Nowadays there is agreement that the descended larynx in humans is a rare anomalous feature, but there is no agreement as to why it evolved. Sir Victor Negus spent his lifetime researching the anatomy of the larynx and he was in no doubt about whether or not man stood to gain from this odd change in his anatomy. It was, Negus declared, ‘greatly to his disadvantage’.
He never found any convincing explanation of why it had happened. He mentioned that man was microsmatic – that is, with a diminished sense of smell – so to that extent he had slightly less to lose by ceasing to be a habitual nose-breather. He noted that the tongue ‘extends for some considerable distance beyond the last tooth’, and speculated whether ‘the larynx may be said to be pushed down the neck by the tongue’. (But unless and until the larynx had descended, there was no reason why the tongue should have acquired its backward projection.)
Finally he leaned towards the theory still in favour today – namely, that the descent of the larynx was in no way adaptive; it was simply one of the many unfortunate unavoidable side-effects of bipedalism. Bipedalism necessitated changing the angle at which the head is set on the spinal column, so that the human gaze is directed forward at right angles to the spine, instead of in the same plane as in quadrupeds. It is argued that this change in angle caused the windpipe to slide down the throat, aided possibly by the force of gravity.
One exposition of this theory points out that ‘… in mammals where the larynx is placed high in the neck such as cats, dogs, monkeys and apes, the basicranium is relatively non-flexed’ – that is, the head is at the normal angle for quadrupeds. The argument sounds persuasive until you appreciate that the relatively non-flexed category includes not only cats, dogs, monkeys and apes, but every terrestrial mammal except man.
There are three reasons for rejecting the ‘unfortunate side-effect’ theory. One is that in a foetus or a new-born human the angle of the head is even farther removed from the quadrupedal angle than in an adult, yet the larynx is placed high, as in animals. Secondly, experiments by Jan Wind in the Netherlands have established that gravity does not affect the issue. Certainly, when a baby’s larynx descends the baby is still in the horizontal phase of its life, so gravity can hardly be pulling it down.
But the strongest argument against the theory was pointed out to Negus by his greatest admirer and supporter, Sir Arthur Keith. If there had been any tendency to slide, only a slight modification would have been needed to counteract it, and ‘… if Nature had any necessity for maintaining close relations between nose and larynx, it would find means to do so.’
When Negus spoke of ‘great disadvantage’, he was thinking of disadvantage to a land animal, for which an uninterrupted passage from nostrils to lungs is a high priority – provided that what enters the nostrils is going to be air. For an animal that spends much of its time in water, the priorities are somewhat different.
One priority for swimmers and divers is the ability rapidly to inhale and/or exhale large quantities of air, either before diving or when briefly surfacing. This necessitates a wide opening. Negus made this point in connection with the wide laryngeal opening of the horse. ‘If any animal has the necessity of supplying a large volume of air to its lungs, it must have a wide tract, and cannot attain the required result by mere increase in the rate of respiration.’
When we breathe through our nostrils, our own air tract is not merely narrow but – compared with that of a quadruped – also tortuous. In a dog, air travels in a straight horizontal line from nostrils to lungs; in man it goes up, and across, and down. By far the quickest way of increasing the volume inhaled is to breathe through the mouth.
The other requirement for aquatic species is to be able to close the air passages. Any mammal accidentally falling into the water automatically holds its breath by closing the glottis – the space between the vocal cords – and inhibiting respiratory movements of the lungs. This is adequate as an emergency procedure, and most terrestrials do not need to sustain the closure for long, because their mode of swimming, like the dog-paddle, is designed to keep the head above water at all times.
Diving species need more protection. A method many of them have evolved is that of acquiring valvular nostrils like those of a seal, which in a relaxed state are closed, but can be opened at will on surfacing. Aquatic species which are unable to use this method resort to other devices. In birds, for example, the nostrils are openings in the beak, and they cannot be closed. The penguin therefore has a triangular flap of cartilage inside the windpipe which has, like our own larynx, lost all connection with the nasal passages. Like many other aquatic birds, such as pelicans and gannets, the penguin has become a mouth-breather.
