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Does brain size variability provide evidence of multiple species in Homo habilis.

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AMERICAN JOURNAL OF PHYSICAL ANTHROPOLOGY 84~385398(1991)
Does Brain Size Variability Provide Evidence of Multiple Species
in Homo Habilis?
JOSEPH A. MILLER
Department of Anatomy and Cell Biology, University of Southern
California School of Medicine, Los Angeles, California 90033
Endocranial volume, Coefficient of variation,
KEY WORDS
Hominids, Paleoanthropology
ABSTRACT
Endocranial volume (ECV) variability as measured by the
coefficient of variation (CV) has been important in supporting the view that
more than one species is represented in Homo habilis. Supporters of this view
used a CV of 10 as a standard to determine that 1)the H. habilis CV of 12.7
indicates multiple species and 2) there is a low probability of H. habilis
specimens KNM-ER 1470 and KNM-ER 1813 being members of the same
taxon. This study examines published data for ECVs of fossil and extant
hominoids to determine whether CV yields any information regarding species
number in H. habilis. Results indicate that there is no empirical basis for using
a CV of 10 as a standard to detect multiple species in H. habilis. Also,
geography, time, sample choice, sex ratio, and measurement technique are
complicating factors that must be considered when interpreting CVs for fossil
samples. Additionally, the broad 95% statistical confidence limits (5.1-20.3)
indicate that the CV estimate of 12.7 for H. habilis is not sufficiently reliable to
allow biologically meaningful interpretation. However, if the CV for H. habilis
is actually 12.7, it still falls within the range of variation for single species of
modern hominoids. The evidence from ECV variability does not support the
argument for multiple species in H. habilis.
Controversy surrounds the interpretation
of the mor hological variability of specimens
attribute to Homo habilis. Some interpret
this variability as indicative of a variable,
polymorphic species (Howell, 1978; Johanson et al., 1987; Tobias, 1987; Johanson and
Shreeve, 1989). Others consider it as evidence of more than one species (Leakey and
Walker, 1980; Walker, 1981; Wood, 1985;
Stringer, 1986; Chamberlain and Wood,
1987; Lieberman et al., 1988; Groves, 1989).
The argument for multiple s ecies is supported by two lines of morpYlological evidence l)craniofacial morphometrics and 2)
brain size as estimated by endocranial volume (ECV).Evidence from craniofacial variability was presented by Stringer (19861,
who examined the pattern of midsagittal
cranial and transverse facial angles of H.
habilis relative to extant hominoids. Lieberman et al. (1988) em loyed a probability
model based on a samp e of gorillas to determine whether sexual dimorphism could ac-
a
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@ 1991 WILEY-LISS. INC
count for the craniofacial difference between
H. habilis specimens KNM-ER 1470 and
KNM-ER 1813.
Endocranial volume variability as measured by the coefficient of variation (CV) has
been considered by some authors to be important evidence supporting the h pothesis
that more than one species may e represented in H. habilis. Stringer (1986) argued
that a CV of 12.4 for H. habilis indicates
multiple s ecies. Wood (1985) used a CV of
10 to calc3ate a low robability that KNMER 1470 and KNM-EE 1813 are members of
the same species. Both arguments are based
on the assum tion that a CV of 10 can be
used effective y to detect multiple species in
H. habilis.
This study reviews published data for
ECVs of extant and fossil hominoids to answer the following questions. 1)Is there an
z
f
Received March 15,1990; accepted September 13,1990
386
J.A. MILLER
empirical basis for using a CV of 10 to detect
mixed-species samples? 2) What factors confound the interpretation of CVs for paleontological samples? 3) How do the statistical
consequences of small sample size affect the
reliability of CV estimates? For this study,
H. habilis refers to Stringer’s (1986) sample
of nine specimens (see Appendix A).
THE COEFFICIENT OF VARIATION
The CV is a measure of relative variability
that is calculated b dividing the standard
deviation (SD) by t e sample mean (CV =
SD/mean x 100). This provides a rough size
correction that allows comparisons of variability to be made between different-sized
animals or species.
CVs are commonlyused to 1) document the
relative variability of a given character, 2)
examine the relative variability of homologous characters between subspecies or species (Sokal and Braumann, 1980; Sokal and
Rohlf, 1981). 3) identify characters of low
variability that might be useful taxonomically (Simpson et al., 1960), and 4) indicate
heterogeneity in neontological samples
(Simpson et al., 1960).The last practice was
described by Mayr (1969:170):“If. . . the CV
for a certain statistic fluctuates around 4.5
in a series of samples, but is 9.2 in one
sample, such a sample should be reinvestigated.” Such heterogeneity might be due to
the inclusion of different subspecies, an additional sibling species, wrongly sexed specimens, or some other source of variability
(Ma r, 1969).
