Gona … Gona … not Gona work here anymore more

The Gona pelvic remains (A-D), and the reconstructed complete pelvis (E-J), Fig. 2 in Simpson et al., 2008.

A few years ago, Scott Simpson and colleagues published some of the most complete fossil human hips (right). The fossils are from the Busidima geological formation in the Gona region of Ethiopia, dated to between 0.9-1.4 million years ago. (Back when I wasn’t the only author of this blog, my friend and colleague Caroline VanSickle wrote about it here)

Researchers attributed the pelvis to Homo erectus on the basis of its late geological age and a number of derived (Homo-like) features. In addition, the pelvis’s very small size indicated it probably belonged to a female. One implication of this fossil was that male and female H. erectus differed drastically in body size.

Christopher Ruff (2010) took issue with how small this specimen was, noting that its overall size is more similar to the small-bodied Australopithecus species. Using the size of the hip joint as a proxy for body mass, Ruff argued Gona’s small size would imply a profound amount of sexual dimorphism in H. erectus: much higher than if Gona is excluded from this species, and higher than in modern humans or other fossil humans. Ruff thus proposed an alternative hypothesis to marked sexual dimorphism, that the Gona pelvis may have belonged to an australopithecine.

Fig. 3 From Ruff's (2010) reply. Australopiths (and Orrorin) are squares and Homo are circles. Busidima's estimated femur head diameter is represented by the star and bar.

Fig. 3 From Ruff’s (2010) reply. Australopiths (and Orrorin) are squares and Homo are circles. Gona’s estimated femur head diameter is represented by the star and bar.

Now, Simpson & team replied to Ruff’s comments, providing a laundry list of reasons why this pelvis is H. erectus and not Australopithecus. They cite many anatomical features of the pelvis shared with Gona and Homo fossils, but not australopithecines. They also note that there are many other bones reflective of body size, that seem to suggest a substantial amount of size variation in Homo fossils, even those from a single site such as Dmanisi (Lordkipanadze et al., 2007).

Interestingly, neither of these parties compared the implied size variation with that of living apes. So I’ll do it! Now, I do not have any acetabulum data, but a friend lent me some femur head measurements for living great apes a few years ago. Gona is a pelvis and not a femur, but there are more fossil femora than hips. Because there’s a very high correlation between femur head and acetabulum size, Ruff estimated Gona’s femur head diameter to be 32.6 mm (95% confidence interval: 30.1-35.2; Simpson et al. initially estimated 35.1 mm based on a different dataset and method). To quantify size variation, we can compare ratios of larger femur heads divided by smaller ones. Now, this ratio quantifies inter-individual variation, but it will underestimate sexual dimorphism since I’m likely sampling some same-sex pairs that aren’t so different in size. But this is just a quick and dirty look. So, here’s a box plot of these ratios for Homo fossils, larger specimens divided by Gona’s estimated femur head size in different time periods:

Ratio of a fossil Homo femur head diameter (HD) divided by Busidima's HD. E Homo = early Pleistocene, Contemporaneous = WT 15000 and OH 28, MP = Middle Pleistocene Homo. White boxes are based on Ruff's Busidima HD estimate, green boxes are based on Simpson et al.'s estimate.

Ratios of fossil Homo femur head diameter (HD) divided by Busidima’s (Gona’s) HD. E Homo = early Pleistocene, Contemporaneous = WT 15000 and OH 28, MP = Middle Pleistocene Homo. White boxes are based on Ruff’s Gona HD estimate, green boxes are based on Simpson et al.’s larger estimate. Boxes include 50% quartiles and the thick lines within are sample medians.

Clearly, Gona is much smaller than most other fossil Homo hips, since ratios are never smaller than 1.14. Average body size increases over time in the Homo lineage, reflected in increasing ratios from left to right on the plot. Early Pleistocene Homo fossils are fairly small, including Dmanisi, hence the lower ratios than later time periods. Middle Pleistocene Homo (MP), represented by the most fossils, shows a large range of variation, but even the smallest is still 1.17 times larger than the largest estimate of Gona’s femur head size. To put this into context, here are those green ratios (assuming a larger size for Gona) compared with large/small ratios from resampled pairs of living apes and humans:

*

The fossil ratios of larger/smaller HD from above, compared with resampled ratios from unsexed living apes and humans. Boxes include the 50% quartiles, and the thick lines within are sample medians. **(05/03/14: This plot has been modified from the original version post, which only included the fossil ratios based on the smaller Gona estimate)

What we see for the extant apes and humans makes sense: humans and chimpanzees show smaller differences on average, whereas average differences between gorillas and orangutans are larger. This accords with patterns of sexual dimorphism in these species. **What this larger box plot shows is that if we accept Ruff’s smaller average estimate of Gona’s femur head size (white boxes), it is relatively rare to sample two living specimens so different in size as seen between Gona and other fossils. If we use Simpson et al.’s larger Gona size estimate, variation is still elevated above most living ape ratios. Only when Gona is compared with the generally-smaller, earlier Pleistocene fossils, does the estimated range of variation show decent overlap with living species. Even then, the overlap is still above the median values.

