#AAPA2017 – Modularity & evolution of the human canine

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!

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Osteology Everywhere: Ilium Nublar

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 fictitious 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).

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.’