Humans are part of the hominin family. Although there are many similarities between all of us, it would still be easy spot a human face in a hominin crowd. One of the recognisable features that makes this possible is how much our face sticks out. Or rather, how much it doesn't.
Every other member of our family has a noticeably protruding jaw, referred to as "prognathism." In fact, this isn't just ubiquitous amongst hominins. All the other apes have a degree of prognathism too.
Except for us. So what's up with that?
Tracking bones
The first step in figuring out why our face is unusual is to identify what makes it unusual. Sure, it's easy to just say "because it's flat" but that doesn't really answer the question. What has changed that makes our face so flat. Did we lose a bone? Get squished as a child?
When it comes to the shape of a bone, there are two main forces at work. The obvious one is that bone grows. However, bone can also be re-absorbed into the body. This process is known as bone resorption. Both of these features work together to help shape our bones, both as we grow and later in life.
Crucially, these features leave behind different looking bones when looked at under a scanning electron microscope. An international team of palaeoanthropologists realised they could use this to study what processes were driving our flat faces. Perhaps our jaw just doesn't grow as much, or maybe there was excessive resorption.
A fossil face
Armed with this information, these researchers examined the faces of young fossil humans. As in, fossils who were young when they died. Not fossils from last Tuesday. Since these young fossils would still be growing, they hoped they could catch face growth "in the act" and determine which processes were involved.
The fossils in question included the Devil's Tower Child. This ominously titled fossil was a female only 4 and a half years old when they died. And she was named after the place in Gibraltar where she was found, FYI. Not because she's particularly spooky. A few other Neanderthal kids were also examined, along with plenty of humans for comparison. For good measure, they also looked at Homo heidelbergensis kids from Spain. These are thought to be the ancestors of (or closely related to the ancestors of) Neanderthals, allowing the researchers to track how things change over their evolution.
Sure enough, their approach was a success and the researchers could identify how the Neanderthal and human face grew.
As that handy image they made shows, the big difference is in bone resorption. It's not that humans stop growing a prognathic face. Rather, what does grow gets remodelled and eventually absorbed away. Notably, these differences are even present in the 4-year-old Devil's Tower Child. The fact that such deep differences are seen at such an early age suggests there were fundamental differences in how our faces grew.
But why?
The interesting follow-up is why did the faces grow like this. For the Neanderthals, the answer is kind of obvious. It's what their ancestors did. The Homo heidelbergensis fossils show a similar pattern of growth. The Neanderthals simply inherited that and continued (relatively) unchanged.
On the other hand, the fact that this pattern of growth is so old suggests that's what the common ancestor of humans and Neanderthals had too. Which means something changed in the human lineage to produce our unique face.
Sadly, this research doesn't shed much light on why that may have happened. However, it does provide some hints. Notably, the fact that our face is different at a young age suggests that this evolved as a response to something that happened whilst we were young. Unfortunately, saying much more would just be pure speculation which, whilst fun, wouldn't be particularly productive.
But that's where my comment section comes in. Leave your wild speculation in the bit below!
References
Lacruz, R.S., Bromage, T.G., O'Higgins, P., Arsuaga, J.L., Stringer, C., Godinho, R.M., Warshaw, J., Martínez, I., Gracia-Tellez, A., De Castro, J.M.B. and Carbonell, E., 2015. Ontogeny of the maxilla in Neanderthals and their ancestors. Nature communications, 6, p.8996.
