Primatology.net

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:

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.

Aside 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.

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:

• Enamel thickness and the topography of the enamel-dentine junction in South African Plio-Pleistocene hominids with special reference to the Carabelli trait.
• Variation in hominoid molar enamel thickness.
• Modern human molar enamel thickness and enamel-dentine junction shape.
• The enamel-dentine junction of human and Macaca irus teeth: a light and electron microscopic study.

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.

“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.

With 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?

A new Nature paper announces a new species of Miocene great ape, Chororapithecus abyssinicus, and both the press and the blogosphere are having a field day with this publication. From our neck of the woods, Afarensis, P.Z. Myers and John Hawks have commented on the paper and from the press Jay Kelly, Peter Andrews, and Richard Potts. No one is fully convinced.

Before, I get into the thick of it, here is a photo of the nine teeth that Suwa et al. say belong to the new Miocene great ape.

These teeth are 10 million year old. And Suwa et al. say the teeth are gorilla-like. Specifically the tooth morphology at the enamel-dentine junction is like that of a gorilla, but christened a new species name to these teeth. Hawks criticizes the findings,

“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.”

In their defense, Suwa and crew are saying the teeth belong to a member of the gorilla family stem from similarities with teeth of modern gorillas.

Others are are criticizing this conclusion because it completely goes against what the genetic evidence has been telling us. Early this year two papers reassessed the time at which hominids diverged from the other great apes, and that was about 7 million years ago. If teeth are the safe-houses of genotype to phenotye, a common understanding in evolutionary studies, the two lines must agree. With a 10 million year old gorilla like ape and a suggestion that the split happened earlier than 10 million years ago, this just screws with the genetic findings, and further complicates the relationship between paleoanthropology and molecular evolution.

Until I read the paper and I’ll leave you with what Afarensis said,

“You would have thought paleoanthropology would have learned something from Ramapithecus. Dental gorillas don’t mean actual gorillas. Just like being a dental hominid didn’t make Ramapithecus a real hominid.”

There’s a new and unique behavior of chimpanzees being documented in the journal Biology Letters that I think the primatologist in you will appreciate. 

Chimps that have a hard time acquiring lots of food purposely busy themselves in order to avoid the temptation of gorging themselves straight away. The study shows that, these chimps welcome a distraction that takes the mind off the impulsive urge to splash out.

Nature has a news article summarizing this behavior,

“Researchers at Georgia State University in Atlanta presented four chimps with a plastic container attached to a tube that gradually filled the container with candy. Opening it, however, would cut off the flow of food. Chimps were kept away from the candy machine but were allowed to observe it, so learning that the longer they waited, the bigger the treat they would get.

But as many of us know, self-control doesn’t come easily. Studies of human children have shown that the average five-year-old is rarely able to resist eating sweets, even if promised that abstinence will be rewarded with even more sweets later on.

The Georgia researchers, Theodore Evans and Michael Beran, guessed that chimps would have a good chance at resisting the candies if given a range of toys and other distractions to play with. “We chose a set of items they are known to have an interest in,” explains Evans. “They enjoy brushing their teeth, for example; we gave them magazines so they could look at the pictures; and they enjoy different types of fasteners, zips and clips that they can take apart.”

The chimps resisted going for the accumulating candies for longer when given access to the toys, showing that play did indeed take their minds off food.

And they were more likely to play with the toys when the candies were accessible than when they were visible but behind a barrier. This suggests that they actively chose to use the toys as a distraction, rather than simply playing for the fun of it.”

The implications of this behavior is summarized by one of the authors, Theodore Evans,

“Controlling their impulses could benefit chimps in the wild when deciding where and when to look for the best food… “Self-control may relate back to their feeding ecology — should they eat this food here now, or should they travel down the road and potentially find some nice fruit?”

Chimps low down in the troop social hierarchy often have to wait their turn for the best fruit… Perhaps self-distraction helps them pass the time…”

The DOI link to the article is currently not active, but that’s because the journal probably hasn’t gotten around to publish it. Check the link in a couple days time, and I bet it’ll work.

John Hawks introduces a new paper in Current Biology with a not too surprising conclusion on male chimpanzee behavior and coalition forming. The title, “Male chimpanzees exchange political support for mating opportunities,” is pretty descriptive. The abstract gives us a bit more,

Male chimpanzees, Pan troglodytes, differ from males in most other mammalian taxa because they remain in their natal communities throughout their lives, form close bonds with one another, and cooperate in a range of activities. However, males also compete fiercely for status within their groups and, and high rank enhances male reproductive success. Males rely partly on coalitions to achieve and maintain status, and shifts in male alliances can have dramatic political effects. It is not known what benefits are obtained by low-ranking coalition partners. Here we report that the highest-ranking (alpha) male in one well-studied community of chimpanzees rewarded his allies by allowing them preferential access to mates.

The chimps that were studied in the 22 month period of observation, are the Kanyawara community of chimpanzees. Observing ten males,

“the alpha male in the Kanyawara community selectively tolerated mating by his allies and exchanged mating tolerance for support in conflicts. The alpha male in this community was a singularly important trading partner because of the disproportionately high value of his mating tolerance.”

So there you have it, male chimpanzees lower in rank to the alpha form coalitions with the alpha to gain access to mating. For the alpha male, having some wingmen, ensures his status in the pecking order as well as boosts his access to mate. Everyone wins.