The taxonomic family Hominidae are defined by one common trait, bipedalism. Although modern humans, Homo sapeins, are the only living hominids, all fossil remains unearthed and identified as having belonged to bipedal hominoids are considered to have belonged to our own evolutionary family tree, if not directly ancestral. (Turnbaugh et al 1996: 234)

This is going to be an examination of how bipedalism expresses itself in the fossil record, and of what it may have meant to the evolution of Homo sapiens.

SKELETAL EVIDENCE OF BIPEDALISM

When a fossil is said to represent a hominid, it is due to the evidence that the creature walked upright, not due to any evidence that it was smarter than a modern ape. Although the apes are said to have a tendency towards erect posture, sitting upright to use their hands in taskwork and occasionally walking on two legs, they use all four limbs in locomotion for most purposes. Their forelimbs are long enough to work as walking limbs, and they do so comfortably and efficiently. When apes walk on the two hind limbs only, they cannot straighten them at the knee for efficient extension and they cannot balance themselves without swinging their entire body weight side to side onto each bearing leg in turn. In short, they are slow and awkward as bipeds, and are therefore not true bipeds.

Humans can walk with the efficient leg extension and balance to be considered fully bipedal. When we do use four limbs for locomotion, our arms are so short that we must use our knees as soles, dragging useless lower legs behind us as we crawl along.

These differences between apes and hominids are fully reflected in our skeletal structures. The significant skeletal markers of bipedalism are these (Turnbaugh et al 1996: 296) :

1. The foot, used for locomotion and load bearing only, has no opposable big toe. Instead, all the toes are parallel and short. The ankle and foot bones form a double arch, a shock absorbing dome structure. (Johanson et al 1996: 86)

2. The lower limbs are entirely elongated, and the femurs converge below the body's centre of gravity at ground level.

3. The knees lock at the position of extension which gives the leg a straight line of bearing, for maximum reach and minimum muscular load on landing. (Turnbaugh et al 1996: 296)

4. The pelvis is shortened top to bottom to lower the body's centre of gravity in the upright position. The pelvis is broadened horizontally to increase the mechanical advantage of the leverage applied by the muscles of the enlarged Gluteus Medius which must support the upper body weight off balance side to side as the weight shifts from leg to leg. The pelvis is also broadened to better support the Gluteus Maximus which is enlarged to control the extension of the thigh and to pull it backwards during running, jumping and climbing. (Turnbaugh et al 1996: 296)

5. The vertebral column is double curved, to send the upper body weight back over the legs, and then allow it to bend over forwards for functional utility. This S-curve is also acting as a shock absorbing structure, which compresses and extends while loading the muscles of the back evenly on all sides. The lumbar curve is not seen in four legged creatures.

6. The head is positioned directly over the supported vertebral column, and the foramen magnum (hole) in the centre of the skull base, through which the upper cervical vertebrae must pass, is on the centre of gravity of the skull assembly, whereas in the apes the head is attached through a foramen magnum at the rear of the skull base. (Johanson et al 1996: 86)

From these series of differences between the great apes and the hominids, we can see what to look for when deciding if a fossil of a hominoid may be of a biped.

The line of bipedal fossil remains has been given two Order names, so far. Ours, Homo, is shared by our species sapiens, preceded by erectus, preceded in turn by habilis. This order of hominids is distinguished by a very large brain, diminishing in size as the fossils originate further back in geological time. The order Homo is preceded in the fossil record by the order Australopithicae, which is distinguished by its antiquity (1.6 - 3.9 million years ago), by its bipedal skeletal structure, and by its apelike skull. If it had not been bipedal, Australopithicus could have been a virtual chimpanzee. There is a series of robust gorilla-like skulls within this order which, it has been suggested, would be better served given an order of their own, being Paranthropis, but this has not been universally adopted yet. (Burenhult 1993: 45)

THE ORIGIN OF BIPEDAL ADAPTATION

This is truly the most interesting and entertaining aspect of this study of human beginnings. The searching out of the fossil record is the most labour intensive project imaginable, given the physical output. As Donald Johanson says of his discovery of "Lucy",

"Until Lucy was found, there just weren't any very old skeletons. The oldest was one of those Neanderthalers...about 75,000 years old...Lucy is approximately 3.5 million..." (Fagan 1996: 59)

The few hundred fossils resulting from this century long search has nevertheless allowed anthropologists to form some solid opinions about who we used to be, apelike bipeds, and that we originated in Africa over four million years ago.

