'Lucy' and The Arc of Visual Perception

Progression from a Gibbon/MacauType Species

This theory has been promoted in recent times however in the complete absence of any hard evidence there are severe difficulties in explaining such a direct progression from arboreal to a savannah existence.

In any case the theory we are concerned with suggests A.Afarensis as our ancestor and accordingly the problem would still remain as to why it evolved from such origins over a long period and ended up with a visually inefficient and typically chimp-like skull structure.

Therefore and in the knowledge of the close similarity of chimpanzee and human DNA it is reasonable to assume for this argument that we evolved from a similar animal.

Evolutionary Progression from Quadrupedal to a Permanently Upright Posture

The progressive development of the locomotive capabilities of our ancestral line would logically have to be from knuckle walking, to walking with fingertips touching the ground, then to a crouched locomotion with the hands just above the ground and ready to support when necessary. From this point in the progression the front limbs contact the ground less and less frequently and the attitude in motion is one of a strongly crouched bent-kneed gait. At this point it can be said that hominid bipedalism began.

In all the early bipedal stages, a severely crouched attitude would be necessary both for good visual coverage of the ground surface and to maintain the centre of gravity of the whole body positioned over the feet. The change from quadrupedal to bipedal could be described as shifting the centre of gravity of the body progressively backwards from between the four supporting limbs to between the two rear limbs.

Chimpanzees bear only 30-40% of the body weight on the front knuckles and the progression to bipedalism would therefore involve a gradual reduction of the weight supported by the front limbs to a point where all the weight was on the rear limbs. The centre of gravity would accordingly move to over the rear feet. The progression then would be a matter of maintaining the centre of gravity over the feet while the gradual straightening of the legs and the body, the re-alignment of the head on the spine, the change in the attitude of the facial plane relative to the spine etc. allowed a more and more erect posture in motion.

This crouched attitude would place an undue stress on the body muscles, in particular of the back and the legs. Consequently the amount of time actually spent in such a position would be kept to a minimum. While such stress would naturally lead to the progressive strengthening of the relevant muscles, this stance would nevertheless consume a lot of energy and accordingly there would be no incentive to maintain it unnecessarily. This resultant high-energy consumption would have a direct effect on stamina and would accordingly severely limit the ability to maintain a bipedal motion for any length of time, at any velocity. Accordingly there would have needed to be a very strong incentive to undertake a journey of any length and leave the security of the home base.

As the anatomical changes permit a more and more erect locomotion the level of muscular stress and the resultant energy consumption would reduce proportionately. The ability to remain in a bipedal stance for longer and longer periods and thus to roam further afield would correspondingly increase. It follows that the increase in efficiency would have an influence on the amount of time regularly spent in a bipedal stance. This would of itself tend to speed up the rate of progression.

The Bipedal Motion of Homo Sapiens Sapiens

When motionless man today stands with the centre of gravity of the whole body positioned over and between the arches of both feet. The head is erect on the upper spine and the spine arched slightly in order to position the weight of the head and the upper torso over the hips. This of course is the most efficient or energy conserving, high alert resting position. The focus of the eyes is about horizontal and the legs bent slightly at the knee.

When running however the spinal arch is straightened and the spine is angled forward from the hips. This moves the centre of gravity of the torso forward to balance the propulsion thrust of the legs. This thrust of the legs is angled upwards and forwards and the angle of thrust, as well as the angle of the torso at the hip, is dependent on the acceleration or velocity at that moment.

Inclining the upper spine forward has a secondary effect in that the comfortable position of the head on the spine also changes the inclination of the facial plane. This brings the comfortable focus of the eyes down from the horizontal to a point on the surface of the ground about 20-30 metres ahead and additionally extends the lower arc of visual perception back to cover the ground vertically under the body. This position of the focus of the eyes on the ground surface at 20-30 metres ahead is ideal for the observation of small obstacles and foot placement programming.

When running the knees are normally bent. The angle subtended between the upper and lower leg at the knee does not normally exceed about 160° in running motion, except when reducing speed.

Modern man therefore in forward motion adopts a crouched, bent-kneed stance, albeit a much more erect or advanced version than that practised by early bipedal hominids.

Co-ordination of A.Afarensis

Of course there is no real evidence as to the mental and physical capabilities of A.Afarensis. A comparison with modern man can therefore only be based on the capabilities generally and hypothetically attributed to the species together with the models and pictorial reproductions of it.

These reproductions show it fully and permanently upright with what is essentially a modern human torso, limbs and extremities, but with a smaller stature and with body hair or fur and with a head that is similar to that of a chimpanzee. It is suggested that it foraged for food on the open Savannah adding to its diet by hunting small game and scavenging. This implies that its upright mobility was good, having an appropriate level of endurance and speed over the ground.

