The Organic Evolution Of The Pinna In Primates And Humans

Charles Darwin (1809-1882), in his masterpiece The Descent of Man, and Selection in Relation to Sex,2 wrote:

The whole external shell of the ear may be considered a rudiment, together with the various folds

CHAPTER CONTENTS Introduction 37

The organic evolution of the pinna in primates and humans 37

Anthropometry of the outer ear 40

Morphological variables 41

The shape of the outer ear as a possible expression of different modeling factors 42

The shape of the ear and the problem of the transcription of points 47

Point zero and the 'principle of alignment' according to Paul Nogier 50

Organic Evolution

and prominences (helix and anti-helix, tragus and anti-tragus &c.) which in the lower animals strengthen and support the ear when erect, without adding much to its weight ...

Rudimentary organs are eminently variable; and this is partly intelligible, as they are useless or nearly useless, and consequently are no longer subjected to natural selection.

One of these 'rudiments' became famous as the 'tubercle of Darwin' but the true story of this distinguishing mark of the helix was told by the author himself:

The celebrated sculptor, Mr. Woolner, informs me of one little peculiarity in the external ear, which he has often observed both in men and women, and of which he perceived the full signification. The peculiarity consists in a little blunt point, projecting from the inwardly folded margin or helix. These points not only project inwards, but often a little outwards, so that they are visible when the head is viewed from directly in front or behind. They are variable in size and somewhat in position, standing either a little higher or lower; and they sometimes occur on one ear and not on the other ...we may safely conclude that it is a similar structure - a vestige of formerly pointed ears - which occasionally reappears in man. (Fig. 3.2)

Darwin2 made several comparative observations in primates and could not at his time understand the reason of the loss of mobility of the outer ear. He wrote:

The ears of the chimpanzee and orang are curiously like those of man, and I am assured by the keepers in the Zoological Gardens that these animals never move or erect them; so that they are in equally rudimentary condition, as far as function is concerned, as in man. Why these animals as well the progenitors of man, should have lost the power of erecting their ears we cannot say. It may be, though I am not quite satisfied with this view, that owing to their arboreal habits and great strength they were but little exposed to danger, and so during a lengthened period moved their ears but little, and thus gradually lost the power of moving them.

Fig. 3.2 Human ear, modeled and drawn by the sculptor Mr Woolner.2 Point marked a = the 'projecting point' mentioned by Darwin.

Darwin's observations influenced all following anthropologists who tried to find an answer to this query. Two famous researchers, the German Gustav Schwalbe (1844-1916) and Rudolf Martin (1864-1925) from Switzerland, demonstrated that the process of regression in primates and in man consisted essentially in a shortening of the tip of the ear, causing an inward curl of the helix and a rise of the upper branch of the anthelix. Schwalbe3 measured the base of the ear (a-b in his study) and the so-called true length (d-c), from the tragus to Darwin's tubercle, and compared the morphological ear index (base x 100/true length) in various mammals and primates (Fig. 3.3; Box 3.1).

Box 3.1 The Morphological Ear Index measured in different mammals: the higher scores belong to primates and humans, according to Schwalbe3

Hare

21.3

Macacus rhesus

93.0

Antelope

27.6

Chimpanzee

105-107

Pig

35.4

Orang-utan

122

Cat

58.8

Gorilla

125

Lemur macaco

76.0

Human

130

Cynocephalus

84.0

Humans had the highest score not only because length was reduced but also because the base of the ear was significantly larger. This increase was actually responsible for the progressive loss of mobility observed by Darwin. Nevertheless, the significance of the raised anthelix and the curled helix remained obscure and it was only several decades later that an explanation was found, thanks to modern technology.

The unanswered questions in recent times were fundamentally two:

• how could the human ear localize the direction of sound without moving the pinna?

• to what extent is the strange shape of the ear involved in the understanding of speech?

Several hypotheses were proposed on both subjects but the most interesting conclusions were probably those reached by the psychiatrist Johann Burchard4-6 of Hamburg, who dedicated 30 years of his life to these issues. In a series of experiments

Fig. 3.3 The human outer ear compared with that of a baboon and that of a calf according to Schwalbe.3 The human outer ear (represented by an unbroken line; a c b); the outer ear of a baboon (dashed line; a c1 b) and the outer ear of a calf (dotted line; a c2 b) were overlapped maintaining the same base length (a d b).

Fig. 3.3 The human outer ear compared with that of a baboon and that of a calf according to Schwalbe.3 The human outer ear (represented by an unbroken line; a c b); the outer ear of a baboon (dashed line; a c1 b) and the outer ear of a calf (dotted line; a c2 b) were overlapped maintaining the same base length (a d b).

Volt 10

Fig. 3.4 (A) Specimen of 2-peak sound-wave transmission: in this case interval A was 0.22 ms = 7.2 cm. (B) Specimen of 3-peak sound-wave transmission: in this case interval A was 0.18 ms = 6.0 cm; interval B was 0.12 ms = 3.9 cm.) (From Burchard.6)

Fig. 3.4 (A) Specimen of 2-peak sound-wave transmission: in this case interval A was 0.22 ms = 7.2 cm. (B) Specimen of 3-peak sound-wave transmission: in this case interval A was 0.18 ms = 6.0 cm; interval B was 0.12 ms = 3.9 cm.) (From Burchard.6)

he noticed that a square-shaped click of 0.14 ms produced by the computer could be recorded with two or three peaks by a miniaturized microphone in the external acoustic meatus (Fig. 3.4A, B). Of 53 people (106 ears) placed in a noiseless room, 14% showed two peaks and 86% three peaks. In both groups interval A (between the first and second peak) and interval B (between the second and third peak) were significantly different when the direction of the sound was shifted from a perpendicular axis to the ear surface to points placed at 45° respectively in front, at the back and below (P <0.01) or up (P <0.05). These data confirmed the evidence that the human ear can actually localize sounds in space without moving the pinna.

Burchard's second hypothesis was that the three peaks could correspond to three different anatomical paths covered by the sound wave transmitted to the external meatus (Fig. 3.5). Calculating the average delays from one peak to the next, he obtained distances in cm which were significantly correlated with the length of the respective anatomical path measured with soft threads (P <0.01). He identified three possible paths: the first (I in Fig. 3.5) was the shortest way to reach the external meatus (about 0.1 ms corresponding to 3 cm) and could seemingly start from the lower concha and the wall of the anthelix; the second (II in Fig. 3.5) had a further average delay of 6.6 cm and could possibly start from the upper-middle portion of

Fig. 3.5 The three anatomical paths (I, II, III) of transmission of the sound wave to the external acoustic meatus according to Johann Burchard.

the scapha and helix tunnel; the third (III in Fig. 3.5) had a further average delay of 3 cm and could possibly originate from the lower portion of the same anatomical parts. The incomplete consistency of peak 3 was explained by Burchard by the lack in these subjects of a sufficiently pronounced depression of this anatomical portion (as for example in Fig. 3.6 or in types 3 and 4 in Fig. 3.9A). Repeating the same experiment on chimpanzees and gorillas he found that these primates had only two peaks which were similar to peaks 1 and 2 in humans. The less curled helix and the distance between anthelix and helix make the third peak, typical in humans, impossible for primates. The author concluded that the triple signal and its variations of amplitude and interval were essential for the cortical recognition of sound which is fundamental in the analysis of speech.

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