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Cranial capacity evolution in Homo erectus and early Homo sapiens.

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AMERICAN JOURNAL OF PHYSICAL ANTHROPOLOGY 8711-13 (1992)
Cranial Capacity Evolution in Homo erectus and Early
Homo sapiens
STEVEN R. LEIGH
Departments of Anthropology and Cellular, Molecular, and Structural
Biology, Northwestern University, Evanston, Illinois 60208
KEY WORDS
Hominid evolution, Brain size, Trend analysis
ABSTRACT
This paper investigates patterns of cranial capacity evolution
in Homo erectus, early Homo sapiens, and in regional subsamples ofH. erectus.
Specifically, models explaining evolution of cranial capacity in these taxa are
evaluated with statistical techniques developed for the analysis of time series
data. Regression estimates of rates of evolution in cranial capacity are also
obtained. A non-parametric test for trend suggests that cranial capacity in
both H. erectus and early H . sapiens may increase significantly through time.
Cranial capacity in an Asian subsample of H. erectus (comprised of Chinese
and Indonesian specimens) increases significantly through time. Other subsamples of H. erectus (African, Chinese, and Indonesian) do not appear to
increase significantly through time. Regression results generally corroborate
results of the test for trend. Spatial and temporal variation may characterize
evolution of cranial capacity in H. erectus. Different patterns of cranial
capacity evolution may distinguish H. erectus from early H. sapiens.
The study of the evolution of cranial capacity in fossil hominids has long occupied the
attention of evolutionists (Darwin, 1871; Tobias, 1971; Weidenreich, 1941). An unmistakable tendency toward increasing cranial
capacity (which reflects the size of the brain
and associated soft tissue) characterizes the
course of hominid evolution in general (Blumenberg, 1983; Godfrey and Jacobs, 1981;
Henneberg, 1987, 1989; Lestrel, 1976;
Lestrel and Read, 1973; Wolpoff, 19801, but
debate surrounds many phylogenetic issues
related t o this increase.
Disagreement about changes in hominid
cranial capacity is especially pronounced
when limited numbers of taxa are investigated. In these cases, the familiar paucity of
specimens and the relative lack of time depth
may obscure evolutionary patterns, severely
restricting our ability to understand factors
involved in the evolution of taxa under investigation.
Analyses of cranial capacity evolution in
Homo erectus and early Homo sapiens typify
the problems entailed in understanding the
finer details of cranial evolution in hominids; these analyses have engendered substantive discord among scholars about the
@ 1992 WILEY-LISS.INC.
patterns of later hominid evolution (Eldredge, 1985; Holt, 1988; Levinton, 1982;
Rightmire, 1981, 1982,1985,1986; Wolpoff,
1984, 1986). The present analysis attempts
to clarify this issue through an investigation
of changes in H. erectus and early H. sapiens
cranial capacity through time.
Previous authors have discussed the significance of various evolutionary models to
the problem of later hominid evolution. Models that specify either a punctuated equilibrium process or a gradualist process have
been advocated by various researchers (Eldredge, 1985; Rightmire, 1981, 1982, 1985,
1986; Wolpoff, 1984, 1986). These contrasting models make very different predictions
about the ways in which evolution occurs.
Briefly, as proposed by Eldredge and Gould
(1972), the punctuated equilibrium model
addresses the tempo and mode of evolution.
Evolutionary change is the result of two
basic processes: stasis of an ancestral species
followed by rapid allopatric evolution of new
species. Traditionally, the punctuated equilibrium model has been contrasted with a
Received March 8,1990 revision accepted J u n e 24,1991
2
S.R. LEIGH
gradualist model that portrays evolutionary
change as a relatively slow, steady, and sympatric process.
The nature of cranial capacity evolution in
Homo has figured prominently in debates
over the adequacy of these contrasting views
as general models of evolutionary change.
Within the Homo lineage, attention has centered upon comparisons of rates of change
between the taxa H. erectus and early H.
sapiens (e.g., Rightmire, 1985).
Stasis followed by rapid evolution in H.
erectus would tend to support a punctuated
equilibrium model of evolutionary change.
On the other hand, significant and sustained
increases in H. erectus cranial capacity
through time followed by significant increases in cranial capacity in those samples
classified as early H. sapiens would approximate a “gradualist” model of evolutionary
change.
Previous literature relevant to this problem is plainly divided between these two
positions. Rightmire (1981, 1982, 1985,
1986) and Eldredge (1985) have strongly
suggested that the evolution of early H. sapiens from H. erectus populations can be best
understood as a process predicted by the
punctuated equilibrium model. On the other
hand, several authors (Cronin et al., 1981;
Wolpoff, 1984, 1986) have argued that later
hominid evolution fails to follow a pattern
predicted by the punctuated equilibrium
model.
