advances.sciencemag.org/cgi/content/full/1/8/1500355/DC1

Supplementary Materials for

Early hominin auditory capacities

Rolf Quam, Ignacio Martínez, Manuel Rosa, Alejandro Bonmatí, Carlos Lorenzo, Darryl J. de Ruiter,

Jacopo Moggi-Cecchi, Mercedes Conde Valverde, Pilar Jarabo, Colin G. Menter, J. Francis Thackeray,

Juan Luis Arsuaga

Published 25 September 2015, Sci. Adv. 1, e1500355 (2015)

DOI: 10.1126/sciadv.1500355

This PDF file includes:

Comparative sample composition

Preservation of early hominin specimens

CT scanning of modern human, chimpanzee, and fossil hominin specimens

Model description

Comparison of present measurements with previous studies

Fig. S1. Virtual (3D CT) reconstruction of the outer, middle, and inner ears in P.

robustus (SK 46).

Fig. S2. Model results for the effects of intraindividual measurement error on the

sound power transmission in two reconstructions of the CSJ 26 H. sapiens

individual.

Fig. S3. Model results for the effects of interindividual measurement error on the

sound power transmission in two reconstructions of the HTB 3434 P. troglodytes

individual.

Fig. S4. Block diagram of the analog electrical circuit model based on (30).

Fig. S5. Model results for sound power transmission in chimpanzees.

Fig. S6. Model results for sound power transmission in modern humans.

Fig. S7. Model results for sound power transmission in the early hominins.

Fig. S8. Model results for sound power transmission in the Middle Pleistocene

Atapuerca (SH).

Fig. S9. Model results for the magnitude of the middle ear gain (|GME|) in

modern humans.

Fig. S10. Model results for the magnitude of the middle ear gain (|GME|) in

chimpanzees.

Fig. S11. Model results for the magnitude of the middle ear gain (|GME|) in early

hominins.

Fig. S12. Model results for the magnitude of the middle ear gain (|GME|) in the

Atapuerca (SH) specimens.

Table S1. Measurements and model results for the influence of intraindividual

measurement error.

Table S2. Measurements and model results for the influence of interindividual

measurement error.

Table S3. Definition of the electrical parameters, their related anatomical

variables, the source of the value used, and the sensitivity analysis for frequencies

above 2 kHz in the model.

Table S4. Measurements in the present study compared with those reported

previously.

References (69–112)

Supplementary Materials

Comparative sample composition Most of the extant hominid individuals were chosen based on adult status and the presence of all

three ear ossicles. In the few instances of missing data, the species-specific mean value was substituted.

H. sapiens comparative sample (n = 10)

Nearly all of the H. sapiens specimens (n = 9) represent recent individuals disinterred from the

Cementerio San Jose near the city of Burgos in northern Spain. One additional individual (n = 1) is from

the Medieval site of Sepulveda near Segovia, Spain. All of the skeletal variables for the model were

preserved in seven individuals. The ossicles were not preserved in two individuals (CSJ 11 & CSJ 59),

while for a third individual (Sepulveda 622) only the ossicle masses were available. For the missing

ossicle measurements, the mean value for the rest of the H. sapiens sample was used.

Pan troglodytes comparative sample (n = 11)

The P. troglodytes individuals are from the collections housed at the Centro UCM-ISCIII de

Evolución y Comportamiento Humanos in Madrid, Spain (n = 2), the Estación Biológica Doñana in

Seville, Spain (n = 4) and the Cleveland Museum of Natural History (n = 5) in Cleveland, United States

of America. Two individuals (EBD 6842 & HTB 3437) did not preserve the ossicles, while the stapes

was missing in three additional individuals (UCM2, HTB 411 & Kiko). For the missing ossicle

measurements, the mean values from a large chimpanzee sample were used (7).

Atapuerca (Sima de los Huesos) sample (n = 5)

The Middle Pleistocene Atapuerca (SH) fossils have been dated to c. 430 kya and are members

of the Neandertal clade (69). The model parameters for five individuals from the Atapuerca (SH) sample

have been published previously (14, 15). In the present study, we have used the chimpanzee value for

the mass of the stapes since the dimensions of this bone most closely approximate those of chimpanzees,

rather than modern humans (70).

Preservation of early hominin specimens

Modeling of auditory capacities was limited to three early hominin specimens: SK 46, attributed

to P. robustus, and Stw 98 and Sts 25, both attributed to A. africanus. These individuals were chosen

based on their adult status, preservation of most of the relevant skeletal variables, absence of matrix

infilling and fairly clear taxonomic attributions. Nevertheless, it was necessary to estimate a few

dimensions in these individuals. For any missing variables, the species-specific mean value was used

relying on data from a number of less-complete early hominin specimens. These measurements were

mainly taken on the original specimens, complemented occasionally by measurements on CT scans.

Here we provide more detail on preservation of the skeletal variables in all of the early hominin

specimens.

Auditory ossicles

A few auditory ossicles are known from both P. robustus and A. africanus (23, 25, 26). The P.

robustus specimen SKW 18 preserves a complete ossicular chain (malleus, incus, stapes), making it

possible to calculate the middle ear lever ratio. A second incus is also known for P. robustus (SK 848),

but was not used in the present study since it is incomplete and does not provide data on the functional

length. For A. africanus, there is a complete malleus and partial stapes associated with individual Stw

255. A second complete stapes is also known from Stw 151.

We used the middle ear lever ratio (1.36) from the P. robustus specimen SKW 18 (26) for all of

the fossil specimens. Although there is no known incus for A. africanus (and, hence, the lever ratio

cannot be measured directly), the preserved malleus is very similar in its overall morphology and

dimensions to the malleus in P. robustus.