Several unrelated aquatic species have evolved some kind of internal movable flap either instead of, or in addition to, valvular nostrils. The penguin has one; and the crocodile has one. Alone among the primates, humans have such a flap – that is, the back of the soft palate, known as the velum, which in our species can be raised and lowered to isolate the nasal passages from the mouth cavity. It could not operate in this fashion if the larynx had not retreated out of its way to its present position below the back of the tongue.
The only other mammals which are known to feature a descended larynx are diving mammals – the sea lion and the dugong. These two species are as about unrelated to one another as they are to humans. The descended larynx must have evolved independently in each of them, after their respective land-dwelling ancestors entered an aquatic environment.
If these unusual features of the human respiratory tract were indeed aquatic adaptations, by now their efficiency has been in some respects eroded. There may have been a time when the velum closure was more powerful than it is today. There may have been a time when the wings of our nostrils (not found in other primates, and muscled in the same way as a seal’s) were fully valvular and could open and close.
The descended larynx, however, has stayed with us permanently, for good or ill. Some of the more unfortunate effects have been mentioned. A more positive aspect is that when we learned to speak we were able to do so more effortlessly and to produce a wider range of sounds.
The question of why one primate, and only one, has acquired the power of speech is another of the unsolved mysteries of human evolution. One approach to the question has consisted of attempts to find out why chimpanzees – although so closely related to ourselves – do not speak.
One early experiment to investigate the chimp’s powers of verbal communication, set up by the American psychologists K. J. and Caroline Hayes, met with limited success. They tried to teach their chimpanzee Vicky to talk, and after six years of hard work the animal had learned to utter only four words – ‘papa’, ‘mama’, ‘cup’ and ‘up’. Later, when Allen and Beatrice Garner set out to teach sign language to the chimp Washoe, the results were much more impressive. Washoe acquired a vocabulary of 34 signs within two years.
These results were not too surprising. Chimpanzees have an intricate social system requiring subtle methods of communication which is conducted chiefly by means of body language. Postures and facial expressions signal intentions and emotions by eloquently conveying threat, anger, fear, and a wide range of other reactions. Washoe was being asked to extend the range of a channel of communication – that is, sign language – already highly developed and in constant use. Vicky was being asked to renounce it in favour of elaborating the vocal channel of communication. Apes do have a vocal repertoire, but it is far more stereotyped and less flexible than their use of visual signals.
To many people, however, the different responses of Vicky and Washoe meant that (a) chimpanzees have the intelligence to understand the concepts behind such words as ‘table’, ‘hug’, ‘food’, ‘open’, ‘bad’, ‘into’, ‘give’, and many others; (b) they have the desire to communicate; (c) therefore there must be some specific obstacle which prevents them from speaking the words.
It was implied, and sometimes stated, that chimps cannot speak because th
e structure of their mouths and throats debars them from producing the requisite sounds. This is not the case. Apes and monkeys can produce a variety of vowel sounds, varying both in pitch and volume. They can say ‘ah’ and ‘ee’ and ‘oo’. They can pronounce ‘k’ and ‘p’ and ‘h’ and ‘m’ and the glottal stop (the sound which in Cockney dialect replaces the ‘tt’ in ‘bottle’). They might have difficulty with ‘t’, ‘f’, and ‘n’, but there is no structural reason why they could not produce ‘b’ and ‘g’. Anatomists who have studied the question agree that ‘… phonation can be carried on perfectly well by means of an intranarial larynx’ – that is, one that has not descended.
The likelihood is that the difficulty Vicky had to overcome was not primarily in her mouth, but lower down in her lungs, where every act of vocal communication starts, and in her brain, because voluntary lung ventilation is a function of the brain.
