evolution

The snail in your ear telling you about evolution

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The bony labyrinth combines two of my favorite things: skull cavities that tell us about living and extinct animals, and Jim Henson dark fantasy films. This ludicrous structure is nestled within each of the two temporal bones of the skull, filled with fluid surrounding the organs of balance and hearing.

Modified movie poster from the 1986 film Labyrinth. The word "Labyrinth" is scrawled fantastically across the top. Beneath, David Bowie playing Jareth the Goblin King holds out a crystal ball containing 5 bony labyrinths.
Bony labyrinths in roughly frontal view, semicircular canals branching toward the top, cochlea coiled beneath.

As former senator Ted Stevens once famously described the internet, the labyrinth is kind of like a series of tubes: namely the cochlear duct and three semicircular ducts, each housed within its own bony canal. These ducts (and canals) meet one another in the bony vestibule, where they’re interconnected with two “otolithic” organs called the utricle and saccule. Movement of fluid within these ducts (and otolithic structures) gets translated into signals that are then sent to the brain. The vestibular system including semicircular ducts and otolithic organs helps you detect your head and body’s movement through space (or as the world falls down), while the cochlear system translates waves of pressure hitting the ear into sound.

As noted by Christopher Smith (the scientist who studies the labyrinth, not the filmmaker who has directed the TV show Labyrinth), this elegant sensory system is present in all vertebrates, inherited from our common ancestor that lived over 500 million years ago. The structure is so important to individual survival that it seems to be fully formed before birth [1], surrounded by the densest bone in the body [2]. This snail in your ear therefore has a lot to say about life on Earth.

The labyrinth has been studied to trace the evolutionary origins of endothermy (warm-bloodedness) in mammals [3]. Because the size of semicircular ducts/canals correlates with sensitivity to head movements, it has been used to reconstruct how extinct primates moved around[4], including the earliest human ancestors to walk on two feet [5]. Because cochlea length and coiling correlates with hearing capacities [6], scientists can use the labyrinth to reconstruct what kinds of sounds extinct organisms could have heard [7]. Some studies have found the labyrinth to be sexually dimorphic in humans [8,9] (though this varies across different populations [10,11]), meaning that it could be used to estimate sex from archaeological or fossil remains, including of non-adults.

As David Bowie sang in the movie Labyrinth, “Down in the underground you’ll find someone true.” He could well have been singing about the bony labyrinth, a gift to paleontologists: a small, strange time capsule brimming with biological information.

References

1. Jeffery, N., & Spoor, F. (2004). Prenatal growth and development of the modern human labyrinth. Journal of Anatomy, 204(2), 71–92. https://doi.org/10.1111/j.1469-7580.2004.00250.x

2. Pinhasi, R., Fernandes, D., Sirak, K., Novak, M., Connell, S., Alpaslan-Roodenberg, S., Gerritsen, F., Moiseyev, V., Gromov, A., Raczky, P., Anders, A., Pietrusewsky, M., Rollefson, G., Jovanovic, M., Trinhhoang, H., Bar-Oz, G., Oxenham, M., Matsumura, H., & Hofreiter, M. (2015). Optimal ancient dna yields from the inner ear part of the human petrous bone. PLOS ONE, 10(6), e0129102. https://doi.org/10.1371/journal.pone.0129102

3. Araújo, R., David, R., Benoit, J., Lungmus, J. K., Stoessel, A., Barrett, P. M., Maisano, J. A., Ekdale, E., Orliac, M., Luo, Z.-X., Martinelli, A. G., Hoffman, E. A., Sidor, C. A., Martins, R. M. S., Spoor, F., & Angielczyk, K. D. (2022). Inner ear biomechanics reveals a Late Triassic origin for mammalian endothermy. Nature, 607(7920), 726–731. https://doi.org/10.1038/s41586-022-04963-z

4. Ryan, T. M., Silcox, M. T., Walker, A., Mao, X., Begun, D. R., Benefit, B. R., Gingerich, P. D., Köhler, M., Kordos, L., McCrossin, M. L., Moyà-Solà, S., Sanders, W. J., Seiffert, E. R., Simons, E., Zalmout, I. S., & Spoor, F. (2012). Evolution of locomotion in Anthropoidea: The semicircular canal evidence. Proceedings of the Royal Society B: Biological Sciences, 279(1742), 3467–3475. https://doi.org/10.1098/rspb.2012.0939

5. Spoor, F., Wood, B., & Zonneveld, F. (1994). Implications of early hominid labyrinthine morphology for evolution of human bipedal locomotion. Nature, 369(6482), 645–648. https://doi.org/10.1038/369645a0

6. Manoussaki, D., Chadwick, R. S., Ketten, D. R., Arruda, J., Dimitriadis, E. K., & O’Malley, J. T. (2008). The influence of cochlear shape on low-frequency hearing. Proceedings of the National Academy of Sciences, 105(16), 6162–6166. https://doi.org/10.1073/pnas.0710037105

7. Coleman, M. N., & Boyer, D. M. (2012). Inner ear evolution in primates through the cenozoic: Implications for the evolution of hearing. The Anatomical Record, 295(4), 615–631. https://doi.org/10.1002/ar.22422