The hypotheses investigated in this paper
attempt to distinguish between alternatives
posed by Rightmire (1981, 1985, 1986) and
Wolpoff (1984, 1986) but also address some
more basic issues. Specifically, the hypothesis that H. erectus cranial capacity is randomly variable through time is tested. A
similar hypothesis is also investigated in
early H. sapiens. This possibility is relevant
because a trend in the change of cranial
capacity through time may characterize
these data. A trend is defined as “any systematic change in the level of a time series’‘
(McCleary and Hay, 1980:31). The presence
of a significant trend in H. erectus cranial
capacity would provide evidence t o reject a
punctuated equilibrium model of later hominid evolution.
A study of the evolutionary history of a
variable is clearly a time series problem, and
such problems should be addressed with appropriate statistical techniques. As stressed
by McCleary and Hay (19801, regression
analyses of time series data may present the
impression that changes in the level of a
variable through time are non-random, even
thoilgh stochastic processes may actually
account for the observed trend.
In addition to investigating the possibility
that change in cranial capacity is random
during later hominid evolution, hypotheses
about differences in rates of change among
and within taxa are addressed. Specifically,
the null hypothesis that rates of change in H.
erectus and early H. sapiens are equal is
tested. The hypothesis that rates of change
among geographically defined subsamples of
H. erectus are equal is also tested. Evaluation of these hypotheses may provide insight
into processes affecting patterns of evolution
in H . erectus and in early H . sapiens. Investigation of regional patterns of change in H.
erectus may be especially important given
the emphasis of punctuated equilibrium
models on allopatric speciation (Eldredge
and Gould, 1972; Gould andEldredge, 1977).
In order to test these hypotheses, this
study utilizes a variety of statistical approaches to measuring rates of cranial capacity evolution. These include a nonparametric test for trend in a time series,-nonparametric and parametric regression
analysis, and analysis of covariance. The test
for trend utilized in this study is of critical
importance because it circumvents some of
the problems inherent in previous statistical
investigations of the nature of cranial capacity changes during later hominid evolution.
MATERIALS AND METHODS
Materials
Materials for this analysis include cranial
capacities estimated for 20 specimens classified as H. erectus (Table 1). The H. erectus
cranial capacities used are published by
Rightmire (1985). A supplemental sample
(Table 2) that includes several poorly dated
and taxonomically debatable specimens (the
Sambungmachan and Ngandong crania) is
also analyzed (see Pope, 1988; Santa Luca,
1980). These crania are not investigated by
Rightmire (1985). Separate analysis of this
sample permits comparisons with previous
investigations without the confounding effects of alternative sample composition.
The Sambungmachan individual is classified here as H . erectus, roughly contemporaneous with Zhoukoudien (following Wolpoff,
1984:Table 1). The Ngandong crania are
treated as H. erectus in some analyses and as
H. sapiens in others.
Measurements of early H. sapiens cranial
3
H. ERECTUS AND EARLY H. SAPIENS CRANIAL CAPACITY
TABLE I. Specimens, estimated geologic age, and
cranial capacity for the Homo erectus sample (N = 20)'
Locality and
specimen
African subsample
SaleOlduvai Hominid 12
Olduvai Hominid 9
East Turkana 3883
East Turkana 3733
Chinese subsample
Hexian
Zhoukoudian V
Zhoukoudian I1
Zhoukoudian I11
Zhoukoudian VI
Zhoukoudian X
Zhoukoudian XI
Zhoukoudian XI1
Gongwangling
Indonesian subsample
Sangiran 10
Sangiran 12
Sangiran 17
Trinil
Sangiran 2
Sangiran 4
Estimated
geologic age
(myr)
Cranial
capacity
.25
.70
1.23
1.52
1.60
880
727
1,067
804
848
.25
.42
.42
.42
.42
.42
.42
.42
30
1,025
1,140
1,030
915
850
1,225
1,015
1,030
780
.62
.62
.62
.62
.76
.93
855
1,059
1,004
940
813
808
(4
Specimen
'These data are published hy Rightmire (1985).
TABLE 2. Specimens included in supplemental
analyses'
Specimen1
Ngandong I
Ngandong V
Ngandong VI
Ngandong IX
Ngandong X
Ngandong XI
Sambungmachan
TABLE 3. Specimens, estimated geologic age,
and cranial capacity for the Homo sapiens
sample (N = 10)'
Estimated
geologic age
(mw)
Cranial
capacity
kc)
.25
.25
.25
.25
.25
.25
.42
1,172
1,251
1,013
1,135
1,231
1,090
1,035
'These specimens werenot analyzed by Rightmire(l985).Analyses
of these crania are presented separately. The Ngandong sample is
analyzed both as H. erectus and early H. sapiens (seetext). Cranial
capacities for Ngandong are published by Holloway(l980) (and by
Weidenreich (1943) for Ngandong IX). The cranial capacity
estimate for Sambungmachan is presented by Pope (1988).
capacities (Table 3) are those published by
Rightmire (1985) and by other researchers
(Table 2). An alternative ordering of early H .
sapiens specimens is also provided and analyzed (Table 4). This alternative ordering is
investigated because several of the early H .
sapiens specimens are insecurely dated. Correction for body size was not attempted (see
''Discussion'').