For the masses of the ossicles, the malleus in both early hominin taxa is human-like in its overall

proportions, while the incus (only known for P. robustus) and stapes are more similar to chimpanzees in

overall size and proportions (26). Thus, we used the mean H. sapiens mass for the malleus (23.7 mg),

based on the human sample used in the present study. We also used the mean P. troglodytes masses

from a large sample of chimpanzee ossicles for the incus (n = 58; mean = 19.1 mg) and stapes (n = 35;

mean = 1.2 mg). These same ossicular masses were used for all of the early hominin specimens.

SK 46

This is a fairly complete adult cranium which derives from the Member 1 Hanging Remnant at

Swartkrans (71) and dates to between 1.8-1.9 Ma (72). This specimen has been attributed to P. robustus

(10, 73, 74). While the left side is largely intact and undistorted, the right side of the cranial vault has

been crushed inward due to taphonomic factors. The middle ear cavity is exposed on the right side,

allowing the tympanic ring, oval window and structures of the middle ear to be measured and

photographed. Rak & Clarke (24) have commented previously on the outer and middle ear anatomy.

Virtual reconstruction has allowed for the analysis of the entire outer and middle ear. The original

specimen was studied in the Ditsong National Museum of Natural History in Pretoria, South Africa.

It was possible to measure all of the model variables in this individual, except the lever ratio and

the masses of the ossicles. These variables were estimated as described above.

Stw 98

This is a fairly complete right temporal bone which preserves the petrous, mastoid and most of

the tympanic portions, but little of the squama. It derives from Member 4 at Sterkfontein and most likely

dates to between 2.0-2.6 Ma (75). The taxonomic affinities of the specimen suggest it is clearly

distinguishable from Paranthropus remains (10, 76), and it can probably be attributed to A. africanus.

The original specimen was studied in the School of Anatomical Sciences at the University of

Witwatersrand in Johannesburg, South Africa.

It was possible to measure all of the model variables in this individual, except for the cross-

sectional area of the EAC, the lever ratio and the masses of the ossicles. For the cross-sectional area of

the EAC, we used the species mean value. The lever ratio and the masses of the ossicles were estimated

as described above.

Sts 25

This is a partial cranium and natural endocast of A. africanus (77). The specimen preserves part

of the cranial base, and the left temporal bone is nearly complete. The specimen derives from Member 4

at Sterkfontein and dates to between 2.0-2.6 Ma (75). The original specimen was studied in the Ditsong

National Museum of Natural History in Pretoria, South Africa.

It was possible to measure all of the model variables in this individual, except for the volume of

the mastoid air cells, the length of the aditus and the radius of its exit, the lever ratio, the stapes footplate

area and the masses of the ossicles. We used the value from Stw 98 for the volume of the mastoid air

cells and the species mean value for the aditus variables and the stapes footplate area. The lever ratio

and the masses of the ossicles were estimated as described above.

SK 47

This is a largely complete cranial base with maxilla and associated dentition which derives from

the Member 1 Hanging Remnant at Swartkrans (71) and dates to between 1.8-1.9 Ma (72). The

taxonomy of this specimen has been a subject of debate. It was originally considered to represent a

juvenile Paranthropus individual (73). Subsequently, Olson (78) attributed it to early Homo, but Tobias

(79) did not include it among his list of early Homo fossils from this site. Analysis of the dental anatomy

suggested affinities with A. africanus (80), but several studies have demonstrated clear affinities with P.

robustus (10, 74, 81, 82). The original specimen was studied in the Ditsong National Museum of Natural

History in Pretoria, South Africa.

The middle ear is exposed on the right side, and the oval window can be photographed and the

stapes footplate area estimated.

SK 52

This specimen consists of a partial cranium which preserves the right temporal bone. It derives

from the Member 1 Hanging Remnant at Swartkrans (71) and dates to between 1.8-1.9 Ma (72). It has

been assigned to P. robustus (74, 83). The original specimen was studied in the Ditsong National

Museum of Natural History in Pretoria, South Africa.

The cross-sectional area of the EAC can be measured directly on the original fossil.

SK 848

This is a fragment of right temporal which derives from the Member 1 Hanging Remnant at

Swartkrans (71) and dates to between 1.8-1.9 Ma (72). It preserves the EAC up to the tympanic

membrane, as well as the aditus of the middle ear cavity. In addition, the right incus was recovered from

this specimen and has been described previously (23). Although Sherwood et al. (84) included this

specimen within H. erectus, it has previously been convincingly identified as P. robustus by Rak and

Clarke (24). The tympanic ring is present at two points, anteriorly and posteriorly, at the medial break of

the EAC, allowing this structure and the external auditory canal to be measured. The incus is missing the

long process, but is otherwise complete. The original specimen was studied in the Ditsong National

Museum of Natural History in Pretoria, South Africa.

It was possible to measure the length and cross-sectional area of the EAC and the tympanic

membrane area directly on the original specimen.

SK 879

This specimen consists of fragments of the right and left petrous portions with the middle ear

structures exposed. It derives from the Member 1 Hanging Remnant at Swartkrans (71) and dates to

between 1.8-1.9 Ma (72). This specimen has been attributed to P. robustus (10, 24). The right oval

window is only partially preserved, but is complete on the left side. The middle ear anatomy has been

described previously (24). The original specimen was studied in the Ditsong National Museum of

Natural History in Pretoria, South Africa.

