Why is our face the flattest in the human family?

Humans have a flat face unlike every other member of our family. New research suggests this is due to the fact our face reabsorbs bone as we grow.


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?

The human face (circled) compared to hominin faces

Humans circled amongst the hominin (and some ape) faces. Check out all the prognathism

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.

Bone growth leaves behind a smooth surface (left) whilst resorption creates a rougher appearance (right)

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.

face growth

Growth patterns in the Neanderthal face (top left) and Homo heidelbergensis face (top right) and in humans of similar ages (bottom, respectively).

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.

Our recent family tree. Changes in face shape must have happened some time on the human branch.

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 communications6, p.8996.

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6 thoughts on “Why is our face the flattest in the human family?”

  1. Anonymous says:

    Maybe flattr face makes a child seem more innocent, so when a dad got angry he would killer more often the prognathic brother

    1. Adam Benton says:

      There may well be something to that. Some self-domesticated species – like dogs – appear to have undergone similar “innocentification”. That said, this might be hard to test.

  2. Brian says:

    Resorption or Neoteny?

    1. Adam Benton says:

      The former, perhaps as a result of the latter

  3. Dr. John says:

    The question breaks down into sub-questions: how did genetic machinery, presumably not so terribly different one ‘species’ to the next, change this, and secondly, why did that version survive? Why was it selected?

    The last question would seem to swing us back, possibly, to the previous two comments — innocentification (love that) and neoteny — inasmuch as the adult human skull resembles that of the infant ape.

    The how is slightly less speculative: the retention of multiple features of juvenile apes could be laid at the doorstep of epigenetic regulation — the genes for adult ape morphology were simply turned off. (This will have affected not just bone absorption in the face, but also bone growth/reabsorption rates in the limbs, as it is the juvenile ape which most closely approximates human limb:body and arm:leg proportions. This later could be tested by the same process as above).

    It may be related to longer childhoods which humans enjoyed. As longer maturation is associated with increases in intelligence, the species as a whole would benefit from this mutation (and prefer to mate with those who showed it). The retention of child-like attributes could, then, have been tied to the longer periods of dependency resulting from longer maturation arcs, evincing patience with junior on the part of the adults.

    Much of the above strikes me as testable, just not by me, as I teach at a very small school.

    1. Adam Benton says:

      You raise good points, though it wouldn’t necessarily have to be epigenetics. There is evidence of selection in humans of genes linked to domestication in other animals. Some of these may include neotony, although more work is needed.

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