Regional subdivisions of the H . erectus
sample are not intended to reflect morphologically based taxonomic differences be-
Laetoli Hominid 18
Omo 2
Broken Hill
Petralona
Steinheim
Swanscombe
Hopefield
Dali
Arago 21
Ndutu
Estimated
geologic age
(myd
Cranial
capacity
kc)
,125
.13
.15
.20
.25
.25
.30
.30
.40
.40
1,367
1,430
1,285
1,200
1,100
1,250
1,225
1,120
1,166
1,100
'Data are from Adam (1985), Brauer (19841, Rightmire(1985), and
Wolpoff (1980).
TABLE 4 . Alternative ordering of early
Homo sapiens specimens1
Specimen
Laetoli Hominid 18
Omo 2
Broken Hill
Ndutu
Dali
Swanscombe
Hopefield
Steinheim
Arago 21
Petralona
Estimated
geologic age
(myr)
Cranial
capacity
,125
.13
.15
.20
.25
.25
.30
.30
.40
.40
1,367
1,430
1,285
1,100
1,120
1,250
1,225
1,100
1,166
1,200
(4
'This ordering is made to account for the uncertain geologic age of
several specimens. Ties are not assumed in tests of trend in this
particular sequence. Dates from Table 2 are retained in the same
order.
tween these specimens. Taxonomic discussions of the H . erectus sample have been
provided by numerous aathors (Bilsborough
and Wood, 1986; Rightmire, 1985, 1986;
Stringer, 1984,1987; Turner and Chamberlain, 1989). It should be noted that the Sale
specimen exhibits unusual (possibly pathological) occipital morphology (Hublin, 1985),
which may have affected cranial capacity,
Accordingly, analyses that both include and
exclude this specimen were undertaken.
Methods
Regression analysis has typically been
used to investigate change in hominid cranial capacity through time (Godfrey and Jacobs, 1981; Lestrel and Read, 1973; Lestrel,
1975;Rightmire, 1981,1985).Ordinaryleast
squares regression has frequently been employed to measure these rates, despite the
4
S.R. LEIGH
possibility that this technique may not be
suitable for modeling change in a variable
through time.
The inadequacy of ordinary least squares
regression may be the result of several factors. First, in a time series, the assumption of
independence between dependent variables
is often violated. Correlation among residuals, or serial correlation, may result if values
of Y in one time period are affected by values
of Y at other time periods (Younger, 1979).
Second, ordinary least squares regression
lines tend to be best-fitted to the first and
last observations in the series. Third, ordinary least squares regressions are heavily
influenced by outliers. These problems lead
to difficulty in obtaining accurate estimates
of both slope and intercept in regressions of
time series. Thus, if a trend toward increasing cranial capacity is present, ordinary
least squares estimation may not detect the
presence of such a trend. Conversely, ordinary least squares regression may indicate
the presence of a trend when stochastic behavior (drift) actually accounts for changes
in a variable through time (McCleary and
Hay, 1980:35-36).
In addition to the general problems of
modeling time series with ordinary least
squares regression, there may be substantial
error in measurement of the independent
variable (geologic age) in this particular
sample. This very serious general problem is
discussed by Sokal and Rohlf (1981).Wolpoff
(1984)considers this problem with respect to
measuring evolutionary rates in H. erectus.
Regression techniques that account for
measurement error in both X and Y variables (such as reduced major axis, major
axis, and related techniques) are not well
suited to the present problem. These methods are subject to the same limitations in
time series applications as ordinary least
squares regression. Moreover, the low correlation between geologic age and cranial capacity in the H. erectus sample (see Rightmire, 1985;) produces extremely narrow
reduced major axis confidence intervals
(Jolicoeur, 1975; see also Jolicoeur, 1973, for
discussion of similar problems with major
axis regression). Analyses with Model I1 regressions would likely minimize the differences in rates of evolution between taxa.
Specifically, the H. erectus rate of cranial
capacity increase would be inflated as a
result of low correlation.
The general complications of time series
analysis and the presence of error in measurement of geologic age place a premium on
methods that are capable of accurately describing patterns of hominid cranial capacity
evolution. Unfortunately, the unequal time
intervals between observations and the presence of more than one cranial capacity for a
given date lead to difficulties in using standard methods for the analysis of time series
(Vandaele, 1983). Although these obstacles
are often encountered in paleobiological context s, little theoretical statistical research
has been undertaken to solve these problems. However, Kitchell et al. (1987)discuss
the problem of unequal intervals in paleobiological contexts, while Quenouille (1957),
and Cleveland and Devlin (1980, 1982) address this problem in econometric applications.
A simple nonparametric test for trend that
minimizes the deficiencies of these data is
provided by Hubert et al. (1985; see also
Konigsberg, 1990). Specifically,this test is a
distribution-free permutational test for randomness in ordered data. The test makes no
assumptions about the length of intervals
between observations. In addition, multiple
observations sharing the same geologic age
(ties) can be accommodated by this technique. It is essential t o note that measurement error in geologic age is minimized because observations are merely ordered from
earliest to latest.