The left oval window was photographed and the area of the stapes footplate was estimated.

SK 14003

This is a distorted cranium which preserves most of the left ear structures, but is missing the

lateral EAC. It derives from the Member 1 Hanging Remnant at Swartkrans (71) and dates to between

1.8-1.9 Ma (72). This specimen has been attributed to P. robustus (85). The original specimen was

studied in the Ditsong National Museum of Natural History in Pretoria, South Africa.

It was possible to measure the cross-sectional area of the EAC, the area of the tympanic

membrane, the volume of the tympanic cavity and the length and radii of the aditus in a virtual

reconstruction based on CT scans.

SKW 18

This is a largely complete cranial base which preserves both right and left temporal bones. It

derives from the Member 1 Hanging Remnant at Swartkrans and dates to between 1.8-1.9 Ma (72). This

specimen has been attributed to P. robustus (82, 86). The top of the petrous portion on the right side has

separated from the cranial base, exposing the tympanic membrane and middle ear. All three ear ossicles

were recovered from the right side (26), and the stapes is visible within the breccia still adhering to the

middle ear cavity on the left. The original specimen was studied in the Ditsong National Museum of

Natural History in Pretoria, South Africa.

It was possible to measure the length and cross-sectional area of the EAC in a virtual

reconstruction based on CT scans. However, the aditus is damaged and the mastoid process is

incomplete, limiting measurements in these regions. The tympanic membrane area was measured

directly on the original specimen. The malleus/incus lever ratio and stapes footplate area were also

measured on the preserved auditory ossicles in this individual.

SKW 2581

This is an adult right temporal bone which preserves the petrous, mastoid and tympanic portions.

It derives from the Member 1 Hanging Remnant at Swartkrans and dates to between 1.8-1.9 Ma (72).

This specimen has been attributed to P. robustus (87). The original specimen was studied in the Ditsong

National Museum of Natural History in Pretoria, South Africa.

Damage to the mastoid process yields only a minimum value for the volume of the mastoid

antrum and air cells. However, the length and cross-sectional area of the EAC were measured directly on

the original specimen.

Sts 5

This is a complete cranium of A. africanus (77) which preserves both temporal bones. The

specimen derives from Member 4 at Sterkfontein and dates to between 2.0-2.6 Ma (75). The original

specimen was studied in the Ditsong National Museum of Natural History in Pretoria, South Africa.

The EAC and middle ear spaces are filled with matrix on both sides. However, it was possible to

measure the length of the EAC and the area of the tympanic membrane in the virtual reconstruction

based on CT scans.

Sts 71

This is the right half of a cranium of A. africanus which preserves the lateral portion of the EAC

and part of the mastoid process (77). The specimen derives from Member 4 at Sterkfontein and dates to

between 2.0-2.6 Ma (75). The original specimen was studied in the Ditsong National Museum of Natural

History in Pretoria, South Africa.

It was possible to measure only a minimum value for the length of the EAC.

Stw 151

This specimen is comprised of numerous cranial fragments and a mixed dentition of a single

five-year-old individual. The remains derive from either Member 4 or 5 at Sterkfontein, and date to

≤2.0-2.6 Ma (75). Both Spoor (10) and Moggi-Cecchi et al. (88) have suggested it represents a hominin

which is more derived toward Homo than the bulk of the A. africanus sample. The petrous portion of the

left temporal bone is preserved and the middle ear is exposed, allowing the oval window, round window

and middle ear structures to be photographed. A stapes was also recovered from the left temporal and

has been published previously (25). The original specimen was studied in the School of Anatomical

Sciences at the University of Witwatersrand in Johannesburg, South Africa.

It was possible to measure only a minimum value for the length of the EAC. Nevertheless, the

cross-sectional area of the EAC and the stapes footplate area were preserved and measured on the

original fossil.

Stw 255

This specimen comprises several fragments of both right (formerly, Stw 254, 255, 259, and 263)

and left (formerly, Stw 256, 260, and 266) temporal bones which belong to a single individual. These

specimens are also likely associated with either the partial cranium Stw 252 or the frontal bone fragment

Stw 265 (76). It derives from Member 4 at Sterkfontein and dates to between 2.0-2.6 Ma (75). This

specimen is said to differ substantially from the A. africanus hypodigm from Sterkfontein, and may

represent a new, as yet undefined, species (10, 76). Clarke (89) has also argued that the Stw 252 cranium

can be distinguished from the A. africanus hypodigm, showing features which align it with P. robustus.

Nevertheless, Moggi-Cecchi & Collard (25) included Stw 255 within A. africanus. The right side

preserves the petrous portion, IAM and exposed oval window, with the broken stapes footplate still in

situ. The right stapes (Stw 255b) was also recovered but missing the footplate. The left side preserves

the petrous portion, including the IAM, EAC and EAM and has also yielded a complete malleus (Stw

266b). The original specimen was studied in the School of Anatomical Sciences at the University of

Witwatersrand in Johannesburg, South Africa.

The stapes footplate area was estimated in this specimen from the preserved oval window.

Stw 329

This is a right temporal bone preserving the petrous, mastoid and tympanic elements attributed to

a juvenile individual. The taxonomic affinities of this specimen have been difficult to establish (10, 76),

partially due to its young age at death. It derives from Member 4 at Sterkfontein and dates to between

2.0-2.6 Ma (75). The original specimen was studied in the School of Anatomical Sciences at the

University of Witwatersrand in Johannesburg, South Africa.