8. Osipov, B., Harvati, K., Nathena, D., Spanakis, K., Karantanas, A., & Kranioti, E. F. (2013). Sexual dimorphism of the bony labyrinth: A new age‐independent method. American Journal of Physical Anthropology, 151(2), 290–301. https://doi.org/10.1002/ajpa.22279

9. Braga, J., Samir, C., Risser, L., Dumoncel, J., Descouens, D., Thackeray, J. F., Balaresque, P., Oettlé, A., Loubes, J.-M., & Fradi, A. (2019). Cochlear shape reveals that the human organ of hearing is sex-typed from birth. Scientific Reports, 9(1), 10889. https://doi.org/10.1038/s41598-019-47433-9

10. Uhl, A., Karakostis, F. A., Wahl, J., & Harvati, K. (2020). A cross-population study of sexual dimorphism in the bony labyrinth. Archaeological and Anthropological Sciences, 12(7), 132. https://doi.org/10.1007/s12520-020-01046-w

11. Ward, D. L., Pomeroy, E., Schroeder, L., Viola, T. B., Silcox, M. T., & Stock, J. T. (2020). Can bony labyrinth dimensions predict biological sex in archaeological samples? Journal of Archaeological Science: Reports, 31, 102354. https://doi.org/10.1016/j.jasrep.2020.102354

Brain size & scaling – virtual lab activity

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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 🙃

#AAPA2017 – Modularity & evolution of the human canine

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I’m recently returned from this year’s AAPA Conference, hosted by Tulane University in New Orleans. What a trip!

Usually my presentations involve fossils and/or growth, but this year I wanted to try a different way of looking at the evolution & development – integration & modularity. In short, biological structures that share a common developmental background and/or function may comprise ‘modules’ that are highly ‘integrated’ with one another, but relatively less integrated with other structures or modules.

I hypothesized that canine reduction in hominins is a result of a shift in modularity of the dentition, such that the canine became more highly integrated with the incisors than with the premolars. I’d thought of this 5 years ago when creating the first rendition of my human evo-devo course (offering again next fall!), but never got to look into it. Interestingly, the results generally supported my predictions, except for one pesky sample…

Screen Shot 2017-04-23 at 8.36.05 AM

As my primatologist friends will tell you, male chimps are the worst.

Here’s a pdf version of the poster. It was fun to dabble with a new methodology, to see my far-flung friends, and to visit a fun historic place for the AAPA conference. Definitely looking forward to next year in Austin!

Osteology Everywhere: Skull in the Stone #FossilFriday edition

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It’s that time of year again.

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It’s the end of the year and I’ve got Homo erectus on the brain somethin fierce. Our precedent-erect first popped up in Africa around 1.9 million years ago, quickly spread throughout much of the Old World, and persisted until perhaps as late as ~ 100,000 years ago in Java, Indonesia. This was a very successful species by all accounts, and as a result of its great range and duration, you can imagine it was also pretty variable.

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Hominin brain sizes. Boxes and whiskers represent sample tendencies and points are individual specimens. 1 = Australopithecus, 2 = Early Homo (cf. habilis & rudolfensis), 3 = Dmanisi H. erectus, 4 = Early African H. erectus, 5 = Early Indonesian H. erectus, 6 = Chinese H. erectus, 7 = Later Indonesian H. erectus, 8 = modern humans.

Despite this great variation, H. erectus skulls generally share a common visage: long and low cranial vault, low forehead, protruding brow ridges, fun tuberosities and tori in the back. You’d recognize them anywhere. Including the sidewalk!

20161220_202353.jpg

Homo erectus in front of Ploenchit Tower, Bangkok (lateral view, front is to the right).

The relief in this sidewalk slat superficially looks like a trace fossil of partial H. erectus cranium, the face either missing (from the lower right) or taphonomically displaced toward the left side of the tile (see here for actual H. erectus trace fossils). This looks really similar to H. erectus from Indonesia, not surprising given its discovery in Thailand. Why, it could have come straight out of Figure 6 from a 2006 paper by Yousuke Kaifu and colleagues:

Bangkok erectus.png

Left lateral views of Javanese H. erectus crania, modestly modified from Kaifu et al. (2006: Fig. 6). Front is to the left this time.

Using my insane photo editing skills, I’ve inserted the Ploenchit Tower trace fossil (reversed) within the horde of heads presented by Kaifu et al., above. Like many of the real fossils, the Ploenchit specimen is missing the face (due to taphonomy), the supraorbital torus or brow ridge juts out from a low-rising forehead, and the occipital bone also projects out about from the otherwise rounded contour of the cranium. Note that there is a good deal of variation in each of these features among the real fossils.

What a happy holiday accident to find a Homo erectus cranium on the street!

seinfeld-its-a-festivus-miracle

ResearchBlogging.org Reference
Kaifu Y, Aziz F, Indriati E, Jacob T, Kurniawan I, & Baba H (2008). Cranial morphology of Javanese Homo erectus: new evidence for continuous evolution, specialization, and terminal extinction. Journal of human evolution, 55 (4), 551-80 PMID: 18635247