Hubert et al.’s test allows detection of a
trend in cases where regression may not
prokide an accurate measure of trend. The
procedure is based on a test statistic (r)
derived from the cross-product (or dot product) of two 1 x N matrices or vectors where:
r =a’b
The first vector (a)is, in this case, a vector of
cranial capacities ordered by time. The second vector (b) consists of an ordered series
(e.g., 1,2,3,4,5)with equal intervals between
each observation. The b vector reflects the
chronological order of the variables in the a
vector. In the case of ties (cranial capacities
from the same date) the cells of the b vector
are “stuttered (e.g., 1,2,2,2,3,4,5).
Hubert et al. (1985) recommend that the
dot product of these vectors serve as a test
statistic for measuring the presence of randomness in ordered data. The significance of
the initial summed cross-product statistic
(r)can be estimated by obtaining a distribu-
H . ERECTUS AND EARLY H . SAPIENS CRANIAL CAPACITY
tion of summed cross-products calculated
from random permutations of the values in
one of the original vectors. In other words,
one vector is randomly re-arranged, and a
cross-product is calculated for each new permutation. The significance level of the test
statistic is defined by (M + l)/(N + 11,
where M is the number of values as large or
larger than r and N is the number of permutations (Hubert et al., 1985). Thus, the significance of the association between vectors
can be estimated.
A significant positive trend is implied
when only a small percentage of randomly
obtained dot product values exceeds the
value of gamma derived from the original
vectors. Conversely, randomness in the order of observations is suggested if the value
of the original gamma lies near the center of
the distribution of permutational gammas.
Up to N! permutations are possible.
In the present study, tests for trend are
undertaken for the H. erectus, early H . sapiens, supplemental samples, and in subdivisions of the H. erectus sample. The subdivisions of the H. erectus sample include
African, Chinese, Indonesian, and “Asian”
(represented by the combined Chinese and
Indonesian samples). All analyses were undertaken with 1,000 random permutations
with the exception of the African (120 exact
permutations) and Indonesian (720 exact
permutations) samples. Exact permutations
utilize every unique cross-product value and
are feasible because the African and Indonesian samples are very small.
The test for trend offers information about
the presence and significance of directional
change in the variable of interest. Unfortunately, it does not provide information about
rates of change. Nonparametric and ordinary least squares regressions are used for
this purpose, but only with the assumption
that estimates of regression parameters are
unbiased. If regressions detect a trend after
the presence of trend has been established
with Hubert et al.’s method, then the regression results may be interpreted more confidently.
Nonparametric spline regression described by Schluter (1988; see also Eubank,
1988; Seber and Wild, 1989) is used for visual assessment of changes in cranial capacity. This method is not constrained by choice
of model (e.g., linear, nonlinear), allowing
flexibility in the modeling of complex distributions. Most importantly, confidence inter-
5
vals with these regressions are calculated
through bootstrap techniques, which are
based upon iterative random selection (with
replacement) of data points from the original
sample. The main assumption of this technique is that the sample analyzed bears a
relatively close relationship to the probability distribution from which the sample was
drawn (Efron, 1982). With iterative sampling, relatively accurate confidence intervals should be obtained. In all cases, 300
iterations are undertaken in construction of
confidence intervals,
Finally, ordinary least squares regression
and analysis of covariance are employed to
estimate the significance of differences between rates of change in H. erectus and early
H. sapiens. These techniques are used only
after tests for trend and modeling of bivariate distributions with nonparametric regression. Thus, their roles can be viewed as
corroborative. Only linear models were used,
facilitating tests of significance between regressions. It should be noted that nonlinear
models may suffer the same limitations as
ordinary least squares linear models in time
series analysis (Seber and Wild, 1989).
Analysis of covariance permits tests of
significance of differences in regression
lines. This analysis is comprised of two parts.
The first is a test for difference in slope,
while the second is a test for difference in
intercepts (see Edwards, 1985). Together,
these tests provide a basis for distinguishing
between several hypotheses that specify the
relation between regression lines. Specifically, analysis of covariance permits identification of 1)identical regression lines (equal
slopes and intercepts), 2) parallel regression
lines (equal slopes, unequal intercepts), or 3)
unequal slopes. Throughout this study, significance for all statistical tests is measured
at the .05 level. Regression significance values are evaluated as two-tailed tests because
previous analyses (Rightmire, 1981, 1985)
suggest that slopes near zero will be obtained in the H. erectus analyses. Tests for
trend are treated as one-tailed tests.
RESULTS
Trend
The test for trend indicates that cranial
capacity in H. erectus may not vary randomly
with respect to geologic age (Table 5). Of
1,000 random permutations, only 58 (5.8%)
of the randomly obtained values of gamma
equaled or exceeded the value of gamma
6
S.R. LEIGH
TABLE 5. Results of tests for trend by taxon, by region, and withing the Homo erectus sample*
Sample or subsample
Probability*
Number of permutations
.058
1,000'
1,000'
1,000'
Homo erectus
Homo erectus, excluding SaleEarly Homo sapiens
Early Homo sapiens, re-ordered
African Homo erectus
Asian Homo erectus
Chinese Homo erectus
Indonesian Homo erectus
.041
,001
,020
.500
1,000'
1202
,014
,079
,200
1,000'
1,0001
7202
*Probability values are based on the No. of randomly permuted gammas equal to or larger than observed gammas.