Tze use of CV as an indicator of heterogeneity in neontological samples has been generalized by some investigators to detecting
multiple species in fossil samples (e.g., H.
habilis). This has been done using CVs of
extant and fossil samples as a basis for determining what a “reasonable” CV “should be”
for a given fossil taxon. If the fossil taxon in
question has a higher than expected CV,
then a mixed-species sample has been inferred by several investigators, but such an
approach ignores other possible sources of
higher variability as indicated by Mayr
(1969).
When calculating CV, Sokal and Braumann (1980) recommended a correction for
bias introduced by small sample size (N):
i
CV* = (1+ 1/4N) x CV.
All CVs in the present study were adjusted
using this correction. Thus for H. habilis, a
corrected value of 12.7 is used rather than
Stringer’s (1986) value of 12.4.
IS CV = 10 A REASONABLE STANDARD TO
DETECT MULTIPLE SPECIES?
Stringer (1986) argued that a CV of ECV
greater than 10 indicates multiple species in
H. habilis. This contention was based on CVs
for small samples of extant and fossil hominoid taxa considered to be analogs for H.
habilis. These values were 9.7 for pygmy
chimps (N = 40), 8.9 for common chimps
(N = 331, 11.0 for gorillas (N = 39), 10.3 for
modern humans (N = 131, 9.0 for H. erectus
(N = 51, and 10.3 for Neandertals (N = 6).
Stringer (1986:270) claimed “it is evident
from these data that a coefficient of variation
of about 10% is a reasonable figure for a
hominoid species.”Because the CV of 12.7for
H. habilis was larger than that for any of the
hominoid samples, he suggested that more
than one taxon was present.
Published data for other small samples of
hominoids, however, often indicate CVs
greater than 10. The gorilla sample (N = 40)
of Lieberman et al. (1988) has a CV of 13.02.
Smaller series of modern humans have CVs
ranging from 6.1 to 13.5 (Table 1).My ECV
data for all the measurable adult orangutans
(N = 10) in the Museum of Comparative Zoology (Harvard University) yield a CV of
18.3.
More importantly, published data for
large, combined-sexSam les of extant hominoids indicate that the 8V of ECV is often
greater than 10 (Fig. 1). Tobias’s (1971) CVs
are 9.7 for common chimps (N = 3631, 11.0
for orangutans (N = 402), 13.1 for gorillas
[N = 668; or 12.9 if the extreme ECV of 752
cc discussed by Tobias (1971:40)is omitted],
and 12.6-14.9 for modern humans ( N =
l,OOOs, males only). The CV for Randall’s
(1943) gorilla sample (N = 151) is 13.2. The
CVs for four large series of modern humans
[Naqada (N = 2061, Hythe (N = 1931, Whitechapel (N = 154)and Farringdon (N = 13211
range from 8.1 to 11.6 (Table 1).These CVs
are more reliable indicators of true population CVs since they are based on larger
Sam les. Thus values for both lar e and
sma 1 samples of extant hominoids emonstrate that there is no empirical basis for
using a CV of 10 to detect multiple species in
H. habilis. Additionally, these data show
that the H. habilis CV of 12.7 is within the
range of variation for single species of modern hominoids.
Employing a different method, Wood
P
%
387
BRAIN SIZE VARIABILITY IN HOMO HABILIS
TABLE 1. Endocranial volumes [mi) and CVs for some modern human populations
N
Mean
SD
CV
206
154
132
52
27
63
52
64
68
92
33
54
15
12
11
85
60
28
38
24
193
30
9
7
1,330.0
1,385.3
1,369.9
1,409.6
1,423.7
1,251.3
1,418.8
1,378.4
1,301.3
1,310.9
1,340.8
1,337.4
1,280.8
1,319.2
1,473.5
1,258.2
1,354.2
1,245.0
1,447.7
1,472.2
1,396.8
1,335.0
1.441.1
1;455.7
108.0
147.3
158.6
134.9
118.3
118.4
137.7
141.9
137.3
139.3
148.6
127.0
127.6
137.6
128.8
102.3
131.1
112.9
113.6
89.0
128.9
107.8
188.8
179.9
8.1
10.6
11.6
10.9
8.4
9.5
9.8
10.3
10.6
10.7
11.2
9.5
10.1
10.6
8.9
8.2
9.7
9.1
7.9
6.1
9.2
8.1
13.5
12.8
Series
Naqada
Whitechapel
Farringdon
Moorfields
Easter isle
Teita
Nepalese
Spitalfield
Congo
Gaboon
Gaboon
Burmese A
Burmese B
Burmese C
Bidford-on-Avon
New Britain
Ninth Dynasty
Tasmanian
Maiden Castle
Scottish
Hythe
First Dynasty
Timor-Laut
Esquimaux
Region
Source of raw data
Africa
Europe
Europe
Europe
Oceania
Africa
Asia
Europe
Africa
Africa
Africa
Asia
Asia
Asia
Europe
Oceania
Africa
Australia
Europe
Europe
Europe
Africa
Australia
North America
Fawcett and Lee (1902)
MacDonell (1904)
Hooke (1926)
MacDonell (1906)
van Bonin (1931)
Kitson (1931)
Morant (1924)
Morant and Hoadley (1931)
Benington (1912)
Benington (1912)
Benington (1912)
Tildesley (1921)
Tildesley (1921)
Tildesley (1921)
Brash and Young (1935)
van Bonin (1936)
Woo (1930)
Wunderly (1939)
Goodman and Morant (1940)
Reid and Morant (1928)
Stoessiger and Morant (1932)
Morant (1925)
Garson (1884)
Duckworth (1896)
range for
males only
H. HABlLlS
CV = 12.7
P E3
4
K
0
9
14:
12
L
0
132
L
0 39
154
t-
z
w
0
LL
402
lo
0 13
0 40
363
0
193
0
206
LL
w
0
0
Fig. 1. A comparison of CVs of ECV for Homo habilis and large samples of extant hominoids.