It was possible to measure all of the model variables in this individual, except for the lever ratio

and the masses of the ossicles. Nevertheless, since Stw 329 is a juvenile individual, the EAC is not fully

formed, and pneumatisation of the mastoid process is incomplete. Thus, these variables were not

included in the calculation of the species mean values.

Stw 370

This is a fragment of the right temporal bone which preserves the roof of the EAC and part of the

mastoid process and has been attributed to A. africanus (76). It derives from Member 4 at Sterkfontein

and dates to between 2.0-2.6 Ma (75). The original specimen was studied in the School of Anatomical

Sciences at the University of Witwatersrand in Johannesburg, South Africa.

It was possible to measure only a minimum value for the length of the EAC.

Stw 499

This is a fragmentary temporal bone associated with the partial skull Stw 498. The specimen was

included in the inventory of hominin cranial fossils from Sterkfontein (76) but was not described. Grine

(65) sees the taxonomic affinities of the Stw 498 partial skull as falling with A. africanus, but Clarke

(90) attributes the same specimen to a “second species” at Sterkfontien.

The cross-sectional area of the EAC and the area of the tympanic membrane were measured in

the virtual reconstruction based on CT scans.

Stw 505

This is a large well-preserved adult cranium which has been attributed to A. africanus (91, 92).

Both temporal bones are present, but the left side is better preserved. It derives from Member 4 at

Sterkfontein and dates to between 2.0-2.6 Ma (75). The original specimen was studied in the School of

Anatomical Sciences at the University of Witwatersrand in Johannesburg, South Africa.

Although matrix filling prevented measuring the volumes of the middle ear spaces, it was

possible to measure the length of the EAC.

CT scanning of modern human, chimpanzee, and fossil hominin specimens Scanning parameters

Scanning parameters of the Atapuerca (SH) fossils have been published previously (14, 15).

Computed Tomography (CT) images for all of the modern humans and most of the chimpanzees in the

comparative samples were captured with a YXLON Compact industrial multi-slice CT scanner

(YXLON International X-Ray GmbH, Hamburg, Germany), housed at the Universidad de Burgos

(Spain). The Field of View (FOV) was restricted to the temporal bone to maximize the spatial resolution

of the scans with FOV values that ranged from 111.2-148.3 mm. Slices were obtained as a 1024 x 1024

matrix of 32 bit Float format for processing, and the number of slices ranged from 211-422, depending

on the individual. Slice thickness was 1.0 mm, with an interslice distance of 0.2 mm and final pixel size

ranged from 0.109-0.145 mm. Five of the chimpanzee specimens were scanned with a

Siemens/Definition medical CT scanner housed at the University Hospital's Case Medical Center in

Cleveland, Ohio (USA). Similar scanning parameters (slice thickness = 1.0 mm; interslice distance = 0.2

mm) and a narrower FOV (50-62 mm) were used for these individuals, and the slices were obtained as a

512 x 512 matrix in Dicom format. Pixel size in these individuals ranged from 0.098-0.121 mm.

The early hominin specimens were scanned with medical CT scanners housed at the Little

Company of Mary Hospital in Pretoria (Siemens Sensation 16 scanner) and the Helen Joseph Hospital

(Philips Brilliance 16 scanner) in Johannesburg. Scanning parameters included a scanner energy of 120

kV and FOV values that ranged from 56-170 mm. Slices were obtained as a 512 x 512 matrix in Dicom

format and the number of slices ranged from 149-443, depending on the individual. Slice thickness was

1.0 mm, with an interslice distance of 0.2 mm and final pixel size mainly ranged from 0.109-0.184 mm.

SK 14003 was scanned at a slightly lower resolution (FOV = 170; pixel size = 0.332 mm).

Thresholding procedure for virtual reconstructions and measurements

We relied on the half maximum height (HMH) thresholding protocol to differentiate bone from

air and to aid in identifying the boundaries in the ear structures (Figure S1). Thresholding was based on

the Hounsfield units (gray values), and the boundary of bone and air was determined as the mean of the

first maximum and minimum grey scale values along a profile line. Given that bone density is somewhat

variable in the skeleton (93), profile lines were drawn for the mastoid process and the EAC separately.

These profile lines were drawn in a transverse slice located approximately at the superoinferior midpoint

of the external auditory meatus and ending in air on either side of the bony structures. Thus, the

thresholding lines sampled both cortical (EAC) and trabecular bone (mastoid air cells). Separate

HMH thresholds were then calculated for each structure, and these two HMH values were averaged to

produce a mean HMH threshold to establish the boundary of bone and air in middle and outer ear

structures. Thus, our thresholding protocol is designed for the outer and middle ear structures and no

measurements of the inner ear were taken on CT scans.

Some of the early hominin specimens contained a matrix infilling in some of the ear structures,

complicating their identification and segmentation. Thus, measurements were avoided when the matrix

infilling was not clearly distinguishable from bone. The specimens we chose to model generally had the

EAC and tympanic cavity largely free of matrix. Matrix infilling was more problematic for the mastoid

antrum and associated air cells and it was only possible to measure this variable in a single individual for

each early hominin taxon.

Intraobserver and interobserver measurement error

For the ossicles, intraobserver error in measuring the stapes footplate area from photographs was

experimentally determined to be 2.5%, in close agreement with intraobserver error in other studies (94).

