Did Homo naledi have big babies?

August 8, 2023December 7, 2024 / zcofran / Leave a comment

I’m working on a project analyzing infant remains of Homo naledi, a species of human that lived in South Africa around 300,000 years ago. In order to paint a full picture of infancy in this species, we need to estimate how big (or small) naledi newborns were. But without fossil neonates that could provide direct evidence of body size at birth, this is a tricky task.

Ideally, we could simply use adult body size estimates for Homo naledi to predict its body size at birth, using the scaling relationship in other primates as a guide. For example, using an average adult body size of 44 kg for Homo naledi (Garvin et al., 2017) yields an estimated newborn size of around 1.5 kg, based on published primate dataset (Barton and Cappellini, 2011). But this approach necessarily overlooks variation within each species, not to mention variation and uncertainty in Homo naledi adult size. In addition, the 95% prediction interval for this estimate ranges from under 1 kg (smaller than an average baboon baby) to almost as large as a human neonate.

Primate body size scaling (Barton & Cappellini, 2011). The black line is the regression for catarrhines (purple squares and blue circles), and the shaded grey area is the 95% prediction interval for newborns at a given adult catarrhine size.

And this gets at the other issue with the regression-based approach to estimating newborn body size in fossil hominins: humans are bad at being primates in some ways, as illustrated here by the fact that we don’t fit the newborn-adult body size relationship that characterizes other catarrhines (apes and monkeys of Africa and Eurasia).

Humans give birth to collosal kids. In contrast, gorillas are the largest living primates as adults, but their newborns are only a little over half the size of human neonates. Why do we have such giant babies? The most proximate reason is that humans are born with adult-ape-sized brains and quite a bit of baby fat as far as mammals go (Kuzawa, 1998). This tells us how babies are big, but it still begs the ultimate question of why—an enduring puzzle that you may have read about in the New York Times last week.

In order to land on a reasonable estimate of newborn body size in extinct humans, we need to figure out when evolution blew up the kid. Unfortunately, the only fossil hominin neonates are two Neandertals from France and Russia dating to under 100,000 years ago—pretty remarkable, but they don’t necessarily tell us about earlier species like Homo naledi.

My colleague Jerry Desilva (2011) worked out a potential solution to this conundrum. He argued that one could work from adult brain size to newborn body size through the following steps. First, in contrast to newborn-adult body size scaling, humans are good catarrhines when it comes to newborn-adult brain size scaling. This means that we can reasonably estimate newborn brain size based on adult brain sizes, which are aplenty in the human fossil record. Second, humans and many other primate newborns have brains roughly 12% of their overall body mass, while the great ape newborns stand out with brains around 10% of their adult size. Putting these two pieces together, one could estimate newborn body size: Adult brain ➡️ newborn brain ➡️ 10–12% newborn body size

DeSilva showed that regardless of whether you use an ape or human model of newborn brain/body size, hominin babies from Australopithecus afarensis 3 million years ago onward were probably large relative to maternal body size, estimated independently using skeletal remains. It’s a bit of a tortuous approach to estimating body size at birth, but the assumptions are reasonable and it’s probably the best way to figure out this important life history variable given the fossil evidence. What does this mean for Homo naledi?

*Virtual reconstruction of brain size and shape of the Homo naledi cranium “Neo” (work in progress). At 610 cm³, this is the largest and most complete Homo naledi endocast.*

There are a few reliable adult brain size estimates for naledi, ranging from 465–610 cm³ (Berger et al., 2015; Garvin et al., 2017; Hawks et al., 2017), which based on catarrhine scaling would predict newborn brain size of around 170–210 cm³ (DeSilva and Lesnik, 2008). These brain sizes would then predict newborn body sizes of around 1.4–2.1 kg: the smol estimate is based on the smallest naledi adult brain size and a human model of newborn brain/body size; the chonk estimate is based on the largest naledi brain size and an ape brain/body model (pinkish stars in the boxplot below, left).

*Boxplots of newborn body size in great apes. Gorilla, Chimpanzee, and Bonobo data from the Primate Aging Database* *(Kemnitz, 2019)*.

So, did Homo naledi have big babies? On the one hand, no: these 1.4–2.1 kg naledi newborns are outside the human range, and within the range of living great apes.

On the other hand, maybe Homo naledi babies were relatively large, though this depends on the size of Homo naledi adults. Recall from earlier that Garvin and colleagues arrived at an average estimated adult size of 44.2 kg. But this is an average of estimates for 20 separate naledi fossils, and each of these estimates has its own range of uncertainty. Garvin and team reported that the extremes of the prediction intervals for these estimates ranged from 28–62 kg. The second boxplot above shows newborn size relative to the adult average (sexes combined) for each species: for naledi, the six labels compare the smol and large newborn sizes (1.4 and 2.1 kg) with the adult average and extremes (28, 44, and 62 kg). Assuming the ‘true’ naledi sizes are somewhere in the middle of the range of estimates, naledi likely gave birth to babies 3–5% of adult body size, somewhat ‘intermediate’ between chimpanzees and humans (and bonobos…?) and similar to what DeSilva found for other hominins.