Open rectangles, Stringer (1986); solid rectangles, Tobias (1971); stipled rectangle, Randall
(1943); black circles, Miller (see Table 1).Numbers beside symbols indicate sample size.
Horizontal line indicates CV = 10.
(1985) used a CV of 10 to calculate a low
probability that KNM-ER 1813(510 ml) and
KNM-ER 1470 (770 ml) are members of the
same population. He used the ECVs of these
specimens to calculate a hypothetical population mean of 640 ml. Using a CV of 10,
Wood (1985) solved the formula CV = SD/
mean x 100 to obtain a standard deviation
of 64 ml. He then determined the approximate 95% opulationlimits to be 512-768 ml
(calculate as 2 2 SD from the mean). Since
the ECVs for KNM-ER 1813 (510 ml) and
B
388
J.A. MILLER
KNM-ER 1470 (770 ml) fall outside these
limits, the probability of either belonging to
the same population would be 5%. The probability of both being members of the same
population would be 0.25%. Disregarding
the possibility that these two specimens
might be members of two different local populations of the same species, Wood (1985:
212) concluded that KNM-ER 1813 and
KNM-ER 1470 “should not be uncritically
regarded as belonging to the same taxon.”
One problem is that Wood (1985) used an
ECV of 770 ml for KNM-ER 1470 (a preliminary estimate attributed to Holloway by Day
et al., 1975) rather than Holloways (1978)
later estimate of 752 ml. When the latter
value is used, the mean becomes 631-ml and
the 95% PO ulation limits are 505-757 ml.
Now, both&M-ER 1813and KNM-ER 1470
can be accommodated in a single PO ulation.
The conclusions of Wood (19857 should
also be reexamined using CVs other than 10.
Employing Wood’s (1985)method, if Tobias’s
(1971)gorilla CV of 13.1, and if the corrected
mean of 631 ml are used, then the 95%
population limits are 466-796 ml (or 472808 ml using Wood’s mean of 640 ml). If, as
Tobias (personal communication) suggests,
the CV of 11.3 for his H. erectus pekinensis
sample (Tobias, 1987)is used, then the limits
are 488-774 ml (or 495-785 ml using Wood’s
mean). If a CV of 10.8 for a sample of H.
erectus erectus (Tobias, 1987) is used, then
the limits are 495-767 ml (or 502-778 ml
usin Wood’s mean). In all the above cases,
the ECVs of KNM-ER 1813 and KNM-ER
1470 fall within the 95% population limits.
Thus, Wood’s (1985) approach provides no
evidence that H. habilis comprises more
than one taxon.