'Random permutations.
'Exact permutations.
based upon the original ordering of data.
When the Sale specimen is removed from the
sample, the trend in H . erectus is more significant. In this analysis, 41 of 1,000permutations (4.1%) exceeded the observed
gamma. Thus, a significant positive trend
toward increasing cranial capacity appears
present in the H . erectus sample, although
this result is not quite significant when the
Sale specimen is included.
The early H . sapiens sample also appears
to exhibit a trend toward increasing cranial
capacity. In this analysis, only 1 of 1,000
randomly calculated gamma values (. 1%)
equaled or exceeded the original observed
gamma.
A positive trend is also present when some
of the less securely dated early H . sapiens
specimens are re-ordered (see Table 4).
Trend in this re-ordered sample is less obvious than in the analysis with the original
ordering, with 20 of 1,000 (2.0%) randomly
calculated gammas exceeding the initial reordered gamma.
The patterns among subsamples of H .
erectus are not as clear. The Chinese subsample may show a significant trend toward
increased cranial capacity, with 79 of 1,000
randomly obtained gammas (7,9%)equal to
or exceeding the observed gamma. Obviously, this is not quite significant at the .05
level. Trend is apparent neither in the Indonesian subsample (143 of 720 exact gammas
equal to or exceeding the observed gamma)
nor in the African subsample (59 of 120 exact
gammas equal t o or exceeding the observed
gamma).
Combination of the Chinese and Indonesian subsamples to form the Asian subsample produces significant results. Only 14 of
1,000 random gammas equal or exceed the
observed test statistic for the combined
Asian sample.
TABLE 6. Results of tests for trend in the
supplemental sample*
__
Samiole or subsamole
Probabilitv*
Homo erectus,
Ngandong a s Homo erectus
Early Homo sapiens,
Ngandong as Homo sapiens
Asian Homo erectus,
with Ngandong
Indonesian Homo erectus,
with Ngandong
-
,003
,003
,001
,001
*Probability values are based on the No. of randomly permuted
gammas equal to or larger than observed gammas, and there are
1,000 permutations in each analysis.
Analyses of the supplemental sample mirror analyses based on Rightmire's (1985)
sample (Table 6). Trend in the H . erectus
sample appears more significant. Inclusion
of the Ngandong crania either with the early
H . sapiens sample or with the Asian H .
erectus sample produces significant trend.
Significant trend seems present in the supplemental Indonesian sample, in contrast to
results based on the original sample.
Regression analysis
Spline regressions of H . erectus cranial
capacity against geologic age suggest little
change, with some possible variation in rates
through time (Fig. 1). The rate of cranial
capacity evolution may fluctuate through
time in H . erectus, although the rate increase
between 1.2 and 1.6 myr is based upon only
three specimens. In contrast, early H . supiens cranial capacity appears to increase
markedly during a short time span. Visual
assessment of predicted lines and confidence
intervals indicates that the rate of change in
early H . sapiens probably exceeds the rate of
change in H. erectus. Unfortunately, regional subsamples are too small to allow
H . ERECTUS AND EARLY H . SAPIENS CRANIAL CAPACYW
7
0.9
0.7
0.5
0
0.2
0.4
0.6
0.8
1
Geologlc Age (Miillons)
-Homo erectus
1.2
1.4
1.6
+Early Homo sapiens
Fig. 1. Predicted regression lines and 95% confidence intervals obtained from spline regression.
Confidence intervals are based upon bootstrap procedures with 300 iterations for each sample. Only
predicted values are shown.
TABLE 7. Ordinary least-squares regresion results by taxon, and by region within the Homo erectus sample
Sample or subsample
RZ
Slope
Intercept
P-value for slope
Homo erectus
Homo erectus,
excluding Sale
Early Homo sapiens
Early Homo sapiens,
re-ordered
African Homo erectus
Asian Homo erectus
Chinese Homo erectus
Indonesian Homo erectus
.14
.18
-124.87
-145.13
1,029.79
1,050.00
<.I09
<.075
.61
59
-864.08
-658.72
1,440.75
1,389.31
<.007
<.069
.01
26.28
-367.48
-523.06
-282.18
837.34
1,172.51
1,233.00
1,125.95
<.a49
<.033
<.123
<.438
.31
.31
.I6
accurate calculation of bootstrap confidence
intervals.
Parametric regressions produce results
relatively consistent with trend analysis
(Table 7). Ordinary least squares regression
(Fig. 2) suggests that H. erectus cranial capacity changes relatively slowly through
time: a rate of about 125 cc per million years
is suggested (cf. Rightmire, 1985).This slope
appears statistically indistinguishable from
zero (p < .log), a result that seems to conflict with the results of the test for trend. It
should be noted that the observed cranial
capacity value for Sale exceeds the value
predicted from this regression. When the
Sale specimen is removed from the H. erectus
regression, the estimated slope increases
slightly to 150 cc per million years. This
slope is not quite significant (p < ,0691.
