Just some musings today
over chimps and humans (Fig. 1) and some other higher primate skulls (Figs. 2, 3). Chimps have not yet made it into the large reptile tree (LRT, 1068 taxa), but they will someday.
The actual starting point
is closer to an extinct ancestor of both, Proconsul (aka: Dryopithecus; Hopwood 1933; 18–14mya; Figs. 2, 3) a genus that resembled a chimp, but did not knuckle-walk (Fig. 3) and lacked brow ridges, both traits retained by Homo.
Question #1 today is…
What changes can we see in the face of a human compared to a chimpanzee?
- Forehead present (housing more cerebral frontal matter)
- Longer and protruding nose with ventral nostrils (better for underwater)
- Shorter nose-to-lip distance with philtrum (medial furrow)
- Chin boss (deeper in adults)
- Internal lip tissue externalized
- Shorter muzzle
- Thicker, less patchy and eternally growing cranial hair (+ beard on males)
- The rest of the face (and most of the body) hairless
- Smaller iris vs. sclera (whites of the eyes)
- Smaller ears
- Fewer wrinkles on breeding adults and babies
- Brow ridges absent, replaced by decorative eyebrows
- Maturity does not include a change of face color
- Not visible: smaller canines
- Lower cheekbones (jugal, zygomatic arch) relative to tooth row
It looks like the ears are lower in humans, but relative to the eyes and nose, they are not.
Question #2 today is…
What changes can we see in the face of a chimp (Pan) compared to Proconsul?
- Loss of forehead in Pan compared to Proconsul
- Nose unknown in Pronsul, but bones are shorter and flatter in Pan
- Longer nose-to-lip distance in Pan
- Chin, absent, as in Proconsul
- Internal lip tissue unknown in Proconsul
- Muzzle the same in Pan, less above, but more below the nose
- Hair unknown in Pan
- Skin unknown in Pan
- Eyes unknown in Proconsul, but note their relatively higher placement in Pan
- Ears unknown in Proconsul
- Wrinkles unknown in Proconsul
- Brow ridges present in Pan, absent in Pronsul
- Skin color unknown in Proconsul
- Canines slightly larger in Pan
- Higher jugal relative relative to tooth row (= taller premaxilla and maxilla) and coronoid process of mandible
And then there’s one more transitional taxon
Ardipithecus (Fig. 4) nesting somewhere between Proconsul and Homo.
compared to Proconsul, we find larger eyes, a larger, lower nose, smaller canines, and an overall shorter/wider face… and a pelvis more appropriate for an upright stance, freeing the long arms to do something else, like carrying everything from infants to water to weapons to belongings. This is where we lost our hair, became long distance runners, developed sweat glands, and became wanderers.
Hopwood AT 1933a. Miocene primates from British East Africa. Annals and Magazine of Natural History (Series 10), 11, 96-98.
Hopwood AT 1933b. Miocene primates from Kenya. Journal of the Linnean Society of London. Zoology 38:437–464.