A potential justification for using a CV of
10 to detect multiple species comes from
Simpson et al. (19601,who observed that the
majority of CVs for linear measurements
tend to fall between 4 and 10. However, this
does not imply that a CV of 10 is a useful
standard for area or volumetric measurements. Lande (1977) and Yablokov (1974,
citing Schmalhausen, 1935) observe that
CVs for traits of different dimensionalities
are not directly comparable and that a CV for
a volumetric measurement may be up to
three times as large as the average CV of its
component linear dimensions. Data from
two samples of modern H. sapiens illustrate
this point (Table 2). In the first sample
(N = 11) from Bidford-on-Avon, England
(Brash and Young, 19351,the respective CVs
TABLE 2. A comparison of CVs for endocranial
volume (ml) and linear cranial dimensions
lmmi for two samwles of modern humans
Bidford-on-Avon
(N = 11)
Length
Breadth
Height
ECV
(Brash and
Young, 1935)
Mean
SD
CV
Timor-Laut
(N = 9)
(Garson, 1884)
Mean
SD
CV
191.0
8.4
141.1
6.3
136.7
6.3
1,473.5 128.8
166.3
144.7
135.6
1,441.1
4.5
4.6
4.7
8.9
7.1
7.8
5.3
188.8
4.4
5.5
4.0
13.5
for cranial length, breadth, and height are
4.5, 4.6, and 4.7. Thus the average CV for
cranial linear dimensions is 4.6. However,
for endocranial volume, the CV of 8.9 is
almost twice as great. In the second sample
(N = 9) from Timor-Laut (Garson, 18841,the
CVs for cranial length, breadth, and height
are 4.4, 5.5, and 4.0, respectively, with an
average CV of 4.6, yet, for endocranial volume, the CV of 13.5 is almost three times as
great. For H. habilis, this means that if a CV
of 10 is an upper threshold for cranial linear
dimensions (after Simpson et al., 1960),and,
if CVs for volumetric measurements are up
to three times as great as those for linear
dimensions (after Lande, 1977; Yablokov,
1974), then the “threshold CV for detecting
multiple species using ECV should be about
30. Alternatively, the CV for the cube root of
ECVs for H. habilis can be calculated (see
Yablokov, 1974). The resulting CV of 4.3 is
comparable to those for the linear dimensions of the human cranium and is below the
threshold CV of 10 for linear measurements.
WHAT FACTORS CONFOUND THE
INTERPRETATION OF CVS?
Although CVs are intended to measure the
relative variability of a given trait, they are
subject to various biasing factors that can
confound their interpretation. These factors
include geogra hic variation, tem oral variation, sample c oice, sex ratio, anEpmeasurement technique. The inability to control for
these factors in paleontological samples
makes it difficult to compare CVs of fossil
samples with those for extant samples.
Simpson et al. (1960) warned that CVs for
fossil samples are usually higher than those
for extant samples.
Geographic variation
Elevated CVs for a sample can result from
combining geographically varying popula-
R
389
BRAIN SIZE VARIABILITY IN HOMO HABILIS
tions of the same species (Mayr, 1969). Samples of modern humans from Tasmania and
Scotland (Fig. 2) differ in mean ECV (Tasmanian 1,245.0 ml, Scottish 1,472.2 ml) and
have CVs under 10 (9.2 and 6.1, respectively). However, the CV for the combined
samples is 11.4. This elevated CV represents
a complex interaction of within-group and
between-group variability. A similar effect
on CV might apply to the H. habilis sample,
since it consists of specimens from localities
that are geographically widely separated. Of
course, it is also possible that the combined
CV for two geographically varying samples
could be intermediate or lower than that of
the individual samples. This argues for caution in uncritically accepting hi h CVs as
evidence of heterogeneity or low Vs as evidence of homogeneity.
Temporal variation
An elevated CV can result for a paleontological sample that includes specimens from
different time periods. This point cannot be
demonstrated convincingly with fossil samples, since the temporal continuity and taxonomic distinctiveness of fossil samples are
roblematical. The data presented
above
Often
or geographic variation, however, can
be reformulated into a hypothetical example
to demonstrate the effect of temporal changes
in ECV and its variability between ancestral
and descendant populations. The Tasmanian sample (Fig. 2) can be treated as a
hypothetical ancestral population at time A
that evolves into a descendent population
(represented b the Scottish Sam le) a t a
later time B. T us, at time A, the V would
be 9.2; at time B, it would be 6.1. A paleontological sample that combined specimens
from these two hypothetical time periods
together would have a CV of 11.4. Such an
elevation of CV due to temporal variability
is, in part, why Simpson et al. (1960) cautioned that CVs for fossil taxa are often
higher than those for extant taxa. For H.
habilis, however, the brief time duration of
the taxon, fragmentary nature of the specimens, and small sample size prevent a reliable assessment of its temporal variability (if
any).
Sample choice
The CV for a given sample depends on the
specimens chosen to represent that sample.
Specimen choice differs among investigators
and depends, in large art, on one’s taxonomic preference. The e fect of sample choice
8
P
K
8
P
on CV is demonstrated with the following
two examples.
Neandertals. The five samples shown in
Figure 3 are of six specimens each taken
from a group of 13 Neandertal specimens
that man consider to represent a single
species. T ese samples were chosen to illustrate the remarkable range in CV that may
potentially occur with sample choice. A CV of
3.8 is calculated for the group consisting of
the Spy 11, Neandertal, Tabun I, Ganovce,
Gibraltar, and La Quina specimens. A CV of
10.2 is calculated for the group consisting of
the Spy 11, Neandertal, Monte Circeo I, La
Chapelle, Tabun 1,and La Quina specimens.