The fossil record actually shows that we were apelike bipeds (Australopithicus Afarensis, including the renowned "Lucy") at the end of the Moicene epoch four million years ago. The Pliocene epoch was taken up with the dominance of these small African hominids until the emergence 2.4 million years ago of a few larger brained bipeds (Homo habilis), who migrated out of Africa into Eurasia and the eastern Pacific. The gradual disappearance of the Australopithicians during the long emergence of the Homo bipeds may have been due to unequal war between the various species. While the fossil record presents us at the end of the Miocene with a completely developed bipedal species, there is no such treasure yet unearthed which takes the story further still back into the past. (Burenhult 1993: 55-59)

Genetic analysis, which includes protein concentration comparisions and amino acid sequence comparisons, points towards a date of six million years ago when the bipeds diverged from the great apes. (Johanson et al 1996: 32)

There are several evolutionary scenarios which have been proposed as explanations for both the divergence theory and the subsequent success of the bipeds intellectually. The four principle models which attempt to explain the origin of bipedalism in apes are the following:

1. The Scavenging Model proposed by Shipman has a tree dwelling ape family being forced, at the edge of the savannah, to leave the cover of the inadequate forest edge to scurry across the grasslands scavenging carrion. Standing up proved so advantageous in this productive enterprise, that the ones who stood best and ate best would breed best, and so the selective pressure for bipedalism weeded out the tree climbing genes, and the apes eventually became fully adapted bipeds. (Taylor-Hollings 1997: 10)

The difficulty here is that there is nothing less advantageous for a famished jungle ape than to stumble awkwardly about the savannah standing up in full view of myriad swift predators. The present day lack of apes on the savannah points to the likelihood that such a survival strategy is now unsustainable, and always was.

2. The Provisioning Model of Kent State's C. Owen Lovejoy would have the female ape with the least inclination to get up and feed herself mating with the male ape having the greatest inclination to do so, and beginning a family with the evolutionary advantage of a prodigiously breeding female lollygagging about with the nursing infants while her furiously relentless hunter mate presents her with a ceaseless cornucopia of edibles. The selective advantage of this breeding family is cemented when the male notices that he can carry twice as much in two hands as in one, and so begins to make the effort to travel back bipedally with his goodies, presumably to save the effort of double tracking. This proves so selectively advantageous that eventually the bipeds in the family evolve a skeletal structure which is better suited to the new biped economy. The key to this ambitious model is the supposed breeding advantage of the bipedal couple, especially in the greater world outside the jungle.(Johanson et al 1996: 88, & Shreeve 1997: 152)

While there is no doubt that the bipeds have outbred the great apes, it is questionable whether this could motivate the great apes to start walking and evolve themselves into bipeds on account of it. This model ignores the fact that females, breeding or not, feed themselves and their offspring without the help of male providers. This is true in both the foraging ape species and in the cultivating human species. Indeed, if we look at the highly dimorphic Australopithicians we might note that the smaller females required far less food than the huge males, and probably fed themselves easily.

3. Jolly's Small Food Foraging Model has hand usage providing a selective advantage in a forest habitat where the main food supply are the seeds, nuts, berries, and bark eaten by the powerfully jawed apes of the Miocene. While the great apes of today exist in marginal populations using this same habitat, the advantageous use of two foraging hands naturally selected for bipedal skeletal function in the ancestral hominids, and once the adaptation had been effected, it facilitated the hominid's ability to function in other habitats and in other environments. (Taylor-Hollings 1997: 11)

This dietary adaptation goes some way to explaining the generalized dentition of the hominoids, and was advantageous in various habitats, savannah or jungle or seaside, when they had to wander far and pick constantly at small foods to sustain themselves. But squirrels and gophers do the same thing, without becoming fully bipedal.

4. The Walk Tall...Stay Cool Model of Peter Wheeler would offer further selective advantage to the upright apes when they ventured onto the savannah to hunt by reducing the heat exhaustion at midday, since the body area exposed to direct sunlight is less when standing than when crawling. He points out that cooling airs one metre off the ground would also help the stamina of the hunter in the sun, whose improved time on the job naturally selected the lineage who evolved into complete bipeds. (Johanson et al 1996: 89)

CONCLUSION

However this was accomplished, our bipedal skeleton was framed long before our brains grew large, and while we evolved slowly, the great apes, and indeed many species on earth, appear to have evolved very little or not at all. It seems from the evidence available that we must conclude that species evolve major functional changes so slowly, and from such tiny beginnings, that no fully matured large species can conceivably be supposed to have evolved any such major changes at all.

The swelling of one single organ, our brain, can be explained exclusively on the grounds of culturally driven selective advantages, biocultural evolution. The selection of the bipedal function and its associated array of skeletal developments must have preceded the growth of the bipedal ape into a large species. Its body type was already too fixed by the time it was fully an ape. It is for this reason that I have chosen to think that the systemic explanation for the origin of bipedalism in apes (hominids) was not divergent evolution from a pre-existing ape species, but convergent evolution towards several ape-like species (hominoids) from a long distant ancestral species who was small and not an ape at all, but in whose own structure lay the seeds of a possible evolution into apes. Perhaps it was this tiny proto-ape which generated the A. afarensis and the Pan troglodyte, while its own ancestors gave rise to the other apes.

We may be looking for a small tail-less monkey from the pre-Miocene. Given our autonomic reflex under water, it has been proposed by some that it was a water monkey, nesting in seaside jungle caves. Lacking conclusive evidence, there remains much scope here for the imagination.

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