Writing in the journal Neurosurgical Focus on Sunday, Filler said one main clue was a bone feature called the transverse process, which sticks out from the side of the hollow, round vertebrae, Filler said in a telephone interview. This is where muscles attach to the spine.
“The vertebra is transformed in a way that literally reverses the mechanics of the spine,” Filler said. “The bone lever of the vertebrae gets switched from bending the spine forward to bending the spine back.”
Most vertebrates are oriented forward, to walk on all fours. The transverse process is at the front of each vertebra, facing the animal’s belly. This is true of monkeys, too.
But in humans and in the 21 million-year-old fossil of a creature called Morotopithecus bishopi, a tree-dwelling, ape-like creature that lived in what is now Uganda, the transverse process has moved backward, behind the opening for the spinal cord.
Great apes, such as chimpanzees, share this feature.
I’ve tracked down the article and am in the process of reading it. But I am a bit skeptical. Below is a picture of a human lumbar vertebra.
At issue is the position of the transverse process. In humans the transverse process attaches to the dorsal part of the pedicle. This is similar to what is seen in apes and Morotopithecus bishopi. In macaques the transverse process arises from the vertebral body. Macaques also have a styloid process, unlike apes and humans. This leads Filler to suggest:
In his study and in a book published last week called “The Upright Ape — a new origin of the Species,” Filler argues that this common ancestor, and ancestors going back many millions of years before, walked upright. Homo sapiens, the human species, continued upright, while apes evolved back toward all fours, he argues.
One major problem, for me, is the picture below:
This is the lumbar vertebra of Megaladapis – a giant subfossil lemur from Madagascar. Just like in humans, the transverse process arises from the dorsal part of the pedicle. It goes without saying that no one has ever claimed that Megaladapis is bipedal…
Update: Karl wants to know what Filler’s explanation for bad backs is. Filler argues, correctly I think, that in humans the posterior superior iliac spine (pointed to by the arrow):
is positioned more dorsally to the longissimus insertion in humans. This, combined with the higher placement of the lumbar transverse process, makes the longissimus a powerful extensor of the lumbar spine adding a lot more stress to it and making the muscle more susceptible to strain. This is one cause for bad backs, it does not support Filler’s contention that:
Morotopithecus bishopi therefore emerges from among the various proconsulid hominoids as a representative of a
new subgroup that can be called the “hominiforms.” The key difference between proconsulid and hominiform apes
of the East African Early Miocene is that the hominiforms have the same basic lumbar spine anatomy as that in modern humans (Fig. 1).
This modified vertebral pattern is quite unusual for a mammal and appears to have emerged fairly abruptly in a
limited period of time. From that point of view, it is almost possible to view this modification as a birth defect, makquadrupedal locomotion. The lack of any intrinsic mechanism to resist extension under the force of gravity in a horizontal pronograde position allowed the lumbar spines of these apes to function well only in upright or orthograde postures. The large modern hominiform apes such as gorillas and orangutans have independently evolved new types of osseous ridges that lock the lumbar facets against extension. 2 This finding suggests a continuous series of upright ancestral apes–with body plans more or less like those in
a modern human–from which other apes have descended to establish diagonograde (between upright and horizontal) postures during knuckle walking and other straightarm/ flexed-leg gaits.4
Filed under: Paleoanthropology