A CV of 16.9 results for the group consisting
of the La Ferrassie I, La Chapelle, Saccopastore I, Tabun I, Amud I, and Gibraltar specimens. These extraordinary differences in
CV are merely a consequence of sampling
and document the potential variability inherent in CVs of small fossil samples.
Homo erectus. Figure 4 shows a more complicated example of 10 different geographic,
temporal, and taxonomic groupings of specimens that might conceivably be used to represent H. erectus. The CVs for these groups
are as follows: East Africa (15.41, all H .
erectus including Solo (14.5), all H . erectus
excluding Solo (13.2), all Africa (13.6), East
Africa excluding OH 12 (13.51, East Africa
plus Java and China (13.31, Zhoukoudian
(12.71, all Asia excluding Solo (12.41, Java
excluding Solo (10.21, and Solo (8.1). This
wide spectrum of values demonstrates how
CVs can vary with sample choice. Given this
variability and the difficulty in controlling
for the effects of geography, time, and other
factors on CV, caution should be employed
when the variability of one fossil or extant
sample is used as an analog for that of another.
Sex ratio
The relative numbers of males and females in a sample may affect the CV when
sexual dimorphism is present. The gorilla
sample (N = 668) from Tobias (19711, for
example, consists of 414 (62%)males (mean
ECV 534.6 ml, SD 57.05 ml) and 254 (38%)
females (mean ECV 455.6 ml, SD 45.13 ml;
combined-sex CV 13.1). This Sam le can be
manipulated to demonstrate the e fect of sex
ratio on CV. This is done by first recalculating the sum of squares for hypothetical combined-sex samples that contain varying roportions of males and females while holling
constant the male and female mean ECVs,
K
P
390
J.A. MILLER
GEOGRAPHIC VARIATION
c
TASMANIAN
SCOTTISH
COMBINED
11.4
Z 12r
TIME A
TIME B
I
COMBINED
+
I
TEMPORAL VARIATION
Fig. 2. Elevation of CV of ECV for combined geographic samples of modern humans
(Tazmanian and Scottish). The same data illustrate elevated CV for the combined tem oral
sample if the samples at times A and B are treated as a single hypothetical population evoking
through time. CVs are listed above bars.
z
Q
*O[
16.9
i-
a
5
15
LL
O 10
Iz
W
g
5
LL
W
0
O O
2, 4, 9,
11,12&13
1 , 3, 4 ,
5,6&7
2,4,
6,
7,9813
1, 3, 5,
9,11&12
3 , 7 , a,
9,10&12
NEANDERTAL SAMPLES
Fig. 3. Range of CVs of ECV for five samples of Neandertals. CVs are listed above bars. Bar
labels refer to specimens listed in Appendix A. Horizontal line indicates CV = 10.
SDs, and total sample size as given above.
The resulting sum of squares is then used to
calculate a new combined-sex SD, which,
when divided by the weighted ECV mean for
that sample, yields the new CV.
Using this method, the following CVs are
obtained for the hypothetical gorilla samples
(Fig. 5): 13.1 (50% female), 13.0 (60% female), 12.7 (70%female), 12.2 (80%female),
11.3 (90% female), and 9.9 (100% female).
391
BRAIN SIZE VARIABILITY IN HOMO HABILIS
E. Africa
I
All H. erectus including Solo
I
All H. erectus excluding Solo
16
E. Africa excluding OH 12
E.Africa, Java, China ( 1 .O- 0.8mya )
Zhoukoudian
All Asia excluding Solo
I
Java excluding Solo
14
I
12
10
8
Specimens
1-5
1-28
Time Span
1.06
1.58 1.52
1-22
1-5
& 14
1-4
5 - 1 3 15-21
1.38 0.45 0.20 0.23
6 - 1 3 6 - 1 2 23-28
15 22
-
0.74 0.20 0.20 ( m y )
Fig. 4. CVs of ECV for 10 samples of Homo erectus. Samples were chosen on the basis of
different geographic, temporal, and taxonomic criteria. Bar labels refer t o specimens listed in
Appendix A. Numbers below bar labels indicate approximate time spans (durations in millions
of years) for each sample calculated from data in Day (1986)and Feibel et al. (1989).Horizontal
line indicates CV = 10.
males 5 0 %
females 5 0 %
40%
60%
30%
70%
20%
80%
10%
90%
0%
100%
SEX RATIO
Fig. 5. Effect of sex ratio on CV of ECV for a gorilla sample from Tobias (1971). Numbers
above bars indicate CV. Bar labels indicate sex ratio for each hypothetical sample. Horizontal
line indicates CV = 10.
