New (old) Australopithecus anamensis cranium

The Fall semester here at Vassar kicks off next week, and so of course a new fossil discovery is published this week that threatens to upend my course plans and throw my syllabi into disarray. Haile-Selassie and colleagues report a very well-preserved hominin cranium, from the Woranso-Mille region of Ethiopia and dating to 3.8 million years ago. The new cranium shares features with Australopithecus anamensis, a species previously mainly known through jaws and teeth. The fossil is therefore really important since it puts a face to the species’ name, and it is the oldest relatively complete Australopithecus cranium known. When I showed a picture of the fossil to my wife, who is not a paleoanthropologist, all she said was that it looked like the face of a dog who got stung by a bee.

anamensis bee sting

The new A. anamensis fossil MRD-VP-1 (left), and a dog that lost a fight with a bee. Fossil photo from the Smithsonian‘s coverage.

The big buzz in many news stories about the fossil (for example, Nature, ScienceNews, etc.) is that it rewrites an evolutionary relationship early in human history, with Australopithecus anamensis no longer the ancestor of A. afarensis, but rather the two being contemporaries. That idea is based on a 3.9 million year old frontal bone attributed to A. afarensis from a site called Belohdelie, also in Ethiopia (Asfaw, 1987): basically, the new A. anamensis cranium reveals a hominin with a narrow frontal region of the brain, which lived 100,000 later than A. afarensis with a relatively expanded frontal region:

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Top views of the reconstructed A. anamensis cranium (left), and the Belohdelie frontal (center), and my crappy photoshopped overlay of Belohdelie on A. anamensis (right). Images not to scale.

The lede, “human evolutionary tree messier than thought,” is not terribly interesting or compelling since it seems to characterize most fossil discoveries over the past several years. And in this case I don’t know how well supported the argument is, since the trait in question (narrow frontal region of the braincase or “post-orbital constriction”) can vary dramatically within a single species. The image below is from the paper itself—compare the difference in “postorbital constriction index” (left graph) between the new A. anamensis cranium (MRD) and A. afarensis (in blue). Both sets of fossils fall within the range of chimpanzees (P. troglodytes), and note the great range of variation within gorillas (G. gorilla).

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Part of Figure 3 from the paper by Haile-Selassie and colleagues. On the top is a view from above of fossil humans: Sahelanthropus tchadensis, Ardipithecus ramidus, the new A. anamensis, A. afarensis, and A. africanus. Below the graphs show how species differ in narrowing of the frontal (left) and length of the skull (right).

What I find most interesting about the new find is the great front-to-back length of the cranium—check out how long and narrow the brain-case is of the fossil compared with the later hominins to the right. This is an interesting similarity with the much earlier (6 million years ago) Sahelanthropus tchadensis, which is the left-most fossil in the figure. It makes me really curious to see the brain endocast of A. anamensis and the Sahelanthropus cranium—what was brain shape like for these ancient animals, and what does that mean for the earliest stages of human brain evolution? The Sahelanthropus endocast was presented at a conference six years ago but remains unpublished. Haile-Selassie and colleagues report that they made a virtual reconstruction of the A. anamensis endocast, so hopefully we’ll get to pick its brain soon.

 

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Quick thought on the Australopithecus deyiremeda maxilla

It will be lots of work to prep my Human Evolution course for the Fall. This past year has seen many major fossil discoveries, and adding to the list is the newly described species Australopithecus deyiremeda (Haile-Selassie et al., 2015). The fossils come from newly announced sites in Ethiopia (here it is on a map!), dating to around 3.4 million years ago. These new fossils are contemporaneous with Australopithecus afarensis, fossils attributed to Kenyanthropus platyops, and whatever the hell the Burtele foot belongs to.

The main specimens are a fairly complete half of a maxilla (upper jaw) and two decent mandibles (lower jaw bones). These fossils do not belong to the same individual (despite all the media pictures of the upper and lower jaws together). One of the most distinctive features of these fossils is how thick, both in absolute and relative terms, the mandibles are, especially given how short they are. What sticks out to me though, is that the upper jaw looks like it might have still had some growing to do. Why on earth would I think so? (The following is based off pictures from the publications, so I could be wrong!)

Extended Figure 1a from the paper. The type specimnen BRT-VP-3/1 maxilla. Front is to the left.

Extended Figure 1a from the paper, the type specimnen BRT-VP-3/1 maxilla viewed from the left side. I’ve added the M2 label for your reading pleasure.

The holotype maxilla (BRT-VP-3/1) is described as coming from a “young adult” in the Supplementary Information. However, it looks like the second molar tooth (M2) is not quite fully erupted and in occlusion, although this could be due to the natural arc of the tooth row. There is no visible wear on the tooth in the pictures, and indeed the Supplementary Information says the tooth is unworn. This means that the tooth is only recently emerged, and may not have passed the gum line, and therefore hasn’t seen much/any use yet. Authors note in the Supplementary Information that there is no M3 (a.k.a. “wisdom tooth”) wear facet on the back of M2 , meaning the last tooth hadn’t yet emerged yet either. So, this all points to a non-adult age by tooth eruption standards.

