Variation in Homo sapiens – reptile evolution continues

Similarities and differences in the face of humans
Each one of us has eyes, ears, a nose and forehead, but a variety of subtle differences in their shape and placement helps define and distinguish us as individuals. Even twins can be identified given enough familiarity with them. Our similarity binds us together. Our differences make us unique. Variation is very basic to evolution. Over time, enough variation produces new species and larger clades. 

Evolution works in very basic ways.
Overall and in their various body parts, progeny can be distinct from their parents in these several ways (among others unlisted):

  1. larger or smaller
  2. wider or narrower
  3. longer or shorter
  4. more robust or more gracile
  5. more permeable or more durable (skin/scales/osteoderms)
  6. adding protrusions (from horns and hair to feathers to fins to feet and toes) or eliminating them
  7. migrating body parts (more anteriorly, more posteriorly, etc.)
  8. more juvenile or more adult
  9. faster maturation, or slower
  10. colors, shades, transparencies
  11. faster or slower speed capabilities
  12. nocturnal, diurnal
  13. urges to swim, urges to dig, urges to climb

Nearly all of the variation between taxa in the large reptile tree can be attributed to these variations overall or in certain body parts.

Two human males. One has deeper eyes, a protruding chin and less distance from the ear to the back of the head than the other.

Figure 1. Two human males. One has deeper eyes, a protruding chin and less distance from the ear to the back of the head than the other.

Here we’ll show some of the basic elements in the human face.

Chin size
In most humans the chin more or less protrudes. This trait separates us from other species within our genus, like Homo neanderthalensis, as well as archaic members of our own species. No other vertebrates have a chin. It is unique to our species. Lacking a prominent chin nowadays does not make anyone less human. That is just in our gene heritage.

Ear location
The anterior rim of the ear flap is found at the apex of the mandible, in keeping with the origin of the ear bones as former jaw bones. The location of this point can be near to the back of the skull or far (Fig. 1).

Eye location
The eyes can be large and close to the profile (Fig. 1) or deep set and further back from the profile. The further back location provides more shade for the eyeballs.

Wide vs. narrow face.

Figure 2. Wide vs. narrow face.

Face width
Here (Fig. 2) we see two faces scaled to match eyes, nostrils and mouths vertically. However, they do not share the same face width (horizontal bar below faces).

Vertical variation in two faces. What appears to be a low forehead is actually a high face and vice versa.

Figure 3. Vertical variation in two faces. What appears to be a low forehead is actually a high face and vice versa. A large forehead and small chin are childlike features retained in the adult, or not.

Face Height
In some people the face is set higher on the skull, reducing the forehead height and raising the location of the eyes, mouth and nose (Fig. 3). These subtle differences, can be the basis for major changes over eons of time.

Wider skulls in Reptiles
One of the major differences between basal crocs and basal dinosaurs is skull width, with crocs trending toward a wider skull posteriorly. The same can be said about the basal reptiles. The plant-eating lepidosauromorphs trend toward a wider skull, while the insect-eating archosaurmorphs do not. Dimorphodon had a tall skull. The flat-head anurognathid had a wide one.

The migration of the nostrils posteriorly on parasuchians is enhanced by the elongation of the premaxilla. Conversely, in the related champsosaurs, the nostril migrates back to the snout tip… all a matter of facial proportions (Fig. 3).

This is just a primer, a jumping off point. You can find many more examples of convergence for nearly every character trait you may wish to list.

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