392
J.A. MILLER
r
8.5
Mustard Seed
Poppy Seed
Manouvrier
Formula
( modified )
Pearson
Formula
( modified )
MEASUREMENT TECHNIQUE
Fig. 6. Four different CVs for a single chimpanzee sample (N = 18) whose ECVs were
measured four different ways (data from Protsch von Zieten et al., 1987). CVs are listed above
bars.
Thus, in this case, the difference between a
CV of 9.9 and 13.1 is not indicative of interspecific variation but is, instead, a reflection
of sex-ratio differences in a sexually dimorphic taxon.
Measurement technique
The CV is affected b how endocranial
volumes are measured. $rotsch von Zieten
et al. (1987)used four different techniques to
measure ECVs of 18 common chimpanzees.
Two methods involved different filling material (mustard or poppy seed) oured into the
crania. The other two metho$s involved the
use of different formulae to estimate ECV
from linear dimensions. The CVs calculated
for these techniques range from 5.5 to 8.5
(Fig. 6). This nonbiological variation in CV
approximates the difference between a CV of
10 and the H. habilis CV of 12.7.
The measurement effects described above
may be compounded when estimating ECV
for fragmentary fossil specimens. For example, from 1964 to the present, the ECV of H.
habilis specimen OH 7 has been estimated
by various workers using a variety of techniques. The fragmentary nature of the s ecimen (two patial parietals) has resulte in
estimates ranging from 560 ml to over 800
ml: 642.7-723.6 (Tobias, 1964); 684 ml (Tobias, 1971); 560 and 650 ml, (Kochetkova,
1978); 700-750 ml (Holloway, 1980a), 580600 ml (Wolpoff 1981);674 ml (Tobias, 1983);
t;
539-868 ml (Vaisnys et al., 1984).This range
nearly equals the entire range of ECV estimates for all H. habilis specimens. As several other H . habilis specimens (e.g., OH 13
and OH 16)are nearly as fragmentary as OH
7, it follows that their ECV estimates may be
as problematical.
The uncertainty of ECV estimates for fragmentary fossil specimens translates into
problems in estimating CV.The CV for the
same group of fossil specimens can vary
markedly between observers who use different measuring techniques. For example,
Figure 7A shows two sets of ECV estimates
for a single group of six Indonesian H. erectus
specimens. A CV of 10.3 is calculated from
Holloway’s (1981a)ECV estimates, whereas
a CV of 13.1 is based on previous ECV estimates made by others including Weidenreich (1943),von Koenigswald (19621,Tobias
and von Koenigswald (1964)) and Tobias
(1971). Similarly, Figure 7B shows that for
Australopithecus africanus, a CV of 5.1 is
calculated from Holloway’s ECV estimates,
whereas a CV of 10.3 is derived from previous ECV estimates by Sche ers (1950) and
Dart (1962). Recently, Hol oway’s 435 ml
estimate for MLD 37/38 was tested by Conroy et al. (1990) who used an independent
technique to make an estimate of 425 ml.
Although there is good agreement between
these two estimates, Holloway (1972,1981a)
himself warns that his technique of estimat-
P
393
BRAIN SIZE VARIABILITY IN HOMO HABILIS
(A)
W
5
3
$
950
di
d
0
850
i
0
750
V = 13.1
4
w
54
S2
T2
S10
S12
517
HOMO ERECTUS SPECIMEN
-
E 550
W
f6 500
>
4
2
450
d
0
0
W
400
1-
CV = 5.1
Taung
STS60
STSS
STS 19 STS71
MLD37!38
AUSTRALOPITHECUS AFRICANUS SPECIMEN
Fig. 7. CVs for different estimates of ECV for samples of Homo erectus (A) and Australopitizecus africunus (B).For both A and B, open squares indicate Holloway’s ECV estimates, black
squares indicate previous estimates. See Appendix for sources of previous estimates.
ing ECV may underestimate the actual variability between specimens, Clearly, CVs are
dependent on determinations of ECVs which
may vary, sometimes quite markedly, between investigators.
WHAT ARE THE STATISTICAL CONSEQUENCES
OF SMALL SAMPLE SIZE?
When considering estimations of CV for
small fossil sam les, the statistical consequences of samp e size must be addressed.
P
This may be accomplished b using the 95%
confidence limits for CV. Tyhese are calculated as CV plus or minus the standard error
multiplied by the appropriate t value ( a =
0.05 for a two-tailed test usingN - 1degrees
of freedom; Sokal and Rohlf, 1981). The formula for calculating the standard error of a
CV is given b Sokal and Braumann (1980).
In general, t e confidence limits around a
given CV may be used as a reliability indicator of that estimate. All other things being
i
394
J.A. MILLER
(A)
N=5
LL
0
N=6
15
t
N=9
( males only )
N=668
N = 1000s
I-
;
-
10
0
LL
L L 5
w
8
0
Fig. 8. Ninety-five percent confidence limits for CVs of ECV. A: Stringer's (1986) samples of
extant and fossil hominoids. B: Homo habilis and Tobias's (1971) samples of extant hominoids.