These results based on living species agree with Ruff’s concern, that including Gona in Homo erectus results in an unusually large range of variation in this species. Such a large size range isn’t necessarily impossible, but it would be surprising to see more variation than is common in gorillas and orangutans, where sexual size dimorphism is tremendous. Ruff suggested that the australopith-sized Gona pelvis may in fact be an australopith. This was initially deemed unlikely, in part because the fossil is well-dated to relatively late, 0.9-1.4 million years ago. However, Dominguez-Rodgrigo and colleauges (2013) recently reported a 1.34 mya Australopithecus boisei skeleton from Olduvai Gorge, so it is possible that australopiths persisted longer than we’ve got fossil evidence for, and Gona is one of the latest holdouts.

So many possible explanations. More clarity may come with further study of the fossils at hand, but chances are we won’t be able to eliminate any of these possibilities until we get more fossils. (also, the post title wasn’t a jab at the fossils or researchers, but rather a reference to the movie Office Space)

References

Dominguez-Rodrigo et al. 2013. First partial skeleton of a 1.33-million-year-old Paranthropus boisei from Bed II, Olduvai Gorge, Tanzania. PLoS One 8: e80347.

Ruff C. 2010. Body size and body shape in early hominins – implications of the Gona pelvis. Journal of Human Evolution 58: 166-178.

Simpson S et al. 2008. A female Homo erectus pelvis from Gona, Ethiopia. Science 322: 1089-1092.

Simpson S et al. In press. The female Homo pelvis from Gona: Response to Ruff (2010). Journal of Human Evolution. http://dx.doi.org/10.1016/j.jhevol.2013.12.004

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The small, big new Dmanisi skull

The 5th skull early Homo skull from the site of Dmanisi was announced last week. The skull was discovered nearly 10 years ago, but is finally (and very comprehensively) published in Science (Lordkipanidze et al. 2013). The ‘new’ D4500 cranium goes with the massive D2600 mandible, making this the earliest and most complete skull of Homo that I know of. It’s really a remarkable specimen, for a number of reasons beyond its age and completeness. I’ve been busy traveling, teaching and writing lately, so I haven’t yet gotten to pore over the details as much as I’d like. So I hope to sporadically post thoughts on this badass new skull as they come to me. In the mean time, several of what I’d consider the top biology/anthropology blogs*** have discussed the skull, so do check those out if you haven’t already.

The first thing I noted about D4500 is its small brain size, estimated at a mere 546 cubic centimeters. For perspective, D4500 is the green point in the following plot showing brain size in early human evolution:

Endocranial volume for various fossil hominin fossils. 1: Australopithecus afarensis, africanus & boisei; 2: Dmanisi specimens; 3: "habilines" 4: early African Homo erectus; 5: Indonesian and Chinese Homo erectus

Endocranial volume for various fossil hominin fossils. 1: Australopithecus afarensis, africanus & boisei; 2: Dmanisi specimens; 3: “habilines” 4: early African Homo erectus; 5: Indonesian and Chinese Homo erectus. D4500 is green with envy.

I got to see (but not study) the cranium a few years ago when I was helping with the Dmanisi Paleoanthropology field school, and I remember noting just how “robust” the specimen was – big mastoid processes, prominent and thick brow ridge, huge attachments for the neck muscles. In humans, and presumably our fossil forebears, these features are more developed in males than females, and so presumably D4500 was a male (consistent with the huge, associated D2600 mandible). In many primates, and 4 to ~1 mya hominins so far as we can tell, males are larger than females. So it is surprising that a robust probable male cranium is in fact not only the smallest in the Dmanisi sample, but also at the low end of early African Homo (i.e. habilis or rudolfensis), comparable to the largest australopiths. Of course, the only other faces known from Dmanisi are either not fully grown (D2700 and D2282) or old and decrepit (D3444), so perhaps the larger-brained specimens would have been at least as robust as D4500. An untestable hypothesis!