The regression for the early H. sapiens
sample suggests a very rapid increase in
cranial capacity through time. A significant
slope of about 865 cc per million years is
obtained in this regression (cf. Rightmire,
1985). Analyses of the re-ordered early H .
sapiens data (see Table 3) indicate a slope
(about 660 cc per million years) which may
not be significantly different from zero.
Other regressions imply the presence of
some geographic differences in the rate of
increase in H. erectus cranial capacity. A
statistically significant increase in cranial
capacity is apparent in the Asian subsample.
As with trend results, smaller subsamples
(African, Chinese, and Indonesian) do not
appear to show statistically significant increases through time. Finally, inspection of
residual plots for all least-squares regressions indicates no obvious correlation among
residuals.
Regression analyses of the supplemental
sample show some differences with those
based on Rightmire’s (1985) sample (Table 8). Change in cranial capacity is signifi-
:I
8
S.R. LEIGH
mania1 Dapacliy (Thousands)
1.5T
+
\
++
t
-----y4
0.9
o
0.2
0.4
-
0.e
0.8
1
1.2
1.4
1.8
Oeologlc Age (Mllllons)
Homo erectus
+ Early Homo saplens
Fig, 2. Predicted ordinary least-squares regression lines and data points for Homo erectus and early
Homo sapiens.
TABLE 8. Ordinary least-squares regression results for the supplemental sample
R2
Slope
Intercept
P-value for slope
Sample or subsample
Homo erectus,
with Ngandong
Earlv Homo sawiens,
wGh Ngandong
Asian Homo erectus
Indonesian Homo erectus
.2B
-206.24
1,111.63
<.004
63
-862.54
1,411.84
<.009
.49
.65
-468.16
-464.30
1,237.96
1,256.31
<.ooo
cant when Ngandong is included in either
the H . erectus or the H . sapiens sample.
Analyses of Ngandong with the early H .
sapiens sample suggests only minimal differences with analyses of the original early H .
sapiens sample. Assignment of Ngandong to
the H . erectus Indonesian subsample produces statistically significant regression results.
Analyses of covariance
Analysis of covariance (Table 9) suggests
that differences in rates of cranial capacity
increase in H . erectus and early H . sapiens
may not be statistically significant (p < .056),
although this level of significance makes
interpretation difficult. Exclusion of the Sale
specimen produces regression lines that appear more similar. For these tests, the potentially insignificant difference in slope combined with significant intercept differences
suggest that these regression lines are parallel.
In other comparisons (Table 9),patterns of
cranial capacity change among various H .
erectus subsamples appear similar. Comparisons of regressions between early H . sapiens
<.004
and regional subsamples of H . erectus may
suggest some differences. The African regression appears significantly different from
the early H . sapiens regression. The rate of
change in the Asian subsample is indistinguishable from the rate of change in the
early H . sapiens sample, but intercepts appear to differ, Regressions for the Chinese
and Indonesian subsamples are indistinguishable from the early H . sapiens regression. It should be emphasized that small
sample sizes may substantially influence
the analysis of covariance results, particularly for the regional comparisons.
Analyses of covariance for the supplemental sample do not alter the results for comparison of H . erectus with early H . sapiens
(Table 10). When Ngandong is classified as
early H . sapiens, the implied relation does
not change, although the probability values
might be close enough to consider the H .
erectus and early H . sapiens slopes significantly different. Other analyses present
some discrepancies with the analysis of the
original sample. Slope differences (rather
than same lines) are implied for the AfricanAsian comparison and for the African-Indo-
9
H . ERECTUS AND EARLY H . SAPIENS CRANIAL CAPACITY
TABLE 9. Analysis of covariance results f o r comparisons of regression lines between taxa, between Homo erectus
subsamples, and between Homo sapiens and Homo erectus subsamples
Comparison
Homo erectus,
Early Homo sapiens
Homo erectus (excluding
Sale', Homo sapiens
African Homo erectus,
Asian Homo erectus
African Homo erectus,
Chinese Homo erectus
African Homo erectus
Indonesian Homo erectus
Chinese Homo erectus,
Indonesian Homo erectus
Early Homo sapiens,
African Homo erectus
Early Homo sapiens,
Asian Homo erectus
Early Homo sapiens,
Chinese Homo erectus
Early Homo sapiens,
Indonesian Homo erectus
Implied
relation
between
regressions
Significance of
sloue
difference*
Significance of
interceut
difference
,056
.000
Parallel
.062
.001
Parallel
,059
,417
Same line
.I3
,301
Same line
,494
,412
Same line
,627
,620
Same line
Diff. slopes
,022
,161
.04
Parallel
.416
.ll
Same line
,161
,756
Same line
* A significant difference in slope obviates the test for intercept differences
TABLE 10. Analysis of covariance results for comparisons of regression lines between taxa, between Homo
erectus subsamples, and between Homo sapiens and Homo erectus subsamples for the supplemental sample*
Comparison
Homo erectus (with Ngandong),
early Homo sapiens
Homo erectus,
early Homo sapiens (with
Ngandong)
African Homo erectus,
Asian Homo erectus
African Homo erectus,
Indonesian Homo erectus
Early Homo sapiens,
Asian Homo erectus (with
Nagandong
Early Homo sapiens (with
Ngandong), Asian Homo
erectus
Early Homo sapiens (with
Ngandong), Indonesian
Homo erectus
Implied
relation
between
regressions
Significance of
slope
difference*
Significance of
intercept
difference
.096
,002
Parallel
.050
,000
Parallel
Diff. slopes
.003
,006
-
,228
,027
Parallel
,160
,077
Same line
I72
,082
Same line
Diff. slopes
*A significant difference in slope obviates the test for intercept differences.
