem is not so much the 10-million-year-old gorilla, as it is the 17-million-year-old orangutan that it necessitates. Here's the very next paragraph of the paper:

Acceptance of Chororapithecus as a basal member of the gorilla clade would push back the gorilla species split to >10.5 Myr ago. Because this is a minimum date established from a meagre fossil record, the actual divergence would have predated this by an unknown time gap. From the currently available evidence, we consider that a species split of 20 Myr ago for Pongo, 12 Myr ago for Gorilla, and 9 Myr ago for Pan are all probable estimates (see Supplementary Information). We consider that the early divergence hypothesis is congruent with both fossil and molecular data, and should be further evaluated using both sides of the evidence.

I think those dates don't really need to be so old. A 10.5-million-year gorilla divergence could easily correspond to a 17-million-year orangutan divergence. Still, for those of us who have gotten used to the idea that Dryopithecus might have something to do with the origin of African apes, this idea might seem a little troubling. So, let's look at the part of the Supporting Information that, well, supports their assertion that all these dates are "congruent":

The above summarized molecular predictions are in concert with the notion that the Pongo lineage existed in Africa prior hominoid migration to the Eurasian continent, the earliest such opportunity for dispersal (barring significant rafting) being at circa 17 Ma (44). If in fact the Gorilla split was 12 Ma, then the OWM split estimate (33.6-43 Ma) largely predates the earliest known definitive occurrence of catarrhines (Propliopithecus and Aegyptopithecus) (45), and many would consider this to be somewhat outside an acceptable boundary condition. However, it may be indicative of variable molecular rates of evolution across lineages (46, 47), with higher mutation rates in the OWMs (48) (and early hominoids) because of their shorter generation lengths (48, 49) and/or higher metabolic rate in relation to smaller body mass (50).

Well, that's a tricky bit of argument. We might believe that African apes never left Africa and that all the dryopithecines are therefore on the orangutan line. At least, that makes some biogeographic sense. But it's hard to argue that any of these dates are "congruent" with genetic evidence as we currently understand it. Many of the recent methods don't make any prior assumptions about "calibrated" divergence times like the orangutan-human divergence. Worse, Hobolth et al. (2007) found a human-chimp speciation time of 4 million years even considering an orangutan-human divergence of 18 million years.

The "shorter generation lengths" explanation doesn't help much -- after all, if we infer that the current great ape lineages existed as early as 20 million years ago, then almost all of the divergence time is occupied by long-generation-length species. Much faster evolution in Old World monkeys should show a strong signal of acceleration in that lineage (with a higher number of derived substitutions), and we don't see it.

If we believe these interpretations of the genes, a 10-million-year-old gorilla did not exist. Chororapithecus was something else.

If we believe that Chororapithecus was a gorilla, then these genetic interpretations are simply wrong. And Dryopithecus was on the orangutan lineage. And hominoids diverged from Old World monkeys in the Eocene.

And Sahelanthropus could have been a hominid.

References:

Suwa G, Kono RT, Katoh S, Asfaw B, Beyene Y. 2007. A new species of great ape from the late Miocene epoch in Ethiopia. Nature 448:921-924. doi:10.1038/nature06113

Hobolth A, Christensen OF, Mailund T, Schierup MH. 2007. Genomic Relationships and Speciation Times of Human, Chimpanzee, and Gorilla Inferred from a Coalescent Hidden Markov Model. PLoS Genet 3(2): e7. doi:10.1371/journal.pgen.0030007