In fact, as far as the spine is concerned, brachiating is at the opposite end of the spectrum from bipedalism. For the ape, the weight of the body and legs tends to stretch the spine and minimise pressure on the discs of cartilage between the vertebrae. That is why back sufferers are sometimes put into traction or advised to hang from the top of a door to minimise pressure in the same way.
But when a human is standing or walking or running, each of the vertebrae has to sustain the accumulated weight of those above it. The discs between them are flattened from above downwards and expand outwards. This flattening is comparatively slight in an individual disc, but it can add up to an overall shortening of almost an inch. In the course of a day’s work, a normally active man or woman decreases in height as the hours go by and the height is restored during sleep when the discs recover their normal shape. In later years the discs become less resilient, so that the flattening process becomes permanent and the height decreases. All people become a little shorter as they grow older, even without the additional problems which can arise through osteoporosis – the decalcification of the actual vertebrae.
Lower back trouble arises because the kink in the lumbar region of the spine makes it structurally weak and unstable. If extra strain is imposed on it, the lowest vertebra is liable to slip backwards along the slope of the next one up. Such displacements may bring pressure on the nerves emerging from the spinal column, giving rise to pain which may be eased or cured by rest, but is liable to recur.
The discs themselves are encased in a tough outer coating which is not easily ruptured, but this can occur. It happens because in the lumbar region the vertebrae do not sit neatly one on top of the other: they meet at an angle, so that the back of the disc which separates them may be subjected at that particular point to a pressure equivalent to a couple of tons to the square inch. (That alarming figure is the kind of calculation that used to be published to explain the damage done to dance floors by stiletto heels.) Occasionally the disc wall ruptures under the stress, part of the contents leak out and solidify, and the resultant bulge causes acute problems which in some cases can only be relieved by an operation.
Apart from changes in the skeleton, the muscular anatomy also had to be remodelled. Walking and standing, plus the strenuous manoeuvre of rising to an erect posture from sitting or squatting, made heavy demands on the musdes of the legs and buttocks which consequently grew in size and strength. The mass of each leg now constitutes one-sixth of the entire body. The newly developed large buttock muscle (the gluteus maximus) needed a new point of anchorage on the skeleton, so a special extension of the pelvic flange evolved to accommodate it.
There was an even trickier problem, to which natural selection has not as yet perfected a solution. That concerned the question of holding the internal organs in place. Above the waist there was no difficulty. In a mammal’s chest, the heart and lungs are neatly packed inside a limited space enclosed by the ribs, and when we stand upright they are held up by a powerful transverse sheet of muscle – the diaphragm, already present in quadrupeds.
However, there is no cage of ribs protecting the abdomen. In primitive vertebrates there were ribs attached to all the vertebrae. This is still true of some reptiles, but not mammals – possibly because in the females the abdomen has to be extensible to allow for pregnancy.
In the abdomen, therefore, the viscera – which include in our case about 25 feet of coiled intestinal tubing – are much more loosely arranged. In a quadruped, such as a horse, the force of gravity causes the entrails to subside into the curve of the belly, where their weight (plus, when necessary, the weight of the foetus) is supported by a big, broad ligament attached to the vertebral arch of the spine.
When we began to make a habit of walking upright, the pull of gravity was in a different direction, towards the hind end of the body, and the ligament was ineffective because it was in the wrong plane.
Drastic evolutionary measures have been taken by way of damage limitation, and in humans the wall of the lower belly is now protected by three sheets of muscle crisscrossing over one another like a bandage applied to an awkwardly placed wound. But the reconstruction is not yet complete. There is a triangular section on each side of the lower part of the abdominal wall which is left unprotected by the bands of muscle, so that a sudden exertion – in some cases even a violent fit of coughing – may cause a piece of the small intestine to breach the wall and result in inguinal hernia.
Also subject to the pull of gravity is the blood in our veins and arteries. Standing on the head brings this home to us because we are conscious of the blood accumulating in the head and face. But we are rarely conscious that whenever we are on our feet the blood is pooling in our legs in exactly the same way. We may be aware of it on special occasions, such as getting up after prolonged bed rest. The sudden descent of the blood into the lower extremities can cause dizziness or faintness, and the patient may be advised to lie down, or put the head between the legs so that gravity will restore an adequate blood supply to the brain.
In all mammals the blood travels out from the heart through the arteries to all parts of the body and returns to the heart through the veins. In most quadrupeds most of this blood flow is along roughly horizontal channels because the body is horizontal; it is only in the legs that the blood returning through the veins has to climb perpendicularly against the pull of gravity.
To achieve this it relies on valves in the veins, which allow the blood to pass through in one direction only (towards the heart), and close to prevent the blood sliding back down again. The valves are therefore much more abundant in the limbs than in the other parts of the body.
Bipedalism imposes extra demands on this system, which it is even yet not fully equipped to bear. Our vertical posture means that the heart is twice as high above the ground as it would be if we were quadrupedal, and throughout most of our body the blood’s return journey to the heart is uphill all the way. The simple act of rising to our feet, by causing the blood to drain away from the heart, can cause cardiac output to drop by 20 per cent.