nesian comparison. All other comparisons
involving the supplemental sample are unchanged.
DISCUSSION
The results of this analysis suggest that
cranial capacity in H. erectus may change
significantly through time. Patterns of
change among regional subsamples of H.
erectus may show limited variability, but
this cannot be demonstrated with analysis of
covariance. Small sample sizes may be partially responsible for variation in the presence of significant increases among various
H. erectus subsamples.
Regression results, taken with the appro-
10
S.R. LEIGH
priate caveats, generally corroborate the re- interpretations of slope comparisons (Huisults of regional trend analyses. Spline re- tema, 1980:146-147). Third, the data do not
gressions may suggest fluctuation in the extend to the Y-axis.
rate of evolution in the H. erectus sample. An
If these intercept differences can be given
early increase in the African subsample may meaningful biological interpretations, then
be present.
their presence would suggest that some inThe lack of statistically significant in- crease in rate of cranial capacity change
creases in cranial capacity in all but the would be necessary to produce the pattern of
Asian subsample suggests that an hypothe- change observed in early H. sapiens. It
sis of stasis for these samples cannot be should also be noted that the Asian subsamrejected. Considered in conjunction with ple is composed of later specimens than the
trend results, regression models (see Levin- African subsample. Thus, the similarity in
ton, 1982; Rightmire, 1981,1985) and other rate of evolution between the Asian subsamstatistics not designed for time series analy- ple and the early H. sapiens sample may
sis in general (see Wolpoff, 1984)may under- reflect temporal rather than regional differestimate the actual amount of change in this ences in rates of evolution.
case.
A directional process probably accounts
Cranial capacity in early H. sapiens ap- for the possibility of trend in H. erectus, early
pears non-static, and both trend and regres- H. sapiens, and in the Asian H. erectus subsion analyses provide consistent results. In samples. This process could be directional
addition, a trend seems present even when selection on brain size, although other proless securely dated specimens are re-or- cesses might account for the observed patdered.
terns. For example, the trend of increasing
Analysis of covariance suggests that dif- cranial capacity in these samples could be a
ferences in rates of change may not be signif- result of evolutionary increase in body size
icant in a comparison between the early H. (cf. Pilbeam and Gould, 1974; Lande, 1979).
sapiens and the entire H. erectus sample.
The hypothesis that evolutionary changes
The rates of increase in the Asian, Chinese, in body size account for the increases in
and Indonesian subsamples cannot be dis- cranial capacity observed in these samples is
tinguished from rates of increase in the early not directly testable. The inability to test
H. sapiens sample. The rate of change in the this hypothesis is a result of the lack of a
African subsample is statistically distin- suitable measure of body size for each indiguishable from the rate of change in the vidual in this sample. Ideally, a measure of
early H. sapiens sample. Again, small sam- body size would be derived from dimensions
ple sizes may have a marked effect on these such as long bone lengths for each individresults.
ual. Such data are unavailable for these
Incorporation of Ngandong and Sambung- specimens. Dental size could provide an almachan into Rightmire’s (1985) sample pro- ternate source of size estimation. Dental
duces a few differences in results. Specifi- measures might be problematic in this concally, trend and regression analysis suggest text, however, because tooth size seems to
significant change in the Indonesian sub- show a trend toward decrease through time
sample.
in the hominid lineage (Pilbeam and Gould,
Differences in rates of change are not sta- 1974; Wolpoff, 1984).
tistically detectable between the early H.
An indirect argument to evaluate this possapiens sample and various H. erectus sub- sibility can be based on an early juvenile H.
samples. However, different overall patterns erectus specimen from Africa, KNM-WT
of change may characterize these compari- 15000, whose stature is estimated at about
sons. Specifically,intercepts differ when the 168 cm (Brown et al., 1985).l The stature
early H . sapiens regression is compared with estimate for this individual is clearly within
the H. erectus sample and with the Asian
subsample.
Intercept differences are difficult to inter‘This specimen was not used in the cranial capacity analysis
pret in this analysis for two reasons. First, because it is a juvenile. However, it can be noted that incorporation of this specimen into Rightmire’s original sample produces
sample sizes are very small. Second, the data only
slight differences in results. Specifically, the significance
for early H. sapiens and H . erectus overlap levels for the slope comparison between African and Indonesian
subsamples changes from ,059 to ,044. In the Homo erectus vs.
only in a relatively restricted range. The lack Homo
sapiens comparison, the slope comparison moves from ,056
of extensive overlap may also complicate to ,047. All other results are unchanged.