Although we have not performed an interobserver error analysis, comparison of ossicular dimensions

across a wide range of studies using disparate methodologies shows considerable agreement in the data

(95). Given the resolution of the CT scans in the present study, measurement error for CT-based

measurements has been estimated as ± 0.1 mm for linear dimensions (96). Volumetric dimensions

mainly relate to the middle ear air spaces (tympanic cavity, and mastoid antrum/air cells) and these have

only a very weak influence on the model results. Variation by as much as 50% produces a difference of

≤1 dB (see Table S3), suggesting the model results are not heavily dependent on the volumetric

dimensions. To assess the influence of intraobserver and interobserver measurement error on the model

results, we performed two analyses.

One H. sapiens individual (CSJ 26) was virtually reconstructed twice by the same researcher.

The measurements are very similar in both reconstructions (Table S1). The largest difference in the

anatomical measurements seems to be in the length of the aditus, but this variable has only a weak effect

on the model results (Table S3; Figure S2). The lower and upper limits of the occupied band varied by

only 25 and 130 Hz, respectively, while the bandwidth itself differed by 105 Hz. The resultant sound

power transmission values from 0.5-5.0 kHz differed by a maximum of 1.6 dB at 5.0 kHz.

In addition, one chimpanzee individual (HTB 3434) was reconstructed twice by two different

researchers. Again, the measurements are very similar in both reconstructions (Table S2). The largest

absolute difference was in the mastoid air cell volume, likely associated with a difference in the

orientation of the plane of the exit of the aditus (P1 in Figure 1). However, this variable has only a weak

effect on the model results (Table S3; Figure S3). The lower and upper limits of the occupied band

varied by only 10 and 5 Hz, respectively, while the bandwidth differed by only 5 Hz. The resultant

sound power transmission values from 0.5-4.0 kHz differed by <1 dB. The differences were higher at

4.5 & 5.0 kHz, but this is primarily related to a slight displacement of the point of minimum sensitivity

(Figure S3).

Model Description

The use of electrical circuits to model sound power transmission through the outer and middle

ear is a common practice in auditory research (30, 97-101). Here we have relied on a slightly modified

version of the model published by Rosowski (30), to estimate the sound power transmission through the

outer and middle ears. Figure S4 shows a simplified block diagram of the model provided by Rosowski

(30), which we describe below. The model described in (30) has been slightly modified, to take into

account more recent knowledge found in the literature.

The concha is modeled as an exponential horn. The smaller cross-sectional area is equivalent to

the cross-sectional area of the ear canal. The model uses a two-port network, described with

transmission parameters that can be estimated from physical measurements. The ear canal is modeled as

a two-port network, with transmission parameters. The values of these parameters can be estimated from

the length of the ear canal and its radius. It performs as a resonant tube, with a resonant frequency that

depends on the length of the canal, and bandwidth related to the cross-sectional area.

The middle ear cavity is modeled with a four acoustic elements circuit, in the same way as that

proposed by Kringlebotn (99) and Rosowski (30). The first element is a capacitor, that accounts for the

compliance of the tympanic air space located directly behind the tympanic membrane. It is connected in

parallel to the equivalent electrical circuit of a Helmholtz resonator as in Rosowski (30), representing the

aditus ad antrum and the mastoid air cell cavities. The first modification we have introduced in the

model refers to the parameters of the elements representing the aditus ad antrum and mastoid air cell

cavities. The electrical model is composed of a capacitor, representing the compliance of the mastoid air

cells, a resistance and an inertance, representing the aditus ad antrum. These parameters are calculated

from physical measurements. For modeling purposes, we have considered the entrance to the

epitympanum as representing the entrance to the aditus ad antrum and the exit into the mastoid antrum

as representing the exit from the aditus ad antrum (see Figure 1). Subsequently, we have calculated the

radius of the neck of the resonator as the average between the radii at both extremes (entrance and exit)

of the aditus ad antrum. The overall middle ear cavity model is connected in a series branch, and is an

antiresonant circuit, which gives rise to a notch at the antiresonant frequency, which depends on the

physical parameters.

No modifications have been introduced in the tympanic membrane-malleus network, which is the

same as that used in (30) and (99). The ossicular chain is modeled with a series branch composed of a

resistance, compliance and mass, that jointly model the mass of the malleus and incus, the compliance

and damping with the supporting ligaments. After that, a transformer is included, and the transformer

parameter is the ratio of the incus-malleus lengths. A shunt branch is connected to the transformer, with

a capacitor and a resistor, that accounts for the loss of stapes velocity from compression of the ossicular

joints. The ossicular chain model is completed with the mass of the stapes, and another transformer,

whose parameter is the stapes footplate area. Most of the elements in Rosowski’s model of the ossicular

chain were chosen by fitting the model to some middle-ear data. We have measured the mass of the

malleus-incus and the stapes, and these have been used to obtain the corresponding circuit elements.

Finally, the model is completed by the annular ligament block, where no modifications have been

introduced compared to Rosowski’s model (30), and the cochlear input impedance (Zc). Another

modification we have introduced into the model refers to the cochlear input impedance, which has been

directly measured in 11 human cadaver ears by Aibara et al. (66), who found a flat, resistive cochlear

input impedance with an average value of 21.1 GΩ from 0.1-5.0 kHz. Since the model yields accurate

results up to 5.0 kHz, we have used this empirical value for the cochlear input impedance, rather than

the value provided in the original model (30).