This is just a preliminary look at infancy in Homo naledi. There is a lot of uncertainty in these size estimates, but we should still be able to make some interesting inferences about growth and life history in our extinct evolutionary cousin.

REFERENCES

Barton, R. A., & Capellini, I. (2011). Maternal investment, life histories, and the costs of brain growth in mammals. Proceedings of the National Academy of Sciences, 108(15), 6169–6174. https://doi.org/10.1073/pnas.1019140108

Berger, L. R., Hawks, J., de Ruiter, D. J., Churchill, S. E., Schmid, P., Delezene, L. K., … Zipfel, B. (2015). Homo naledi, a new species of the genus Homo from the Dinaledi Chamber, South Africa. ELife, 4, e09560. https://doi.org/10.7554/eLife.09560

DeSilva, J. M. (2011). A shift toward birthing relatively large infants early in human evolution. Proceedings of the National Academy of Sciences, 108(3), 1022–1027. https://doi.org/10.1073/pnas.1003865108

DeSilva, J. M., & Lesnik, J. J. (2008). Brain size at birth throughout human evolution: A new method for estimating neonatal brain size in hominins. Journal of Human Evolution, 55(6), 1064–1074. https://doi.org/10.1016/j.jhevol.2008.07.008

Garvin, H. M., Elliott, M. C., Delezene, L. K., Hawks, J., Churchill, S. E., Berger, L. R., & Holliday, T. W. (2017). Body size, brain size, and sexual dimorphism in Homo naledi from the Dinaledi Chamber. Journal of Human Evolution, 111, 119–138. https://doi.org/10.1016/j.jhevol.2017.06.010

Hawks, J., Elliott, M., Schmid, P., Churchill, S. E., Ruiter, D. J. de, Roberts, E. M., … Berger, L. R. (2017). New fossil remains of Homo naledi from the Lesedi Chamber, South Africa. ELife, 6, e24232. https://doi.org/10.7554/eLife.24232

Kemnitz, J. W. (2019). Database for indices of aging in nonhuman primates. Innovation in Aging, 3(Suppl 1), S957. https://doi.org/10.1093/geroni/igz038.3472

Kuzawa, C. W. (1998). Adipose tissue in human infancy and childhood: An evolutionary perspective. American Journal of Physical Anthropology, 107(S27), 177–209. https://doi.org/10.1002/(SICI)1096-8644(1998)107:27+<177::AID-AJPA7>3.0.CO;2-B

Brain size & scaling – virtual lab activity

March 3, 2021December 7, 2024 / zcofran / Leave a comment

Each year in my intro bio-anthro class, we start the course by asking how our brains contribute to making us humans such quirky animals. Our first lab assignment in the class uses 3D models of brain endocasts, to ask whether modern human and fossil hominin brains are merely primate brains scaled up to a larger size. In the Before Times, students downloaded 3D meshes that I had made, and study and measure them with the open-source software Meshlab. But since the pandemic has forced everyone onto their own personal computers, I made the activity all online, to minimize issues arising from unequal access to computing resources. And since it’s all online, I may as well make it available to everyone in case it’s useful for other people’s teaching.

The lab involves taking measurements on 3D models on Sketchfab using their handy measurement tool, and entering the data into a Google Sheets table, which then automatically creates graphs, examines the scaling relationship between brain size (endocranial volume, ECV) and endocast measurements, and makes predictions about humans and fossil hominins based off the primate scaling relationship. Here’s the quick walk-through:

Go to the “Data sources” tab in the Google Sheet, follow the link to the Sketchfab Measurement Tool, and copy the link to the endocast you want to study (3D models can only be accessed with the specific links).

Following the endocast Sketchfab link (column D) will bring you to a page with the 3D endocast, as well as some information about how the endocast was created and includes its overall brain size (ECV in cubic cm). Pasting the link when prompted in the Measurement Tool page will allow you to load, view, and take linear measurements on the endocast.

*Hylobates lar* endocast, measuring cerebral hemisphere length between the green and red dots.

Sketchfab makes it quite easy to take simple linear measurements, by simply clicking where you want to place the start and end points. The 3D models of the endocasts are all properly scaled, and so all measurements that appear in the window are in millimeters.

The assignment specifies three simple measurements for students to take on each endocast (length, width, and height). In addition, students get to propose a measurement for the size of the prefrontal cortex, since our accompanying reading (Schoenemann, 2006) explains that it is debated whether the human prefrontal is disproportionately enlarged. All measurements are then entered into the Google Sheet — I wanted students to manually enter the ECV for each endocast, to help them appreciate the overall brain size differences in this virtual dataset (size and scale are often lost when you have to look at everything on the same-sized 2D screen).

Feel free to use or adapt this assignment for your own classes. The assignment instructions can be found here, and the data recording sheet (with links to endocast 3D models) can be found here — these are Google documents that are visible, but you can save and edit them by either downloading them or making a copy to open in Docs or Sheets.