Horizontal bars indicate CV. Rectangles indicate 95% confidence limits. Horizontal line
indicates CV = 10.
equal, the larger the sample, the tighter the
confidence limits and the better the sample
estimate is likely t o be of the true population
value.
The reliability of the CVs for Stringer's
(1986) small hominoid samples can be
checked by computing the 95% confidence
limits. These limits for his samples are as
follows (Fig. 8A): Gorilla gorilla (8.4-13.4),
modern Homo sapiens (5.6-15.0), H. erectus
(Solo) (0-18.31, Neandertals (1.5-19.11, Pan
troglodytes (6.6-11.21, and Pan paniscus
(7.4-12.0). These broad confidence intervals
support Andrew's (1978:203-204) claim that
for smaller samples, the confidence limits
are often so broad that they make CVuseless
395
BRAIN SIZE VARIABILITY IN HOMO HABILIS
as a measure of variability. These broad What has been shown is that brain size and
limits also contradict Stringer’s (1986)use of its variability do not seem to be useful for
a CV of 10 to detect multiple species in H. distinguishing whether specimens from
more than one species have been included in
habilis.
How reliable is the CV estimate for the H. H. habilis. Claims for multiple species in H.
habilis sample (N = 9) itself? The 95% con- habilis have been made on the basis of other
fidence limits for the CV of 12.7 for H. habilis characters and approaches (see Lieberman
are 5.1-20.3 (Fig. 8B). Therefore, relative et al., 1988; Stringer, 1986). However, such
variability in H. habilis cannot be statisti- lines of evidence are not unequivocal (see
cally distinguished from comparative sam- Miller, 1990) and are in need of additional
ples having a CV of 6, 10, 15, or 20. Indeed, consideration. Future fossil discoveries and
the broad confidence limits for H. habilis analyses should clarify the situation. For
encompass the entire range of CVs for large now, it is clear that the evidence from endosamples of extant hominoids (Fig. 8B). cranial volume variability does not support
Clearly, even if a CV of 10 was an appropri- the argument for multiple species in H. haate upper limit for a single species, small bilis and that it may be premature to reject
sample size would now allow statistically the hypothesis that H. habilis re resents a
meaningful distinctions to be made between single, sexually dimorphic, a n 8 perhaps
this threshold CV of 10 and the H. habilis CV even polytypic species.
of 12.7.
ACKNOWLEDGMENTS
CONCLUSIONS
A more complete, statistically rigorous examination of the coefficient of variation for
endocranial volume does not support the
hypothesis of multiple species in H. habilis.
Published data for extant hominoids indicates that there is no empirical basis using a
CV of 10 as a standard for detecting mixedspecies samples of fossil hominoids. The
same data also indicate that the CV of 12.7
for H. habilis is within the range of CVs for
single species of modern hominoids.
Furthermore, interpretation of CVs for
fossil samples may be confounded by sources
of variability such as geographic variation,
temporal variation, sample choice, sex ratio,
and measurement technique. The difficulty
in controlling for these factors suggests that
CVs for fossil samples may not be comparable to, and may often be higher than, those
for extant samples.
Additionally, the broad 95% confidence
limits (5.1-20.3) calculated for the small H.
habilis sample indicate that its CV of 12.7 is
not statistically significantly different from
CVs of extant hominoid taxa. Neither is it
statistically different from a CV of 10, the
supposed threshold for detecting multiple
species. The broad confidence limits also
mean that the CV of 12.7 for H. habilis is
insufficiently reliable for biolo ically meaningful interpretation. These Endings suggest that there is no information from the CV
for endocranial volume regarding species
number in H. habilis.
This does not mean that it has been proved
that H. habilis represents a single species.
My sincerest thanks to Gene H. Albrecht,
who provided invaluable guidance on all
aspects of this paper. I also thank Phillip
Tobias, Chris Strin er, Don Johanson,
Bernard Wood, Bruce elvin, Eric Scott, and
two anonymous reviewers for providing insightful comments on the manuscript. A special thanks is given to Donna Yankovich,
Eleanor Bates, Doroth Libby, and the late
Stewart Shermis for t eir support and encouragement in the early stages of this work.
This research was funded by a National
Science Foundation Graduate Fellowship
and was first presented in preliminary form
at the 58th Annual Meeting of the American
Association of Physical “Anthropologists
(Miller, 1989).
