Whereas non-human primates have a more mobile midfoot (e.g., a mid tarsal break) which is adaptive for tree climbing (See Photos below):
Foot Contact made by a Primus Metatarsus Supinatus Foot Structure
(Photo courtesy of Posturology Blog)
(3) The Plantargrade Foot is the end stage of this evolutionally process where the obligate bipedalism has fully developed its structure where it is very stable and functional in a strong gravitational field (mother earth).
When you carefully study the ontogenetic (embryonic) development of the human foot, you see that these embryonic foot structures developing in linear fashion (e.g., continuum)
I believe 3 million years ago the predominant foot structure in our prototypes was the PreClinical Clubfoot structure (for all intense purposes, nearly 100% of the phenotype). Although I am sure there was a foot here and there that was either a PMS foot and even possibly a plantargrade foot.
Today, based on my clinical research I find approximately 80% of the patients I have seen and screened at various locations (Mexico, United States, Italy, Spain and Portugal with patients from 5 countinents) to have either the PCF structure or the PMS foot structure - most of these predominantly the PCF structure. The Plantargrade structure is the exception representing less than 20% (of what I project to be) of the world population.
I believe in another 1-2 more million years, humans will have predominantly the plantargrade foot where the PCF structure and PMS foot structure are the exception.
Philip Tobias's concepts of bipedalism (The Tottering Biped 1982) closely parallels my beliefs that modern homo sapien bipedalism is still imperfect. He states: "After perhaps four million year or more, we have not yet evolved a fault-free mechanism. Our bodies are still subject to what Sir Arthur Keith called the ills of uprightness. They include flat feet, slipped discs, hernias, prolapses and malposture."
Tobias belief's dovetail into what I have written above regarding the three embryological foot types (PreClinical Clubfoot Structure, Primus Metatarsus Supinatus foot structure and the Plantargrade foot) all being present (I suggest) in the Au.sebida feet, as they are today in the h. sapien feet (just in different proportions).
These three foot structures function very differently, which is seen in the foot prints they leave behind. The PreClinical Clubfoot Structure may show a mid tarsal break (depending on its' severity). The Primus Metatarsus Supinatus and Plantargrade foot structures will not.
For more information - See Tracing the Human Lineage
Mid Tarsal Break - Its' Significance
Rothbart's Ontogeny - Phylogeny Evolution Model
Then DeSilva and Gill made what they considered to be a shocking observation.
Examining the video analysis of 32 (human) hyperpronators, they found that many of them also possessed a mid tarsal break which they believe reflects a common ancestry with apes (See photo below):
Is their finding surprising? From an Ontogeny - Phylogeny Evolution Model, No!
Rothbart Ontogeny - Phylogeny Evolution Model
Not from an embryological point of view. The old adage Ontogeny recapitulates Phylogeny brings clarity to Desilva and Gill's observation of hyperpronators gait patterns:
I believe we are still in the very early stages of this evolutionary process. My reasoning is based on my clinical work and research into the embryological development of the human foot:
(1) I believe the PreClinical Clubfoot Sructure is a very very old structure seen in hominis, going back several million years (Au. sebida fossil foot bones have this foot structure).
November 26, 2014
All apes have a mid tarsal break in which the middle of the foot bends as it rocks forward (see photograph below). The absence of this mid tarsal break has been assumed to be a defining feature of the human foot, separating the human foot from the ape foot.
Anthropologists reasoned that humans possess a rigid midfoot which acts as an efficient lever during the push-off phase of gait (See Photos below):
(2) The Primus Metatarsus Supinatus structure is a later developing foot structure, probably not to be commonly seen in the anthropological records, possible only 100,000 years ago (plus or minus).
This foot has an inner (cavus) and lateral arch, the height depending on the severity of the PMS structure (See Photo Below):
Foot Contact made by a PreClinical Clubfoot Structure
(Photo courtesy of Posturology blog)