Last week in my Human Evolution class we looked at whether we could estimate hominin brain sizes, or endocranial volumes (ECV), based on just the length and width of the bony brain case. Students took these measurements on 3D surface scans…
Maximum cranial length in Australopithecus boisei specimen KNM-ER 406.
… and then plugged their data into equations relating these measurements to brain size in chimpanzees (Neubauer et al., 2012) and humans (Coqueugniot and Hublin, 2012).
The relationship between cranial length (x axis) and ECV (y axis). Left shows the chimpanzee regression (modified from Fig. 2 in Neubauer et al., 2012), while the right plot is humans (from the Supplementary Materials of Coqueugniot and Hublin, 2012).
So in addition to spending time with fossils, students also learned about osteometric landmarks with fun names like “glabella” and “opisthocranion.” More importantly, students compared their estimates with published endocranial volumes for these specimens, based on endocast measurements:
Human and chimpanzee regression equations don’t do great at predicting hominin brain sizes. Each point is a hominin fossil, the x value depicting its directly-measured endocranial volume and the y value its estimated volume based on different regression equations. Black and red points are estimates based on chimpanzee cranial width and length, respectively, while green and blue points are based on human width and length, respectively. The dashed line shows y=x, or a correct estimate.
This comparison highlights the point that regression equations might not be appropriate outside of the samples on which they are developed. Here, estimates based on the relationship between cranial dimensions and brain size in chimpanzees tend to underestimate fossils’ actual values (black and red in the plot above), while the human regressions tend to overestimate hominins’ brain sizes. Students must think about why these equations perform poorly on fossil hominins.
Most of the fossil scans come from AfricanFossils.org, but a few are from Artec’s sample gallery. One of the cool, fairly recent humans at African Fossils (KNM ER 5306) will give students something else to think about:
“Why doesn’t this look like the rest of the human crania we’ve seen this semester?”
Here are the lab materials so you can use and adapt this for your own class:
Lab 4-Brain size (Instructions & questions)
Lab 4 data table (with equations)
References
Coqueugniot, H., & Hublin, J. (2012). Age-related changes of digital endocranial volume during human ontogeny: Results from an osteological reference collection American Journal of Physical Anthropology, 147 (2), 312-318 DOI: 10.1002/ajpa.21655
Neubauer, S., Gunz, P., Schwarz, U., Hublin, J., & Boesch, C. (2012). Brief communication: Endocranial volumes in an ontogenetic sample of chimpanzees from the taï forest national park, ivory coast American Journal of Physical Anthropology, 147 (2), 319-325 DOI: 10.1002/ajpa.21641
Lawn Chair Anthropology 
Could the poor fit for the human equation have to do with the fact that Coquegniot & Hublin equations were for up to age 7? and why did you not use the height of the skulls?
Hi Alison,
The poor fit is probably due to the fact that fossil hominins and living humans have differently-shaped brains (not to mention the fact, as you mention, that the Coqueugniot sample includes non-adults). Because of the different overall shapes, each metric (width, height, length) will scale with overall size differently between humans and other species.
The two reasons I didn’t include height were first because I hadn’t figured out a simple way to measure basion-bregma in Meshlab, and second because I didn’t want the lab to take too long. But my students got it done pretty quickly, so the next time I do the activity I think I will include cranial height.