Extended Figure 1d from the paper. Same fossil, but from the bottom, like a dentist peering into its mouth. Back is to the bottom.

Extended Figure 1d from the paper. Same fossil, but from the bottom; pretend you’re a dentist peering into its mouth. Back is to the bottom.

In addition, the M2 roots don’t look fully formed. This is especially apparent in Extended Figure 1h, a CT section through the teeth:

Extended Figure 1h from the paper, with a Demirjian developmental stages, modifed from Table 2 from Kuykendall et al., 1996. Compare the M2 roots with  completed roots of the M1 (to the left).

Left side: Extended Figure 1h from the paper. From left to right, the teeth are P3, P4, M1, and M2. For comparison, to the right are Demirjian tooth development stages, modified from Table 2 of Kuykendall, 1996. Also compare the M2 roots with completed roots of the M1.

 

In many human populations, this stage of M2 development is reached (on average) between 11-13 years (Liversidge et al., 2006). In the wild Taï Forest chimpanzee sample, two individuals with M2 root completely formed (Stage H) are 10 and 11 years old (Smith et al., 2010). These apes would not be fully mature and their facial dimensions would likely have increased had they reached adulthood (Zihlman et al., 2007).

So what this suggests to me is that this maxilla may not accurately represent adult anatomy in this newly described species. In humans, the face continues to grow downwards from adolescence into adulthood, and in apes the face continues growing both forward and downward. In the differential diagnosis of A. deyiremeda, Haile-Selassie and team state, in layman’s terms, that the cheeks are positioned more toward the front than in A. afarensis, and that the front of the face doesn’t stick out as much as in A. garhi. If this specimen was not fully grown, it is likely that the true adult anatomy would have had a face that sticks out more and has less forward-positioned cheeks than in this specimen.

But, this is all speculative, and I’d like to reiterate that these observations of dental development are based only on the published pictures. Just a thought!

Busidima female pelvis

Yay! A female, mostly complete, Homo erectus pelvis has been found (Simpson et al 2008)! I say “yay” because female pelves are the best way to learn about the effects of birthing on pelvic morphology, despite the numerous studies that claim to glean this knowledge from male pelves (I’m basing this mainly on Neandertal examples as I am admittedly not as well versed with H erectus papers).

Quick summary of the facts: Found in Gona, Ethiopia, BSN49/P27a-d (a.k.a. the Busidima pelvis) consists of both os coxae (hipbones) and the upper part of the sacrum of what has been identified as an adult female Homo erectus dated to 0.9 to 1.4 Ma. The hipbones are mostly complete, except for portions of the ilium missing on both sides and portions of the ischial tuberosities. Overall, the pelvis has been reconstructed so that measurements can be taken (see image below, taken from the Simpson et al 2008 article).


Why I’m excited: Since this fossil is female and has the first complete early Pleistocene pubis, and thus the first complete pelvic inlet, it means legitimate inferences about birthing can be made. The initial paper addresses this by exploring neonate (i.e. infant at birth) head sizes compared to inlet dimensions. The authors found that Busidima would have been capable of birthing infants 30% larger than predicted based on the Homo erectus pelvis KNM-WT 15000 (male subadult). Already this find is showing why making assumptions about birth based on male specimens is flawed! Yay!!

Why I’m concerned: The authors draw conclusions about the width of the trunk based on the bi-iliac breadth. As the picture shows, the ilia are largely reconstructed, making any measurement between the widest points on the iliac crest dubious. I’m not saying it’s a bad reconstruction; in fact it looks reasonable to me. I just believe caution must be used when drawing conclusions based on reconstructed measurements. Especially since I cannot find details on how the ilia were reconstructed (if I’ve missed something in my reading of the online supporting material, please, someone correct me). Given that this is the case, it seems extreme to use incomplete ilia as evidence against an endurance running hypothesis and as evidence for what type of environment this specimen lived in.

What others are saying: Hawks goes into more anatomical detail in his blog post about the article. He discusses questions raised by the study and points out that you really have to read the supporting stuff to get the full picture as the Science article is “superficial”. Beast Ape’s blog post discusses the bi-iliac interpretation and what it means. For a multilingual look, Mundo Neandertal also discusses the article, and based on my imperfect Spanish capabilities, I think they discuss the infant head size findings. Finally, New Scientist describes the findings in a newsy way, detailing the bigger birth canal (compared to WT 15000) and the squat proportions of Busidima. Enjoy!

References:
Simpson et al. 2008 A Female Homo erectus Pelvis from Gona, Ethiopia. Science 322: 1089-1092.