The new skull really highlights the overlap, or continuous variation between later australopiths and early Homo known also from eastern Africa. In association with the postcranial remains known from Dmanisi, the authors the paper posit that early Homo may have been distinguished from Australopithecus not so much in brain size as in body size. We could probably add body shape (limb proportions) and tool use to that list of distinguishing features, and to be sure there are Oldowan tools and small but human-like body size and shape indicated by postcrania at Dmanisi. But then, evidence for body proportions and for/against tool use in Australopithecus, especially later in the record, is somewhat equivocal…

More thoughts to follow.

*** https://blogs.wellesley.edu/vanarsdale/2013/10/17/uncategorized/the-new-wonderful-dmanisi-skull/; http://johnhawks.net/weblog/fossils/lower/dmanisi/d4500-lordkipanidze-2013.html; http://ecodevoevo.blogspot.com/2013/10/how-many-human-species-are-there-is-it.html

Reference: David Lordkipanidze, Marcia S. Ponce de León, Ann Margvelashvili, Yoel Rak, G. Philip Rightmire, Abesalom Vekua, and Christoph P. E. Zollikofer. 2013. A Complete Skull from Dmanisi, Georgia, and the Evolutionary Biology of Early Homo. Science: 342 (6156), 326-331.

More FREE badass bioanthro science resources!

Hark! There’s been quite a long silence here, as I’ve been busy preparing manuscripts related to this post and this post. Also teaching; my new Intro to Biological Anthropology students are writing posts over at nazarbioanthro.blogspot.com – check them out!

Anyway, some more FREE DATA have come to my attention that I figured people may find useful (I’ve posted links to other great resources here and here).

First, my buddy and advisor Milford Wolpoff has helped compile an open online dental dataset. This consists of length and breadth measurements for teeth from humans, fossil humans and non-human apes. And promises of more to come! You can read about the data, and online data-sharing more generally, in this paper at the Paleoanthropology Society website.

Secondably, Herman Pontzer has put together a website, Australopithecus, with lots of great information about human evolution for teachers and students, as well as a datamine of links and metrics and pictures of fossil hominins and apes. Pretty boss.

Third, announced in the American Journal of Physical Anthropology just yesterday is a database of cranial non-metric data, pioneered by Nancy Ossenberg. This is a very comprehensive dataset, with info about up to 84 non-metric traits on over 8,000 individual crania from all over the world. Ossenberg also links to the WW Howells craniometric dataset (thousands of cranial measurements of individuals all over dodge); I’m not sure if/how much Ossenberg’s and Howells’ datsets overlap, but the covariance of size, shape and non-metric traits could be a very interesting investigation (if it hasn’t been done already; sorry for my ignorance!).

Finally, if you’re looking to analyze these or any other tantalizing data, you’ll want to download and learn to use R. This free statistical computing program will let you analyze pretty much anything with either traditional statistics, or you can be a badass and make up your own custom tests. I’ve been blabbing incessantly about how awesome this program is since at least 2009, but here’s the link just in case. takes some time to figure out how to use, but its help files are all online, and you can probably find out how to do anything else your dreams can concoct on the Internets.

Now you are ready to take on the world. Go forth!

Evo-devo of the human shoulder?

It’s a new year, and while my mind should be marred by a hangover, instead all I can think about are fossils and scapulas.


A pretty cool study was published online in the Journal of Human Evolution last week, and I’ve finally gotten to peruse it. Fabio Di Vincenzo and colleagues analyzed the shape of the outline of the glenoid fossa on the scapula (not to be confused with the glenoid on your skull), from Australopithecus africanus to present day humans. The glenoid fossa is essentially the socket in the ball-and-socket joint of your shoulder. The authors found that there is pretty much a single trend of glenoid shape change from Australopithecus through the evolution of the genus Homo: from the fairly narrow joint in Australopithecus africanus and A. sediba, to the relatively wide joint in recent humans. The overall size and shape of the joint influences/reflects shoulder mobility, so presumably this shape change hints that more front-to-back arm motions became more important through the course of human evolution (authors suggest throwing in humans from the Late Pleistocene onward).



The finding of a single predominant trend in glenoid shape evolution is pretty interesting. On top of that, the authors add an ‘evo-devo’ twist by comparing species’ average “shapes” (first principle component scores, on the y-axis in the figure at right) with their estimated ages at skeletal maturity (which appears scaled to the modern human value, on the x-axis). Though it’s not an ideal dataset for running a linear regression, the figure at right shows that there appears to be a fairly linear relationship across human evolution, such that groups with an older age at skeletal maturity tend to have a more rounded (modern human-like) glenoid fossa (note that the individuals in the analysis were all adults). Overall size does not contribute to shape variation among these glenoids.