Ah, teaching in the pandemic 🙃

Osteology Everywhere: Aerial Ossicles

May 22, 2017January 28, 2024 / zcofran / Leave a comment

Last month I was flying down to New Orleans for the AAPA conference. I was excited to try authentic beignets & sazeracs, present new research, and catch up with colleagues. Midway through the flight I glanced out the window, not expecting to see much. But lo!

twilight

Thankfully there wasn’t something on the wing. But there was something strange out there in the sparkle of sprawling city lights:

What’s that I spy outside the city center?

A bit outside of the main jumble of street lamps appears to be a concentration of light superficially similar to an incus, one of the three auditory ossicles of the middle ear:

Left: An osteologist’s nightmare at 20,000 feet. Right: Ear ossicles from White et al. (2012).

As a good mammal, there are three small bones inside your middle ear. These are fully formed at birth, and help transfer and amplify sound vibrations from your eardrum to your inner ear. It’s nuts. What’s even more nuts is that paleontologists and anatomists have figured out that the tiny, internal incus and malleus of mammals evolved from larger, external pieces of the jaws of our pre-mammalian ancestors. INSANITY!

Cross section of a right ear, viewed from the front. Image credit.

Being so tiny, it’s not surprising that auditory ossicles are not often recovered from skeletal remains, and are pretty rare in the human fossil record. Nevertheless, some are known and their comparison with humans’ ossicles is pretty interesting. The oldest incudes (yes, the plural of incus is incudes) I know of are from SK 848 and SKW 18, Australopithecus robustus fossils from Swartkrans in South Africa (Rak and Clarke, 1979; Quam et al., 2013). SK 848 is on the left in the set of images below:

Incus bones in three different views of SK 848, human chimpanzee, gorilla, sock puppet (left to right). Modified from Rak and Clarke, 1979.

SK 848 to differs from humans and African apes in looking more like a screaming sock puppet with a horn on the back of its head. Additional ossicles are known from South African australopithecines, including the older A. africanus from Sterkfontein (Quam et al., 2013). Interestingly, malleus of these hominins is very similar to that of humans, and Quam et al. (2013) think this ossicle may be one of the first bones in the entire skeleton to take on a human-like configuration during hominin evolution. Functionally, this may mean that the frequency range to which human ears are adapted may have appeared pretty early in our lineage as well (Quam et al., 2015).

Who’d’ve thunk we’d learn so much just from looking out an airplane window?

Bioanthro lab activity: Hominin brain size

November 15, 2015March 3, 2025 / zcofran / 2 Comments

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 meshes…

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 estimating hominin brain sizes.

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.

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

Osteology Everywhere: Ilium Nublar

June 30, 2015November 5, 2025 / zcofran / 1 Comment

Jurassic Park is objectively the greatest film ever made, so I don’t need to explain why I recently watched it for the bajillionth time. Despite having seen this empirically excellent movie countless times, I finally noticed something I’d never seen before.

Hold on to your butts. What's that on the screen in front of Ray Arnold?

Hold on to your butts – what’s that on the screen in front of John Arnold? (image credit)

The film takes place on the fictitional island “Isla Nublar,” a map of which features prominently in the computer control room when s**t starts to go down. Here’s a clearer screenshot of one of Dennis Nedry‘s monitors:

Isla Nublar from the JP control room. Quiet, all of you! They’re approaching the tyrannosaur paddock…. (image credit)

It dawned on me that the inspiration for this island is none other than MLD 7, a juvenile Australopithecus africanus ilium from the Makapansgat site in South Africa:

Figure 1 from Dart, 1958. Left side is MLD 7 and right is MLD 25. Top row is the lateral view (from the side) and bottom row is the medial view (from the inside). These two hip bones are from the left side of the body (see the pelvis figure in this post). Note the prominent anterior inferior iliac spine on MLD 7, a quintessential feature of bipeds.

Isla Nublar is basically MLD 7 viewed at an angle so that appears relatively narrower from side to side:

MLD at a slightly oblique view (or stretched top to bottom) magically transforms into Isla Nublar.

MLD 7 at a slightly oblique view (or stretched top to bottom) magically transforms into Isla Nublar.

It’s rather remarkable that some of the most complete pelvic remains we have for australopithecines are two juveniles of similar developmental ages and sizes from the same site. In both, the iliac crest is not fused, and joints of the acetabulum (hip socket) hadn’t fused together yet. The immaturity of these two fossils matches what is seen prior to puberty in humans and chimpanzees. Berge (1998) also noted that MLD 7, serving as an archetype for juvenile Australopithecus, is similar in shape to juvenile humans, whereas adult Australopithecus (represented by Sts 14 and AL 288) are much flatter and wider side to side. Berge took this pattern of ontogenetic variation to match an ape-like pattern of ilium shape growth. This suggests a role of heterochrony in the evolution of human pelvic shape, or as Berge (1998: 451) put it, “Parallel change in pelvic shape between human ontogeny and hominid phylogeny.” In layman’s terms, ‘similar changes in both pelvic growth and pelvis evolution.’

Lawnchair Anthropology

Biological anthropology, paleontology, evolution, and development, by Dr. Zachary Cofran

hominin