Model results for sound power (dB) at the entrance to the cochlea relative to P0=10-18 W for an

incident plane wave intensity of 10-12 W/m2 for all of the individuals in the current study are presented in

Figures S5-S8. To ensure the reliability of our model, we have compared the theoretical middle ear

pressure gain (GME) we have obtained for modern humans (14) with those measured experimentally

(66, 102), finding no significant differences. The GME for all specimens in the present study are

provided in figures S9-S12. The electrical parameters used in the model are associated with anatomical

structures of the ear. Some of these parameters are related with skeletal structures accessible in fossils,

while others are related with soft tissues which are not preserved in fossil specimens. Table S3 shows

the relationship between the electrical parameters and the anatomical structures, together with an

analysis of the sensitivity of the model above 2 kHz to each variable.

We have measured or accurately estimated in the fossil specimens all of the skeletal variables

included in the model (Table S3). Since the model requires values for all the variables, the respective

value for modern humans (30, 66) has been used for the remaining soft-tissue related variables which

cannot be measured in fossil specimens (Table 3). It is important to note that only seven of these have an

appreciable effect on the model results above 2 kHz (labeled as medium and high in Table S3).

Although our results are not a true audiogram, there is a strong correlation between sound power

transmission through the outer and middle ear and auditory sensitivity to different frequencies (31-33).

Indeed, the results for sound power transmission in the modern human and chimpanzee comparative

samples agree with the published audiograms for these species. Thus, it is reasonable to conclude that

the skeletal differences between humans and chimpanzees can explain an important part of the

interspecific differences in their patterns of sound power transmission in the outer and middle ear.

Therefore, these skeletal differences can be used to approach the sound power transmission pattern in

closely-related fossil human species. This model has been previously applied to reconstruct the auditory

capacities in the Middle Pleistocene hominins from the Sima de los Huesos in the Sierra de Atapuerca in

northern Spain (14, 15).

Analysis of sensitivity of the model

We performed an analysis of sensitivity of the model to determine the influence of the individual

variables on the model results above 2 kHz (Table S3). Sensitivity is related to the difference in the

value for sound power at the entrance to the cochlea (in dB) obtained by increasing and decreasing the

individual anatomical variable or electrical parameter by 50%. Sensitivity has been classified into three

broad groupings: low (≤1dB difference), medium (>1 to ≤3dB difference), and high (>3 dB difference).

Regarding the skeletal variables which can be measured or estimated in fossil specimens, the

model has a high sensitivity to the length (LEAC) and cross-sectional area (AEAC) of the external auditory

canal, the middle ear lever ratio (LM/LI) and the area (ATM) and mass (LT1) of the tympanic membrane.

The model results show a medium sensitivity to the masses of the malleus and incus (MM + MI) and the

area of the stapes footplate (AFP). The mass of the stapes (MS), the volumes of the tympanic cavity

(VMEC) and mastoid antrum and connected air spaces (VMA) and the aditus ad antrum variables (LAD,

RAD), all show only a weak influence (low sensitivity) on the model results.

Although the model results show a high or medium sensitivity for a few additional variables,

these were held constant in all of the taxa under study. Thus, differences between taxa in the model

results reflect variation in the skeletal variables only. In general, variables of the outer ear and ear

ossicles have a stronger influence on the model results, while the middle ear spaces (tympanic cavity,

mastoid antrum and air cells and aditus ad antrum) have a much weaker influence on the results.

Comparison of present measurements with previous studies

Table S4 compares the data in the present study with that available from the literature (14, 26,

34-37, 39, 40, 70, 97, 103-112). For modern humans, comparative data are available for all of the

measurements except those related with the aditus ad antrum. Most of the mean values in the present

study compare favorably with those reported previously by other researchers using a variety of

measurement techniques. The mean volume of the mastoid air cells is toward the lower end of the range

of variation reported previously, but this is partially affected by the presence of one very small

individual (CSJ 20 = 0.52 cm3). Nevertheless, the values reported in other studies are also highly

variable, suggesting a large degree of variation exists in this variable in modern humans. The length of

the EAC is somewhat shorter than reported in previous studies, but most of the specimens do fall within

the range of variation reported previously. The stapes footplate area in the present sample is also lower

than reported previously, but within the range of variation in modern humans. For chimpanzees, fewer

data are available, mainly related to the length of the EAC and the dimensions and masses of the

ossicles. All of the mean values in the present study compare favorably with the data reported previously

(Table S4).

Figure S1. Virtual (3D CT) reconstruction of the outer, middle, and inner ears in P. robustus (SK

46). The external auditory canal (yellow), middle ear cavity (green), aditus ad antrum (dark blue),

mastoid antrum and connected air cells (purple), inner ear (red) and a portion of the Eustachian tube

(brown) are indicated.

Figure S2. Sound power (dB) at the entrance to the cochlea relative to P0=10-18 W for an incident plane

wave intensity of 10-12 W/m2 in two reconstructions of the CSJ 26 H. sapiens individual.

Figure S3. Sound power (dB) at the entrance to the cochlea relative to P0=10-18 W for an incident plane

wave intensity of 10-12 W/m2 in two reconstructions of the HTB 3434 P. troglodytes individual.

!

Equivalent+pressure+

source+

Concha+horn+

Ear+canal+tube+

Tympanic+membrane+and+

ossicles+Middel+ear+cavity+

Annular+ligament+

Cochlea+

HEAD,"BODY(AND(EXTERNAL(EAR" MIDDLE(EAR" INNER(EAR"

Figure S4. Block diagram of the analog electrical circuit model based on (30).

Figure S5. Model results for sound power transmission in chimpanzees. Results for EBD 15772, EBD

15774, HTB 1769 and UCM2 were published previously (Martínez et al., 2012).