In most mammals breathing is a function as involuntary as digestion or the beating of the heart. The rapidity of breathing and the amount of air inhaled are controlled by chemical, physical and hormonal factors. For example, a low oxygen level in the blood stream triggers deeper breathing; adrenalin speeds the rate of respiration as well as the heart beat. In higher vertebrates pain, or heat, or cold may alter the respiration rhythm; immersion of the head in water causes cessation of breathing. The receptors which receive these messages are no more within the animals’ conscious control than the receptors controlling the knee-jerk reflex. We ourselves are subject to involuntary respiratory reactions of this kind when we sneeze, or sob, or hiccup, or respond to a sudden fright with a sharp intake of breath.
Aquatic mammals, however, have necessarily acquired more conscious control over the operation of their lungs. Unlike land mammals, they can voluntarily regulate the timing of the breaths they take and the amount of air they inhale. Land mammals’ breathing reflexes respond only to events which have already happened, as in the case of an animal which accidentally falls into the water. A diving mammal like a seal or a dolphin purposefully regulates its breathing in relation to actions it intends to perform, as explained in a report by R. Elsner and B. Gooden on diving asphyxia:
Some major component of the diving response is determined by the intention, conditioning, or psychological perspective of the animal being studied. Thus the diver acts as though it produces the most intense diving response when the need for achieving maximum diving duration is anticipated.
Homo sapiens, unlike apes and most land mammals, acquires conscious control of breathing at an early age. A young child, when it is learning to know its own body, discovers that it has power to control its breathing, just as it learns that it has power to control the sphincter regulating its bowel movements. At this stage it enjoys practising and exercising these powers in ways often disconcerting to those around it. If it deliberately refuses to breathe, as some children do, it can hold its breath long enough to terrify them. Some children learn to exploit the trick to express resentment or to practise blackmail.
Adult humans find it easy and natural to respond to a command such as: ‘Breathe in – hold it for a count of five – breathe out slowly.’ They find it so easy that they seldom appreciate that only a small minority of mammals are capable of this feat. When Vicky was urged to speak she was being asked in effect to perform an act of yoga – to attain conscious control over a function not normally subject to it, as humans do when they are taught to lower their blood pressure by an act of will. Such tricks can be performed, but they are not easy.
The really indispensable pre-adaptation for speech is the enhanced degree of conscious breath control which we share with all diving mammals and no purely terrestrial ones. The pattern of inhaling deeply and quickly, and exhaling slowly at a controlled rate, is characteristic of aquatic mammals when they dive – and of humans when they speak.
12
Sex in Transition
‘I never saw an oestrus fossil.’
Tim White
In the field of sex and reproduction, man differs from the African apes in a wide variety of ways. Among the most basic and far-reaching is the absence of oestrus.
In virtually all mammals the oestrus cycle is a vital and efficient system of regulating sexual activity by causing the females to become receptive at certain fixed intervals. It conduces to species survival by ensuring that the greatest procreative efforts take place at the time when they are most likely to produce results, namely, around the time of ovulation. At other times sexual activity is, for the purposes of natural selection, a waste of time and energy and semen.
Oestrus provides a clear signal – olfactory, visual or behavioural – of the time when the female will welcome and co-operate with sexual initiatives by the male. In many species the signal is the trigger of desire in the male. It thus promotes harmony between the sexes by maximising the chances that all sexual encounters are reciprocal, amicable and mutually rewarding.
Since oestrus does not normally occur during pregnancy and lactation, it ensures that the female can concentrate on her maternal role at the proper time without being distracted either by her own desires or by male importunity. Among larger animals native to temperate zones, the cycle is an annual one, and sexual activity is so timed that all the young are born at a time of the year when food will be plentiful. In regions where seasonal variations in the food supply are less extreme, the cycle can be shorter; within a single breeding population, the timing of oestrus and childbirth in different females is staggered. In the African apes the average length of the cycle is 31–32 days in the gorillas and 35–36 days in the chimpanzee, as compared to 29 days in humans.
