Honing in on why humans have slighter heads than other hominins

Neanderthals were so closely related to humans that many of us alive today still carry portions of their genome in our own DNA. The differences between them and Homo sapiens are subtle enough that as details of their nasal structure had to be used to delineate them as a separate species. These small differences can be important though, as anatomical differences may reveal more than a creature’s shape, but also their life cycle and ecology. A recent study of hominin skulls compared not only the relative proportions of Neanderthals, humans and a third group found in Spain, but also the growth rates that would have created such morphology.

In general, skulls of Homo sapiens have slighter, narrower faces than their relatives. The bone structure below our nose is actually indented compared to the flatter, wider skull of Neanderthals and the Spanish hominins, at least as adults. As children, human skulls more closely resemble these other two species’ growth patterns, adding new bone around the face and jaw. But while our hominin relatives continued this growth through their teens, human facial growth halt and and actually reverses towards puberty.

Building and breaking bone tissue

The mechanism that scaled back our upper lips is a type of cell called a osteoclast. Osteoclasts are special cells that move to different parts of our bones and help dismantle, dissolving the tissue with amino acids. This is usually done to clear damaged bone so that it can be repaired, and fortunately for this study, leaves distinct, formations when scrutinized through an electron microscope. Osteoclasts usually work hand in hand with osteoblasts, which are cells that construct new bone matrices for tissue growth. While osteoclasts leave crater-like structures in the bone, osteoblasts create smooth, young bone. By comparing the ratios of these structures, the researchers were able to judge how much skull grew at different ages, indicating the point when human osteoclasts carved our faces slightly.

Face of a winner

With this framework, scientists are now left with questions about the significance of these differences. Humans are the odd-man-out with narrower faces, but also as the only extant species. This could mean that our skull shape provided some kind of advantage over our brethren, although at this point that advantage isn’t at all obvious. To start their search, the researchers will look for the point that this growth pattern first emerged in our ancestors, hopefully indicating what evolutionary pressure triggered it in the first place.

Source: Why Humans Have Slender Faces and Neanderthals Don't by Laura Geggel, Live Science