Figure S6. Model results for sound power transmission in modern humans. Results for CSJ 2, CSJ 16

and CSJ 20 were published previously (Martínez et al., 2012).

Figure S7. Model results for sound power transmission in the early hominins.

Figure S8. Model results for sound power transmission in the Middle Pleistocene Atapuerca (SH)

hominins.

Figure S9. Model results for the magnitude of the middle ear gain (|GME|) in modern humans.

Figure S10. Model results for the magnitude of the middle ear gain (|GME|) in chimpanzees.

Figure S11. Model results for the magnitude of the middle ear gain (|GME|) in early hominins.

Figure S12. Model results for the magnitude of the middle ear gain (|GME|) in the Atapuerca (SH)

specimens.

Table S1. Measurements and model results for the influence of intraindividual measurement error

VMA VMEC LAD RAD1 RAD2 ATM LEAC AEAC LM/LI AFP MM+MI MS

Volume Volume Radius Radius Area of Length of Cross-Sectional Malleus/Incus Area of Mass of

Mastoid Tympanic Length of of Aditus of Aditus Tympanic External Ear Area of Lever Stapes Malleus+ Mass of

Air Cells Cavity Aditus Exit Entrance Membrane Canal EAC Ratio Footplate Incus Stapes

Species cm3 cm3 mm mm mm mm2 mm mm2 mm2 mg mg

CSJ 26 (1) Homo sapiens 6.58 0.47 3.7 2.4 2.9 60.7 21.8 33.5 1.16 2.51 41.3 1.4

CSJ 26 (2) Homo sapiens 6.95 0.46 4.7 2.0 2.7 57.9 22.8 33.9 1.16 2.51 41.3 1.4

Difference -0.37 0.01 -1.0 0.4 0.2 2.8 -1.0 -0.5

Lower Upper SPC @ SPC @ SPC @ SPC @ SPC @ SPC @ SPC @ SPC @ SPC @ SPC @

Limit Limit Bandwidth 500 Hz 1000 Hz 1500 Hz 2000 Hz 2500 Hz 3000 Hz 3500 Hz 4000 Hz 4500 Hz 5000 Hz

(Hz) (Hz) (Hz) (db) (db) (db) (db) (db) (db) (db) (db) (db) (db)

CSJ 26 (1) Homo sapiens 820 4260 3440 2.0 11.4 8.3 9.4 9.7 9.7 9.9 9.5 6.0 0.5

CSJ 26 (2) Homo sapiens 795 4130 3335 2.0 11.1 8.7 10.1 10.9 11.0 10.2 8.2 5.3 2.1

Difference 25 130 105 0.1 0.3 -0.4 -0.7 -1.2 -1.3 -0.2 1.4 0.7 -1.6

OCCUPIED BAND Sound Power at the entrance to the Cochlea (SPC)

Table S2. Measurements and model results for the influence of interindividual measurement error

VMA VMEC LAD RAD1 RAD2 ATM LEAC AEAC LM/LI AFP MM+MI MS

Volume Volume Radius Radius Area of Length of Cross-Sectional Malleus/Incus Area of Mass of

Mastoid Tympanic Length of of Aditus of Aditus Tympanic External Ear Area of Lever Stapes Malleus+ Mass of

Air Cells Cavity Aditus Exit Entrance Membrane Canal EAC Ratio Footplate Incus Stapes

Species cm3 cm3 mm mm mm mm2 mm mm2 mm2 mg mg

HTB 3434 (1) Pan troglodytes 8.06 0.35 6.2 1.9 2.3 84.3 39.8 23.0 1.76 3.04 41.0 1.0

HTB 3434 (2) Pan troglodytes 10.31 0.41 6.1 2.1 2.6 82.2 39.0 24.3 1.76 3.04 41.0 1.0

Difference -2.25 -0.07 0.2 -0.2 -0.2 2.1 0.8 -1.3

Lower Upper SPC @ SPC @ SPC @ SPC @ SPC @ SPC @ SPC @ SPC @ SPC @ SPC @

Limit Limit Bandwidth 500 Hz 1000 Hz 1500 Hz 2000 Hz 2500 Hz 3000 Hz 3500 Hz 4000 Hz 4500 Hz 5000 Hz

(Hz) (Hz) (Hz) (db) (db) (db) (db) (db) (db) (db) (db) (db) (db)

HTB 3434 (1) Pan troglodytes 565 2955 2390 4.2 12.6 7.1 8.5 8.7 5.8 -1.1 -7.7 -14.6 -16.5

HTB 3434 (2) Pan troglodytes 575 2960 2385 4.4 13.0 7.5 9.1 9.3 6.2 -1.0 -8.5 -19.0 -7.1

Difference -10 -5 5 -0.2 -0.4 -0.4 -0.6 -0.6 -0.4 -0.1 0.8 4.4 -9.5

OCCUPIED BAND Sound Power at the entrance to the Cochlea (SPC)

Table S3. Electrical Parameters and Anatomical Variables in the Physical Model

Electrical Parameters

Related Anatomical Variables

Definition Value Used Sensitivity

(≥ 2kHz)

OU

TE

R E

AR

Two-port network that

models the concha horn

Concha length Rosowski (41) High (A)

Cross-sectional area of wide end Rosowski (41) High (A)

Cross-sectional area of narrow end Measured as

AEAC

High (A)

Two-port network that

models the ear canal tube

Ear canal length Measured as

LEAC complete

High (A)

Cross-sectional area of the ear canal Measured as

AEAC

High (A)