The oestrus system, in short (like fur and quadrupedalism), is a device which has evolved over millions of years, is almost universal among mammals, has multiple advantages and works very successfully.
These advantages have been lost to our species since oestrus is not a feature of the human sexual cycle. In most other respects the female cycle in humans conforms to the anthropoid norm. As in apes, menstruation marks the end of the cycle, and ovulation occurs roughly halfway through it. But in humans alone ovulation is unaccompanied by any peak of desire in the female, and is not detectable by the male.
From time to time attempts are made to prove that oestrus is still operative in humans, but the evidence offered is usually anecdotal rather than scientific. One survey, based on a questionnaire distributed among students, reported a tendency for sexual activity in females to be slightly more frequent somewhere around the time of ovulation rather than during other parts of the cycle. But this fact is largely accounted for by avoidance of sexual activity during and immediately prior to menstruation. In any case, the tendency if it exists at all is so vestigial as to be of no practical value. This peculiarly human phenomenon is known to scientists as ‘concealed ovulation’.
As with other anomalous human features, there is a tendency to interpret the suppression of oestrus not as the loss of a valuable heritage, but as a kind of evolutionary Great Leap Forward which only one species is sufficiently advanced to have achieved. Oestrus is sometimes thought of as one of the more ‘bestial’ aspects of animal sex, and associated with promiscuous couplings. But it is equally adaptive in strictly monogamous species, such as the wolf and the gibbon, for a female’s peaks of desire to coincide with her peaks of attractiveness and fertility. It is hard to think of anything better designed to cement the pair bond.
The need to prove the superiority of an anoestrous species was hampered by the fact that no one was quite sure what had happened to the formerly periodic female libido. One theory holds that it has vanished. Another suggests that human females are not merely ‘permanently receptive’ but in effect permanently on heat. Both versions are represented as advantageous for the species.
The first version proposes that oestrus became counterproductive during the hunting/gathering stage of human evolution because the presence of sexy females interfered with male bonding. The males had to develop a
spirit of comradeship in order to co-operate on the hunt for game, and ‘… consequently they could no longer afford socially disruptive aphrodisiacs. Among the males, there would be occasion enough for tension – disputes over distribution of meat, quarrels over desirable mates – without having the tension increased by the sight and scent of oestrous females.’
The second version suggests that food supplies on the savannah were scarcer and less dependable than in the forest; that rearing the young successfully demanded active co-operation from the males, best assured by a system of pair-bonding; and that the best way of cementing the pair-bond and inducing the males to contribute to the food supply was for the females to reward them by becoming ‘permanently receptive’ and ‘making sex sexier’.
The theory won widespread acclaim, and it is often retailed in the textbooks as representing the conventional wisdom. It was not embraced because of any inherent probability. It was welcomed because if you start with the axiom that the loss of oestrus must be interpreted as a gain rather than a loss, then you are reduced to grabbing at straws. The theory depends on a chain of implicit assumptions. Not one of them is proven, and some of them are absurd.
The theory implies, firstly, that being ‘permanently receptive’ was a new departure in hominid females, not found in other apes. If ‘receptive’ means merely permitting copulation rather than desiring or soliciting it, this is not true. A young female chimpanzee not in oestrus will frequently defuse the anger of an aggressive mate by presenting her rump and allowing him to mount her, though the mounting does not always lead to intromission. The interaction has more to do with appeasement than with sexual bonding (a young male will perform exactly the same gesture when threatened with attack), but in this sense female chimpanzees are receptive most of the time.
Secondly, it implies that male co-operation in the care of the young must be secured by a system of rewards such as unlimited sexual access. This is a fallacy. Male Emperor penguins do not have to be bribed to huddle fasting on the ice, incubating eggs through the blizzards of the long, dark Antarctic winter. Where paternal instinct operates, it is a categorical imperative in the same way as maternal instinct. There is no known species in which it has to be reinforced by sweeteners of any kind.
The Scars of Evolution Page 14
