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What's up with the schnoz on Neanderthals?

I'm happy to announce the release of a paper that has been cooking for a few years now, and also my second foray into the realm of Mammalia.

I'm also happy to see that the study has seen its fair share of press over the past couple of weeks, including Live Science, Scientific American, Gizmodo, and even my own institution.

The study started, as so many do, with a simple question: why are Neanderthal faces so different from human faces?

This question has been asked before, with many researchers tossing in potential hypotheses for why our nearest human relative was so much more robust than us. However, few of these hypotheses have ever really been tested.

This led my colleagues down south, in Australia, to tackle this subject. First they looked up the most common hypotheses behind Neanderthal facial morphology. They found three big ones (well, two big ones, and a third, smaller one):

  1. The prognathic (jaw-forward) face and robust skull were adaptations for enhanced bite force.

  2. The enlarged nasal cavity and its consequent extension of the face was an adaptation to the more polar environment that these near-humans evolved in.

  3. The enlarged nasal cavity was an adaptation that allowed more conditioned air to reach the lungs during strenuous activity.

Next, they asked how best to test these hypotheses. The first hypothesis was an engineering problem. As such, it could be tackled using some tried and true engineering methods that Wroe and his team have been pioneering for over a decade now (e.g., here, here, and here). The other two hypotheses were also engineering problems, but they were from a different field of engineering that looks at the relationship of fluids. This is where I came in.

I've worked with Dr. Wroe in the past, and a chance encounter at the Society of Vertebrate Paleontology conference in 2014, laid the groundwork for this collaborative project. Wroe and his team had already worked on the models for Neanderthal in comparison to modern humans and an outlier species, Homo heidelbergensis. They were just getting into the Finite Element work, but still needed someone who could test heat transfer in the nasal passages. It just so happened that I was presenting on a similar project in ankylosaurs at the time, so I was already versed in how to tackle this problem, making me a good fit for the team.

To properly ascertain the shape of the nose in Neanderthal, we enlisted the aid of biological anthropologist, Todd Yokley. Yokley did his PhD work on human nasal passage evolution and variation in extinct relatives like Neanderthal. His familiarity with the specimens we were using, and human nasal passage shape made him a perfect edition to round out the team.

La Chapelle Skull

Over the next few months, I worked with Wroe, Ledogar, Parr, and Yokley on getting the shape of the nasal passage just right in our representative Neanderthal specimen. The specimen we went with was the famous La Chapelle Aux Saints specimen (i.e., the "old man". Skull shown on the right), as it had the most complete nasal cavity of the bunch.

Once the model was made, I went and tested heat transfer against a known model: modern humans.

Although we probably don't think much about it, the nose of modern humans does show variation with latitude. Humans from more equatorial locations (Africa, Australia, South American) tend to have nasal passages that are wider and lower; whereas humans that evolved in more subarctic localities show nasal passages that are more constricted and taller. This is reflected in the piriform apertures on the skull (i.e., the bony opening for the nose). For our study, I had access to people of both African (equatorial) and European (subarctic-ish) descent. This let me simulate heat transfer in the nasal passages of two modern humans and test it against empirical studies that had done the same. This served to ground-truth the approach and ensure that the models were working correctly.

After that, it was time to test Neanderthal. We tested our modern humans against Neanderthal, as well as an older species of hominid called Homo heidelbergensis.

Broken Hill 1 Skull

Well, I should say we tested a specimen called Broken Hill 1, or Kabwe (skull shown on left). It's an earlier relative of modern humans that is believed to be very close to the base of the split between modern humans and Neanderthals, thus making it a prime candidate to compare our tests with. It's taxonomic status is apparently controversial (and indeed, was something we were dinged on during one of our submissions), so not everyone is on board with the Broken Hill specimen being H. heidelbergensis, but for the purposes of our study, that is what we called it.

We tested the different species using an environmental temperature of 0°C, which was a temperature that the modern humans and Neanderthal would have experienced (H. heidelbergensis was unlikely to). We looked at a resting inhale and a resting exhale. That let us see how much air could be heated on the way in, and then (more importantly) how much heat could be reclaimed on the way out.

I don't intend to rehash the entire paper here, so I'll just summarize that our results showed that Neanderthal's weren't the best biters when compared to modern humans. Similarly, they did not heat and cool air as effectively as modern humans either. It seems like the enlarged nasal cavity of H. neanderthalensis evolved for reasons other than what was initially proposed.

At least that is what it seems like when we don't take a paleontological point of view.

The reason we chose the Broken Hill specimen was specifically to gauge our findings in the light of evolution. H. heidelbergensis represented the starting, or plesiomorphic condition for both Homo sapiens, and H. neanderthalensis. Current thinking strongly suggests that Neanderthals are our closest hominin relatives, making them sister taxa to us. That is to say we did not evolve from Neanderthals, but instead shared a very close common ancestor to them. The Broken Hill skull represents that common ancestor (note I said: represents. It is not the actual common ancestor, or at least we have no way to prove that it is). Broken Hill 1 was an equatorial species that did not show signs of a life geared towards cold temperatures. As such, it became our starting point.

When we compared our results to the results we obtained from the Broken Hill specimen, our interpretations changed dramatically. Yes, Neanderthal did not perform as well at air conditioning as a modern human, but it did quite a bit better than H. heidelbergensis. Notably, both Neanderthal and H. heidelbergensis, had similarly large piriform apertures. This suggests that the large apertures were inherited from the last common ancestor of modern-humans and Neanderthals. Neanderthals expanded the nose from this starting point, producing larger surface areas for heat to transfer from the body to the air (and vice versa during exhalation), whereas in humans, the large piriform aperture was reduced along with most of the rest of the skull. Our noses became better at conditioning the air largely because we just got smaller.

This then led into the last hypothesis, one that has been bandied about here and there, but was never a major front runner until now: The large nose of Neanderthals allowed them to be more active at cold temperatures. Again, I don't want to rehash the entire paper, but our data suggests that Neanderthal noses could push more air through them before turbulence within the airways increased the energetic costs/benefit ratio. In humans, we typically breathe through our noses at rest, and when mildly active (e.g., walking around and climbing low inclines). As soon as we start to get into the realm of moderate exercise, turbulence in our nasal passages makes it too costly to push air through them anymore, and we switch to mouth breathing. The upshot to this is that we can get more air into our lungs faster, but the downside is that none of that air gets treated on the way in and out. This is why marathon runners need a lot of water to keep going, and why you get a dry mouth if you have been speaking too long. Neanderthals would have switched to mouth breathing too, as soon as the exercise level got high enough, but our data indicates that the threshold for doing so was much higher than it is in modern humans. This suggests that Neanderthal lifestyles were more energetically intense than modern Homo sapiens, which is a hypothesis that jibes well with the more robust build of Neanderthals compared to modern humans.

So the nasal passage in Neanderthal looks to apparently be a compromise between a streamlined air transit system and an efficient air conditioner.

Or to put it another way: Neanderthal noses were actually pretty cool.

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