This work raises two issues, and ultimately leads to a testable evo-devo hypothesis. The first issue is to what extent we can trust their estimates of age at skeletal maturity. These estimates were allegedly taken from a chapter by Helmut Hemmer (2007) in the prohibitively expensive Handbook of Paleoanthropology. Cursorily glancing at this chapter, I can’t find age at skeletal maturation estimated for any hominids. It is possible that in my skimming I missed the estimates, or alternatively that Di Vincenzo and colleagues misinterpreted another variable as skeletal development. Either way, these estimates would still need to be taken with a grain of salt, given that it is almost impossible to know the true age at death of a fossil (but see Antoine et al. 2008), especially if there are no associated cranio-dental elements.


That said, it is perfectly reasonable to suppose that the age at skeletal maturation has increased over the course of human evolution; life-span increased through human evolution, and so all else being equal (which it almost certainly isn’t) we could expect that maturation would occur later over time, too. So this leads to a second issue: given the “evo-devo change” the authors hypothesize, what is the evo-devo mechanism? That is, how was development modified to effect the evolutionary changes we see in the hominid scapula? Because they found adult glenoid shape correlates with estimated age at skeletal maturity, this leads to the hypothesis that postnatal skeletal growth accounts for the shape difference. Indeed, they state:

“If functional and static allometric influences are unlikely, we…interpret the trend…as reflecting growth and developmental factors. A major, albeit gradual, trend of ontogenetic heterochrony occurred in the evolution of the genus Homo… and thus differences within and between taxa in overall growth rates may have produced the pattern of variation between samples, as well as the overall temporal trend observed. The regression of life history variables [they only looked at 1]… with PCA [principle components analysis] scores supports this ‘ontogenetic’ hypothesis.”

The authors suggest that humans’ slower growth rates but longer growth period “led to longer periods of bone deposition along the inferior-lateral edge of the [glenoid fossa]”  The heterochronic process they suggest is “peramorphosis” – the descendant reaches a shape that is ‘beyond’ that of the ancestor.

The figure above is from a seminal “heterochrony” paper by Pere Alberch and colleagues (1979), portraying how peramorphosis can occur. In each, the y-axis represents shape and the x-axis is age. A the descendant’s peramorphic shape (“Ya”) could result from accelerated growth (left graph) or from an extension of growth to later ages than in the ancestor (right graph).


And so this leads to a testable hypothesis. Di Vincenzo and colleagues cite (dental) evidence that humans’ overall body growth rates are slower than earlier hominids’, undermining the hypothesis that acceleration is responsible for humans’ glenoid peramorphosis. Rather, they hypothesize that humans’ slower growth rates coupled with a longer period of skeletal development, to result in a relatively wider glenoid, due to increased development of the secondary growth centers (e.g. the graph at right, above). This developmental scenario predicts that subadult human glenoids should resemble earlier hominid adults’, that “ontogeny recapitulates phylogeny” as far as glenoid shape is concerned. Analyzing glenoid growth can even be extended to include fossils – the >3 million year old human ancestor Australopithecus afarensis has glenoids preserved for an infant (DIK-VP-1; Alemseged et al. 2006) and 2 adults (AL 288 “Lucy” and KSD-VP-1; Johanson et al. 1982, Haile-Selassie et al. 2010). An alternate hypothesis is that species’ distinct glenoid shapes are established early during life (i.e. in utero), and/or that no simple heterochronic process is involved.


ResearchBlogging.orgDi Vincenzo’s and colleagues’ study points to the importance of development in understanding human evolution, and their hypothesized “evo-devo change” in glenoid shape is ripe for testing.


References
Pere Alberch, Stephen Jay Gould, George F. Oster, & David B. Wake (1979). Size and shape in ontogeny and phylogeny Paleobiology, 5 (3), 296-317


Alemseged, Z., Spoor, F., Kimbel, W., Bobe, R., Geraads, D., Reed, D., & Wynn, J. (2006). A juvenile early hominin skeleton from Dikika, Ethiopia Nature, 443 (7109), 296-301 DOI: 10.1038/nature05047


Antoine, D., Hillson, S., & Dean, M. (2009). The developmental clock of dental enamel: a test for the periodicity of prism cross-striations in modern humans and an evaluation of the most likely sources of error in histological studies of this kind Journal of Anatomy, 214 (1), 45-55 DOI: 10.1111/j.1469-7580.2008.01010.x