Accordingly, when in motion here, it would need to concentrate visually, as do other species, on the terrain as a whole, planning it’s route as well as looking for potential food sources, and maintaining a watchful lookout for predators, etc. Simultaneously it would need to keep a sharp eye on the surface conditions ahead, noting difficulties and obstacles and where necessary adjusting pace, or direction to avoid them. Such vigilance would have been essential to survival in the circumstances, as one wrong step resulting in an injury could undoubtedly have had serious and even fatal consequences. Any inadequacy in this respect, of course would not only be potentially fatal for the individual, but in the long term would be fatal for the species in an evolutionary sense.

Here it has to be noted that its feet, while no doubt toughened with calluses, were by comparison with many other species living in the same environment, unprotected.

We can only conclude from all this that the level of bipedal co-ordination needed by A.Afarensis to be able to run safely in the Savannah at a speed sufficient to be able to hunt and to attempt to evade predators, would not and could not be far different to that of man today.

So if we accept the fully erect theory together with the capabilities generally attributed to A.Afarensis, we have a fully upright hominid with a torso and limbs similar to that of man today and with the head of an ape. Its attributes include good stamina and mobility, enabling it to roam at will on the open Savannah, all implying that it had good bipedal co-ordination. This suggests that A.Afarensis was 3.5 million years ago living in a manner not far removed to that of some ‘primitive’ tribes today.

This leads to the question as to how A.Afarensis evolved to this state and how this was achieved with a brain volume slightly larger that that of a Chimpanzee.

A Permanently Erect A.Afarensis?

The theory that A.Afarensis was our ancestor, was permanently and completely upright at 3.5 million years ago and was fully erect in motion as depicted has various implications, firstly for its apparent evolution to this state.

How long the implied evolution of A.Afarensis took from a quadrupedal to an upright locomotion would, in the current absence of any fossil evidence, be pure speculation. It is however clear that the necessary changes in the alignment, the shape and the dimension of most of the skeletal bones would take a very long time. Some indication of the period of time necessary to effect such evolutionary changes to the torso would be indicated by the time taken for the changes in the structure of the head as shown by successive dated fossil skull specimens of hominids from A.Afarensis to date. While some of these skull specimens may not be of our direct line, they generally show a significant and progressive change in structure over the period in question.

If this remarkable change in the dimension, shape and alignment of the skull components including the jaw took 3.5 million years, then one could reasonably assume that the equally significant changes in the skeletal structure below the neck would take a comparable period of time.

Accepting this theory therefore means accepting that the anatomical configuration of the body of A.Afarensis’ predecessors below the neck altered significantly in a previous long evolutionary period while in the same period the structure and the cranial capacity of the head remained ape-like. Also in the subsequent period of 3.5 million years to date, while the head altered profoundly, the implication is that there was no significant improvement or change in either the structure of the torso or in the level of bipedal co-ordination.

These changes to the skeletal structure below the neck over the last 3.5 million years would, according to the theory, have been mainly one of dimension and not alignment, some components having increased in length and cross section, while others to some extent have decreased. The most apparent result of these changes would be an increase in stature of about 15-20%.

By comparison in the same period the actual, recorded changes to the structure, shape and dimensions of the head are dramatic. There has been an expansion of the cranium and projection of it forward over the eye sockets. The capacity of the cranium has increased by more than 200%, represented by an increase from the 400ml of A.Afarensis to 1350ml today, and there has been a marked reduction in the size and projection of the jaw.

In referring to the models and representations of A.Afarensis and considering the question of muscular stress another factor is highlighted relating to the posture of the head and neck on the body.

Referring to the diagram below the approximate position of the centre of gravity of the body can be said to lie on the line A-A’ and that of the head, as a separate entity, lies on the line B-B’.

It is clear that in an erect posture maintaining the attitude of the head as depicted would put a continual stress on the muscles of the neck and the upper back. This would therefore be an energy consuming and therefore inefficient attitude.

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This highlights a problem that would have faced Johanson and his associate White when designing a model of an erect A.Afarensis. If the neck and skull were placed at an anatomically balanced and more appropriate attitude on the torso then the facial plane would be at an even more acute angle to the horizontal (see diagram below). This clearly does not suit the assumption that it was fully erect and led to the necessity of depicting that the vertebrae in the neck were arched forward allowing a more suitable positioning of the head and in particular of the attitude of the eyes.

Therefore if we accept that A.Afarensis was permanently and fully erect in locomotion, it would also follow that the long evolutionary process of becoming upright resulted in it having a restricted visual coverage of the ground and an inefficient attitude and posture of the head on the spine.

Such inefficient developments would clearly contradict the principles of Natural Selection.

However it is quite clear that for A.Afarensis to have a good safe, visual coverage of the ground surface it would have to have the facial plane of its ape-like head vertical. This would suggest therefore that it was not permanently erect in locomotion for any extended perambulation. The very ape-like structure of the head would indicate that it was still in the initial stages of the evolution to upright and that it would accordingly adopt a strongly crouched bipedal attitude when in motion on the ground.

All this leads to the re-examination of the hard evidence.

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