(5
i
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APPENDIX A. Endocranial volumes (ml)for fossil h,ominids
Specimen
ECV'
Region
Source
S. Africa
Holloway (1972)
S. Africa
Holloway (1972)
Schepers (1946)
Holloway (1972)
Schepers (1950)
Holloway (1972)
Schepers (1950)
Holloway (1972)
Schepers (1950)
Holloway (1972)
Dart (1962)
Conroy et al. (1990)
Australopithecus africanus
1. Taung
2. STS 60 (Ples. 1)
3. STS 5 (Ples. 5)
4. STS 71 (Ples. 7)
5. STS 19 (Ples. 8)
6. MLD 37/38
Homo habilis4
1. O H 7
7. KNM-ER 1805
8. KNM-ER 1813
9. KNM-ER 3732
Home erectus
1. KNM-ER 3733
2. KNM-WT 15000
440
562.52
428
435
485
480
428
500
436
560
435
480
4253
S. Africa
S. Africa
S. Africa
S. Africa
582
510
(700)
E. Africa
E. Africa
E. Africa
E. Africa
E. Africa
E. Africa
E. Africa
E. Africa
E. Africa
Holloway (1978)
Holloway (1973)
Stringer (1986)
Holloway (1973)
Holloway (1978)
Stringer (1986)
Holloway (1978)
Holloway (1983)
Stringer (1986)
848
900
E. Africa
E. Africa
Holloway (1983)
Holloway (1988)
687
650
6675
590
752
(750)
(continued)
398
J.A. MILLER
APPENDIX A. Endocranial uolumes (ml) for fossil hominids (continued)
Saecimen
3.
4.
5.
6.
KNM-ER 3883
OH9
OH 12
Sang2
7. S a n g 4
8. Sang 10
9. Sang12
10. Sang 17
11. Trinil 2
12. SambungI
13. Lantian 2
14. Sale
15. Zhoukoudian I1
16. Zhoukoudian 111
17. Zhoukoudian V
18. Zhoukoudian VI
19. Zhoukoudian X
20. Zhoukoudian XI
21. Zhoukoudian XI1
22. Hexian
23. Solo I
24. Solo V
25. Solo VI
26. Solo IX
27. Solo X
28. Solo XI
Neandertals
1. S D V 1
2. s p y 2
3. La Ferrassie 1
4. Neandertal
5. Shanidar 1
6. Monte Circeo 1
7. La Chapelle
8. Saccopastore 1
9. Tabun 1
10. Amud 1
11. Ganovce
12. Gibraltar
13. La Quina
ECV’
Region
Source
804
1,067
727
813
775
908
750
855
975
1,059
915
1,004
1,029
940
850
E. Africa
E. Africa
E. Africa
Java
1,035
780
880
1,030
915
1,140
850
1.225
11015
1,030
1,025
1,172
1,251
1.013
1;135
1,231
1,090
Java
China
N. Africa
China
China
China
China
China
China
China
China
Java
Java
Java
Java
Java
Java
Holloway (1983)
Holloway (1983)
Holloway (1983)
Holloway (1981a)
Tobias (1971)
Holloway (1981a)
von Koenigswald (1962)
Holloway (1981a)
von Koenigswald (1962)
Holloway (1981a)
Tobias (1971)
Holloway(l98la)
Tobias (1971)
Holloway (1981a)
Tobias and von
Koenigswald (1964)
Pope (1988)
Woo (1966)
Holloway (1983)
Weidenreich (1943)
Weidenreich (1943)
Weidenreich (1943)
Weidenreich (1943)
Weidenreich (1943)
Weidenreich (1943)
Weidenreich (1943)
Wu and Dong (1982)
Holloway (1980b)
Holloway (1980b)
Holloway (1980b)
Weidenreich (1943)
Holloway (1980b)
Holloway (1980b)
1.553
11305
1,681
1,408
1,600
1,550
1,626
1.200
11271
1,740
1,320
1,296
1.367
Europe
Europe
Europe
Europe
Middle East
Europe
Europe
Europe
Middle East
Middle East
Europe
Europe
Euroue
Holloway (1981b)
Holloway (1981b)
Heim (1974)
Bode (1912)
Stewart (1977)
Sergi (1948)
Boule (1912)
Sergi (1948)
McCown and Keith (1939)
Suzuki and Takai (1970)
Vlcek (1955)
Boule (1912)
B o d e (1912)
Java
Java
Java
Java
Java
’Numbers in parentheses are Stringer’s (1986) estimates only and are not based on actual measurements.
2This value is the midpoint of the range of ECV estimatesfor this specimen as given by Holloway (1972). For a historical summary of the
various ECV estimates for Taung, see Tobias (1971).
jThis value is not used in Figure 7B.
“Stringer’s(1986) sample of nine H. habdis specimens. The CV for this sample is 12.7.
SStringer(1986) averaged the lowest (633 ml) and highest (700 ml) values mentioned by Tobias (1971)and Holloway (1978),respectively.
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