~~
H . ERECTUS AND EARLY H . SAPIENS CRANIAL CAPACITY
the range of modern adult humans (Tanner,
1978), despite a dental developmental age
which suggests a chronological age of 11-13
years (Brown et al., 1985). Thus, large body
size seems apparent at an early geologic age
(1.6 million years), implying that increases
in H. erectus and early H. sapiens body size
may not directly account for the observed
increases in cranial capacity.
A related explanation is that the trend in
cranial capacity evolution reflects selection
for increased body size in taxa ancestral t o H.
erectus. In other words, brain size may lag
well behind increases in body size (Lande,
1979). In the case of a lag, Lande suggests
that selection directly on brain size may
“adjust brain size to accumulated changes in
body size” (1979:412). Investigation of this
possibility requires evaluation of trends in
brain and body size in taxa ancestral to H.
11
Regional analyses do not greatly clarify
the situation. Significant trend is present in
the Asian subsample, but this sample is also
late relative to most of the African subsample. Analysis of additional specimens suggests that significant increases in cranial
capacity may characterize the Indonesian
subsample. Small sample sizes, taxonomic
and chronometric problems, plus limitations
on regressions restrict the strength of these
interpretations.
Further evaluation of contrasting evolutionary models must be based on further
analyses of fossil material and on molecular
analyses. It should be emphasized that finer
estimates of rates of evolution within H.
erectus may be necessary, particularly between .8 and 1.6 myr.
CONCLUSIONS
erectus.
Changes of cranial capacity through time
The present analysis cannot distinguish in H. erectus, regional subsamples of H. erecbetween these (and other) possibilities. How- tus, and early H. sapiens are measured using
ever, the apparent presence of trend in early time series techniques and regression. Tests
H. sapiens and H. erectus may indicate that for trend indicate that cranial capacity may
there are similarities in the nature of the increase significantly in H. erectus, contrary
directional process or processes that cause to some previous suggestions. A trend tocranial capacity increase. In other words, ward increasing cranial capacity is present
factors that cause cranial capacity increase in early H. sapiens. Trend is not apparent in
in H. erectus may also affect the rate of most H. erectus regional subsamples, but an
increase in early H. sapiens. The apparent Asian subsample may show significant inincrease in rate of cranial capacity evolution creases. In general, the test for trend utilized
in early H. sapiens could be due either to in this study provides more reliable informastronger selection for increased brain or body tion about cranial capacity change in H.
size or to a more sensitive response to selec- erectus than previous investigations. The
tion. In any case, these results provide no presence of a statistically significant trend
clear evidence that substantial differences in in some samples of H. erectus implies that
causes of cranial capacity increase existed previous models which specify a punctuated
between early H. sapiens and at least some evolutionary pattern of later hominid evoluH. erectus subsamples. Limited variation in tion may not be accurate.
trend among the H. erectus subsamples may
Nonparametric regressions could indicate
imply differences in the precise evolutionary variation in the rate of cranial capacity
mechanisms that affect cranial capacity change through time in the H. erectus samamong H. erectus subsamples.
ple. Specifically, a rapid increase in cranial
The results of this analysis provide a basis capacity may be present in the early African
for refinement and clarification of macroev- subsample. The rate also seems to increase
olutionary models that have been proposed late in the H. erectus sequence in a sample
to explain relations between H. erectus and that is predominantly Asian.
early H. sapiens. The likelihood that a trend
Regression analysis results are consistent
toward increasing cranial capacity is present with some trend analysis results, but limitain the H. erectus sample indicates that stasis tions on regression analysis hinder biologimay not characterize cranial capacity evolu- cal interpretations. Analyses of covariance
tion in H. erectus (cf. Eldredge, 1985; Right- suggest that rates of evolution in H. erectus
mire, 1981, 1985). This result implies that and in early H. sapiens cannot be statistipreviously proposed punctuated equilibrium cally distinguished. However, the overall
models do not adequately describe later pattern of cranial capacity change (as indihominid evolution.
cated by significant intercept differences)
12
S.R. LEIGH
may vary between taxa. Regional differences
among subsamples of H. erectus seem
present.
ACKNOWLEDGMENTS
I thank Dr. Lyle Konigsberg for providing
substantive comments throughout the
course of this research. His computer programs for analysis of trend are also greatly
appreciated. Drs. Brian Shea, Larry Cochard, and Jill Bullington offered constructive comments on this manuscript. I also
thank Drs. Malcolm Dow, Fred Smith, Maceij Henneberg, and G.N. van Vark for comments on earlier versions of this paper. The
valuable comments of anonymous reviewers
and editors contributed much to this manuscript.
An earlier version of this paper was
awarded the 1988 Ales HrdliEka Student
Paper Prize at the annual meeting of the
American Association of Physical Anthropologists, Kansas City, MO.
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