MID

DL

E E

AR

Middle ear

cavity

CTC Volume of the middle ear cavity Measured as

VMEC

Low (A)

CMC Volume of the mastoid air spaces Measured as VMA Low (A)

RA Surface area of the aditus ad antrum and

mastoid air spaces

Rosowski (41) Low (E)

LA Length and radius of the aditus ad antrum Measured as LAD

RAD1 and RAD2

Low (E)

Tympanic

membrane and

mallear

attachment

ATM Area of the tympanic membrane Measured as ATM High (A)

LT1 Mass of the tympanic membrane aEstimated from

ATM

High (A)

CT Structural properties of the tympanic

membrane and mallear attachment

Rosowski (41) Low (E)

RT Low (E)

LT Low (E)

CT2 Low (E)

RT2 Medium (E)

CTSM High (E)

RTSM Medium (E)

Malleus, incus,

ligaments and

stapes

lM:lI Functional lengths of the malleus and

incus

Measured as

LM / LI

High (A)

LMIM Masses of the malleus and incus Measured as

MM + MI

Medium (A)

RMIM Non-articular surface area of the malleus

and incus

Rosowski (41) Low (E)

CMIM Structural properties of the malleus and

incus

Rosowski (41) Low (E)

LSM Mass of the stapes Measured as MS Low (A)

RJM Structural properties of the ossicular joints Rosowski (41) Low (E)

CJM Low (E)

AFP Area of the stapes footplate Measured as AFP Medium (A)

INN

ER

EA

R

Annular

ligament

CAL Structural properties of the annular

ligament

Rosowski (41) b

RAL High (E)

Cochlea ZC Structural properties of the cochlea Aibara et al. (96) High (E)

Table S3. Definition of the electrical parameters, their related anatomical variables, the source of the

value used, and the sensitivity analysis for frequencies above 2 kHz in the model. Definitions and

abbreviations of the electrical parameters generally follow Rosowski (30), except Zc (Cochlear input

impedance) which follows Aibara et al. (66). Anatomical variables are as in Figure 1. aMass of the

tympanic membrane was estimated based on its area, extrapolating from the values for modern humans

provided by Rosowski (30). bThe value provided for this variable in Rosowski (30) is infinite, and it is

not included in the sensitivity analysis.

Table S4. Measurements in the present study compared with those reported previously.

VMA VMEC ATM LEAC (Com) AEAC LM/LI AFP MM+MI MS

Complete Cross

Volume Volume Area of Length of Sectional Malleus/Incus Area of Mass of

Mastoid Tympanic Tympanic External Ear Area of Lever Stapes Malleus+ Mass of

Air Cells Cavity Membrane Canal EAC Ratio Footplate Incus Stapes

Species cm3 cm3 mm2 mm mm2 mm2 mg mg Reference

Homo sapiens

Present study mean ± s.d. 4.43 ± 2.27 0.46 ± 0.09 65.1 ± 5.5 21.0 ± 2.0 36.4 ± 7.0 1.26 ± 0.08 2.92 ± 0.21 49.2 ± 4.4 2.2 ± 0.6

Present study range (n) 0.52-8.02 (10) 0.33-0.62 (10) 56.6-74.0 (10) 17.7-23.8 (10) 26.5-52.0 (10) 1.16-1.40 (7) 2.51-3.13 (7) 41.3-53.0 (8) 1.4-3.2 (8)

6.5 ± 3.8 45

1.6-22.4 (35) 45

7.9 ± 2.3 46

4.0-14.0 (100) 46

4.0 (n = 1) 0.73 (n = 1) 47

5.9 (n = 1) 1.24 (n = 1) 47

0.430 48

0.5-1.0 (8) 49

64.9 ± 6.2 50

52.7-79.9 (66) 50

64.4 ± 3.7 26.6 1.23 3.20 ± 0.14 51

43.8-86.7 (14) 20.4-31.9 (14) 2.8-3.5 (4) 51

22.50 52

25.00 53

25.00 54

25.70 55

25.20 56

23.4 ± 2.4 41.2 ± 7.7 13

20.1-30.6 (30) 27.8-56.7 (30) 13

41.9 ± 8.3 57

30.3-54.9 (14) 57

1.23 ± 0.08 3.39 ± 0.32 21

1.02-1.39 (42) 2.95-4.29 (41) 21

52.78 58

50.40 2.50 59

52.30 3.38 ± 0.48 60

44.50 1.9 ± 0.4 61

Pan troglodytes

Present study mean ± s.d. 8.89 ± 4.73 0.42 ± 0.11 82.1 ±8.2 37.9 ± 2.6 23.0 ± 4.4 1.67 ± 0.11 2.79 ± 0.39 42.0 ± 6.2 1.4 ± 0.5

Present study range (n) 2.25-18.73 (11) 0.26-0.62 (11) 71.0-102.8 (11) 34.2-40.8 (11) 16.4-30.3 (11) 1.52-1.79 (9) 2.40-3.48 (7) 35.0-53.0 (8) 1.0-2.2 (6)

91.5 (n = 1) 62

82.2 ± 3.55 38.30 1.61 2.40 ± 0.04 51

71.9-91.9 (6) 33.5-41.7 (7) 2.30-2.60 (7) 51

39.7 ± 4.5 39.1 ± 5.0 1.2 ± 0.4 *

29.3-52.1 (89) 29.7-54.0 (53) 1.0-2.5 (28) *

1.71 ± 0.11 2.72 ± 0.29 21

1.46-1.96 (41) 2.09-3.48 (30) 21

*Quam-unpublished data