A More Thorough Reading of the Chororapithecus abyssinicus paper

Thanks to Afarensis, I got my hands on Chororapithecus abyssinicus paper, “A new species of great ape from the late Miocene epoch in Ethiopia,” and I have read it. A lot of things have been clarified, such as my misunderstanding that the nine teeth were from one individual. The nine teeth are from at least three and at most six or more individuals. In all honesty, this was a stupid assumption for me to make. Other than finding the teeth inside a mandible or jaw or, it is hard to say with any confidence if any loose fossil teeth are from one individual.

For your viewing pleasure, here’s a line up of the culprits:

The Nine Chororapithecus Teeth

That said, the big thing we should be concerned about is the claim that,

“Phylogenetically, these fossils represent the first Miocene ape species to be recognized as a strong candidate for membership in the modern gorilla clade.”

The claim that these fossil teeth represent a Miocene ape is made by the ‘subtle’ similarities in size and proportions they have to a modern gorilla’s teeth. A quick disclaimer, the size of these teeth are not entirely the same to a modern gorillas… especially the molars. The various molars in this sample fall right in the largest and smallest range of modern gorilla size variation. On my other blog, Anthropology.net, I posted a fairly high resolution photograph with three of the teeth compared up to a gorilla’s jaw. In that image you can see how the overall size and form are similar.

Tooth Section IllustrationAside from the size, the morphology of the teeth is the other argument that supports a Miocene ape claim. The exact morphological condition is observed well below the surface, at the enamel-dentine junction, or EDJ.

You ask, “What is the enamel-dentin junction?” The EDJ is a landmark in teeth where the surface enamel, the hardest substance in body, meets dentin, the less mineralized and less brittle of the two. Unlike dentin, enamel does not contain collagen. The image to your right illustrates where enamel meets dentin. Another difference between enamel and dentin is that enamel has two types of proteins called amelogenins and enamelins, which most likely serve as a framework support.

Amelogenesis, or enamel formation, is first seen in the crown stage, which happens after the first establishment of dentin. Cells known as ameloblasts lay down amelogenins and enamelins matrix to form a partially mineralized enamel. This differentiates the two tissues in morphology. In another view, seen in image below, you see a histologial cross section of enamel (A) meeting dentin (B). Note how dentin is different, it is tubular.

Tooth Cross Section

Enough histology and development, talk. I can go on for days on how important teeth are in paleontology. The whole point of studying EDJ is that as a tooth forms, the growing enamel takes on the distinctive shape which was first shaped by the forming dentin. The EDJ is commonly studied in paleoprimatology, paleoanthropology, and even developmental biology and dental medicine.

Here’s a round up of articles that used EDJ to study human and primate evolution:

To study the EDJ of these nine teeth, three dimensional micro-computed tomography (micro-CT) was used for visualization. Micro-CT is usually used in medicine as a minimally invasive system. Micro-CT uses X-rays to create high resolution images. In the following excerpt you will read what was found from the mirco-CT analysis. I’ve attached figure 2 because if you are like me, you wanna see what what 3-D micro-CT scans look like, since it is such a high tech fancy pants methodology.

Suwa et al., 2006 - Figure 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). “

So only two ratios were used to measure cusp dimensions beneath the enamel cap. I don’t know why only two ratios were compared, maybe it is because the teeth were so deteriorated that only two measurements could be extracted. That’s okay though, the authors were more thorough in their cross species comparisons. In the supplemental materials, the comparison of internal cusp dimensions of Chororapithecus and gorillas was extended to chimpanzees and other apes… even Orrorin and Sahelanthropus, all with pretty large sample sizes.

Suwa, et al. 2007 - StratigraphyWith all that said, I am somewhat convinced these teeth represent Chororapithecus as an ancestor to the gorilla lineage. Why I am uncertain, is as I said earlier… the big unresolved issue… These ape-like set of teeth come from 10 million years ago.

This shakes up our understanding of primate evolution.

Check out the illustration of the stratigraphy where the fossils came out from… it is to your right. I won’t doubt where in the ground these teeth came from.

I was never a subscriber to the hypothesis that apes arose in Eurasia and migrated to Africa, and this 10 million year old African ape is a fitting blow to that hypothesis. I was taught to keep it simple. Old World monkeys gave rise to hominoids, during the Eocene, about 60 million to 34 million years ago and dryopithecines are orangutan ancestors.  The orangutan lineage was in Africa prior to the first migration of Miocene, around 23-25 million years ago of apes from Africa to Eurasia. With the exception of a 9.5 million year old maxilla, no other ape fossils have been found in Africa between 12 million and 7 million years ago. This finding is important in that it fills in that gap of the fossil record.

I was subscribed to the 4 million year old human-chimp speciation time and the orangutan-human divergence of 18 million years that was found earlier this year through sequence comparisons of genomes. In the paper, Suwa et al. challenge this, they say,

“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… We consider that the early divergence hypothesis is congruent with both fossil and molecular data…”

As you can see, these dates are not congruent. There is a 2 million year gap between what the molecular evidence tells us and the estimation made by Suwa et al for Pongo and a even larger spread for Pan. If I am to believe the genetic evidence, a 10 million year old ancestor to gorillas would not have existed. But, the teeth are convincing and look like a gorilla. Can the genetic studies have calculated the times of divergence incorrectly? Yes, it is possible if the molecular clocks they used weren’t properly calculated.

What is one to do?

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