The greatest strain is on the veins of the legs. As with the lumbar vertebrae, they are under stress because they are at the bottom of the heap. Sometimes a valve gives way under the pressure. This results in twice the weight of blood pressing on the next pair down, which may also give way. This is the cause of varicose veins, where the accumulated pressure causes the walls of the veins to bulge outwards.
Varicose veins are especially likely to occur in pregnant women. One reason is that, because of our erect posture, the weight of the foetus presses down on the large blood vessels in the pelvis, causing a tendency to increased venous pressure, slowing of the rate of flow, varicose veins and swelling of the legs.
When varicose veins occur in the rectum or anus the condition is known as haemorrhoids or piles. The bulging and bleeding may be more frequent than in the legs because the veins in this area are not equipped with valves, since in the average quadruped they are higher than the heart, and the valves are not needed.
It is not surprising that erect posture has knock-on effects on the skeleton and the bloodstream. Its effects on the hormones, however, take a bit more explaining.
Endocrine glands sited above the kidneys produce hormones which respond to actual or potential physical ‘emergencies’. The best known is adrenalin (epinephrine), the ‘fight-or-flight’ hormone. In situations arousing fear or anger it causes sugar to be released into the bloodstream supplying instant energy, plus clotting agents in case the situation leads to violence and bloodshed.
A hormone which responds to ‘emergencies’ of a different kind is aldosterone. Its function is to regulate blood pressure, and to inhibit the excretion of salt. The ‘emergencies’ which stimulate the production of aldosterone are listed in medical textbooks as:
1 surgery;
2 anxiety;
3 a diet deficiency in salt;
4 haemorrhage;
5 standing up.
&nbs
p; The first four items apply to all mammals and are readily understandable. If there is a deficiency of salt in the body tissues, aldosterone conserves salt by preventing its excretion into the urine. Loss of blood due to surgery or haemorrhage means that the blood volume must be restored; the action of aldosterone increases the fluid volume of the blood. As for acute anxiety, some of the physical manifestations (sweating, vomiting, loosening of the bowels) are notorious squanderers of salt reserves. An increase in aldosterone production will guard against all these contingencies, since this hormone can inhibit salt excretion in the sweat and the faeces as well as in urine.
The fifth ‘emergency’ (standing up) applies particularly to bipeds. Rising from bed or from a chair produces a six-fold increase in the amount of aldosterone in the blood. This bears no relation to the amount of exertion needed to attain or sustain erect posture. Experiments have been conducted where a volunteer is strapped to a pivoting board which can be tilted on its axis, thus raising the body to the perpendicular with no physical effort involved. The production of aldosterone still rises just as steeply.
The explanation once again lies in the pull of gravity on the bloodstream. When we stand up, the blood tends to drain away from the head and the heart and pool in the lower limbs. It so happens that the main baroceptors which monitor changes in blood pressure are situated in the neck. For a quadruped this is a perfectly appropriate site; blood pressure at this point will be fairly representative of the pressure throughout the body; when these baroceptors register a change in blood pressure, they trigger appropriate responses such as increased output of aldosterone.
But the receptors are incapable of distinguishing between the 20 per cent drop in pressure which would result from a massive haemorrhage in a quadruped, and the 20 per cent drop in the head and neck region which signals in a biped the act of standing up. The aldosterone levels respond in the same manner to standing up as to surgery. Salt excretion is temporarily inhibited, and total blood volume is increased until it reaches a level which will satisfy the baroceptors that all is well – that is, a level which is adequate for the head and chest and overabundant for the legs. Once that point is reached, aldosterone production ceases, and the blood volume stabilises at the higher level appropriate to the bipedal posture. It is a system which works smoothly and is in general benign. If we could not attain this instant boost to the blood volume level, a feeling of faintness on rising would be much more common and would last longer.
It does, however, mean that our endocrine glands have a lot more work to do. (To a lesser degree, standing up also increases the output of adrenalin.) A quadruped in a secure environment may spend weeks at a time stress free and with no need for these glands to be activated until the rutting season comes round. But in a normal human being the blood volume and the hormones in control of it zoom up and down like a yo-yo many times in a day’s work. In patients who are subject to hypertension or circulatory disorders, this side-effect of bipedalism does not help. It is not what the doctor would have ordered if he had any choice in the matter.
The purpose of underlining the debit side of bipedalism at this length is not to whine about it. If these things are part of the cost of being human, few people would consider the price too high.
The evolutionary point is this: the first human ancestors to walk erect were still animals, in the popular as well as the biological sense. The compensating advantages which have since accrued to us were not available to them. The disadvantages of walking erect in the savannah would have been instant, and infinitely more uncomfortable for the beginners than they are today.
From the evidence of the skeletons and the footprints, standing and walking cannot have been as easy for the early Australopithecines as it is for us, even though modifications in the skeleton are interpreted as meaning that they had been practising bipedalism for a considerable time. Lucy’s feet, for example, were relatively broader and larger than ours (35 per cent of leg length instead of 26 per cent), and this would have given her a different gait, described by Roger Lewin as ‘… not quite as bad as trying to walk on dry land wearing swimming flippers, but in the same direction’. According to the French palaeontologists Christine Tardieu and Yves Coppens, the automatic knee-locking mechanism found in modern man was not fully developed in Lucy, so that even standing still would not be quite as easy for her. To appreciate the difference it would make, we should have to imagine standing for any length of time in a situation where the locking mechanism could not easily come into play – for example, in a room where the ceiling was just a couple of inches too low.