Di Vincenzo, F., Churchill, S., & Manzi, G. (2011). The Vindija Neanderthal scapular glenoid fossa: Comparative shape analysis suggests evo-devo changes among Neanderthals Journal of Human Evolution DOI: 10.1016/j.jhevol.2011.11.010


Haile-Selassie, Y., Latimer, B., Alene, M., Deino, A., Gibert, L., Melillo, S., Saylor, B., Scott, G., & Lovejoy, C. (2010). An early Australopithecus afarensis postcranium from Woranso-Mille, Ethiopia Proceedings of the National Academy of Sciences, 107 (27), 12121-12126 DOI: 10.1073/pnas.1004527107


Hemmer, Helmut (2007). Estimation of Basic Life History Data of Fossil Hominoids Handbook of Paleoanthropology, 587-619 DOI: 10.1007/978-3-540-33761-4_19


Johanson, D., Lovejoy, C., Kimbel, W., White, T., Ward, S., Bush, M., Latimer, B., & Coppens, Y. (1982). Morphology of the Pliocene partial hominid skeleton (A.L. 288-1) from the Hadar formation, Ethiopia American Journal of Physical Anthropology, 57, 403-451 DOI: 10.1002/ajpa.1330570403

Look inside bones for free on the interwebs

I forget how I stumbled upon this badass resource, but Kyoto University’s Primate Research Institute made a “Digital Morphology Museum: an awesome online database of CT scans of sundry primate skeletal parts. Ever wonder what an articulated siamang skeleton looks like? Or whether the flaring bony snout of a mandrill is hollow or filled with bone (below)? If you’re a normal person, probably not. But either way, this website provides easy access to the internal views of all sorts of body parts.

Coronal slice through a male mandrill face.  You can see a bone-filled lower jaw,  internal views of some teeth, the nasal cavity. The pics above and on the right give an idea of where in the skull we are. Note the fat flanks above the nasal cavity are filled with bone (they hollow out as you move further into the face).

What’s cool is you can view and manipulate 3D views of these things on the website, or you can register with KUPRI to download the raw CT data. Really a great resource.

A few weeks ago, a paper came out wherein researchers used CT scans to compare the the sides of the nasal opening in skulls of Australopithecus species (Villmoare and Kimbel 2011). They found that although the external nose of the South African Australopithecus africanus and A. robustus appear similar in looking like rounded “pillars,” on the inside these pillars differed between the two species. A. africanus‘s (and the earlier, east African A. afarensis‘s) nasal pillar was hollow, while A. robustus‘s was filled with “spongy” bone, like the contemporaneous A. boisei in East Africa. So the early (and “gracile”) australopiths had hollow pillars while the later (and “robust”) ones had a bony pillar, hmm… It’d be neat to try to see how such bone-filled or hollow pillars develop (i.e. are they hollow in babies but then fill with trabecular bone during growth in the “robust” group? Does this difference arise for functional (e.g. chewing) reasons, or could it be a developmental ‘byproduct’ of the tall robust australopithecine face [cf. McCollum 1999]).

It’s a neat study, and they include lots of great CT images of the hominid sample. But another question arises – what is the inside of the bony nose like in modern primates, and how much variation is there within a species? (NB Villmoare and Kimbel found pretty much no variation within each fossil species, save for two curious examples, but which were based on casts). If I had the time (i.e. weren’t dissertation-ating) I’d love to peruse the KUPRI files to see what “pillar” variation is like in, say, chimps (paleoanthropologists’ go-to referent species). Cursorily looking at just one (female chimpanzee, left), it looks like the sides of the nose are empty higher up, but then fill with bone to form the tooth socket surrounding the canine root. I’ll leave it to someone else to see what the rest look like.

But just lookit what other fun stuff you can see! At the top (anatomically toward the back) are the bone-filled mandibular condyles, beneath (anatomically a bit more toward the front) and between them are the pterygoid plates, and beneath them is a big gross maxillary sinus. Man, if only I had the time, I’d make an anatomy scavenger hunt on this site, and it’d be pretty epic.
ResearchBlogging.org


Those papers I mentioned
McCollum, M. (1999). The Robust Australopithecine Face: A Morphogenetic Perspective Science, 284 (5412), 301-305 DOI: 10.1126/science.284.5412.301

Villmoare, B., & Kimbel, W. (2011). CT-based study of internal structure of the anterior pillar in extinct hominins and its implications for the phylogeny of robust Australopithecus Proceedings of the National Academy of Sciences, 108 (39), 16200-16205 DOI: 10.1073/pnas.1105844108