For absolute beginners the difficulties were very much greater. This is what the orthodox theory asks us to believe: that millions of years ago a population of apes on the savannah chose to walk on two limbs, instead of running rapidly and easily on four like a baboon or a chimpanzee. They stood up, with their unmodified pelves, their inappropriate single-arched spines, their absurdly under-muscled thighs and buttocks, and their heads stuck on at the wrong angle, and they doggedly shuffled along on the sides of their long-toed, ill-adapted feet.
They would not have done this in the blind faith that after the first few hundred thousand years the process would become easier and might in some way pay off. The incentive must have been immediate and powerful.
The next chapter examines some of the theories about what that incentive may have been.
4
Explaining Bipedalism
‘We have to admit to being baffled about the
origin of upright walking. Probably our thinking
is being constrained by preconceived notions.’
Sherwood Washburn and Roger Lewin
One factor contributing to the bafflement is the lack of a parallel to human bipedalism. Other modes of mammalian locomotion, such as hopping, gliding, swimming, burrowing through the earth or flying through the air, have evolved over and over again, in separate species often totally unrelated to one another. But man is the only extant bipedal mammal. For purposes of comparison scientists have to fall back on contemplating mammals that are bipedal for at least part of the time, and see what advantage they gain from it.
One promising model of bipedalism was thought to be the patas monkey. Its home is on the open savannah, and like many of the small mammals of plains and grasslands, such as the American gopher or the African meerkat, it has acquired the habit of standing upright to peer into the distance and detect the approach of danger. The idea that man had the same incentive was a favoured theory at the time when David Attenborough produced his TV epic Life on Earth. For the ape-men, he reasoned, ‘a permanent upright posture must have been very useful. The ability to stand upright and look around might make the difference between life and death.’
The only thing wrong with that argument is the word ‘permanent’. Every one of these horizon-scanning mammals, as soon as it spies the approach of danger, takes flight. It does not stay upright, or retreat backwards on two legs so as to keep the enemy under observation. It dives into a burrow, or shins up a tree if there is one handy. Otherwise it turns and runs away at top speed. And for a savannah ape, as for the patas monkey, top speed would certainly have meant using all four limbs to cover the ground.
Other theories about bipedalism which have surfaced during the last couple of decades have drawn their inspiration from (1) meat, (2) seeds, (3) sex, and (4) sunshine.
The oldest – the proposition that man stood upright in order to kill animals – dates back to Raymond Dart, who published a paper in 1953 called ‘The Predatory Transition from Ape to Man’. He was impressed by the fact that in the cave at Makapansgat where a hominid fossil had been discovered there were numerous bones of other animals, including crushed baboon skulls. He took these to be the prey that the hominids had carried back to their lair.
It seemed reasonable enough to suppose that whereas an ancestral forest ape, surrounded on all sides by its chosen food, would remain strictly vegetarian, a savannah ape would be driven by t
he scarcity of lush vegetation to hunt animals. A whole mystique grew up around the idea that the human race (‘Man the Hunter’) owes its origin to the lust for red meat. When a small piece of skull from a young Australopithecine was seen to have been pierced twice by some sharp instrument, murder was added to meat-eating as one of the factors in hominid evolution.
While some of Dart’s ideas were vindicated, the predatory thesis has not stood the test of time. It is now thought more likely that the hominid in the cave was not a predator, but one of the victims of a carnivore. The two holes in the hominid’s skull exactly fit the size and distance apart of the lower canines of a leopard. Later, a detailed survey was carried out over some years comparing the behaviour in the wild of a band of savannah-dwelling chimpanzees with another population of chimps dwelling deep in the forest. The forest chimps ate more meat than the savannah ones, and were noticeably more skilled and co-operative in hunting and killing.
But it was above all the discovery of Lucy which cut the ground from under the hunting hypothesis. The fossil remains at Hadar proved that bipedalism had become a specialised form of locomotion long before the hominids evolved big brains or the use of tools and weapons for chasing and killing big game.
The picture of the ape-man as big game hunter began to be modified. Perhaps, it was argued, he fed off small game, or perhaps he scavenged the remnants from the carcases of animals slain by the big cats. Indeed, on closer inspection, the fossil teeth of Australopithecus did not look like the teeth of a carnivore.
In 1970 Clifford Jolly made a move towards breaking the link between meat and bipedalism when he published a paper entitled ‘The Seed Eaters’. He saw that the ape-men’s molars were flatter than a chimpanzee’s, and suggested that they were serving as millstones to crush small objects such as seeds. Long, sharp canine teeth would have been invaluable to a carnivore, even a scavenger, in tearing flesh from bone, but in a seed eater they would impede the sideways grinding motion essential to the crushing process. Jolly suggested that this was why the canines were smaller in the hominids than in the other African apes.
The Scars of Evolution Page 4
