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Early hominid body weight and encephalization.

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Early Hominid Body Weight and Encephalization
KEY WORDS Encephalization . Australopithccus . Plio-Pleisto
cene hominids Body weight Brain size.
The body weight of the Plio-Pleistocene hominids of Africa is
estimated by predicting equations derived from the Terry Collection of human
skeletons with known body weights. About 50% of the variance in body weight
can be accounted for by vertebral and femoral size. Predicted early hominid
weights range from 27.6 kg (61 lb) to 54.3 kg (119 lb). The average weight for
AusIralopithecus is 43.2 kg (95 lb) and for Homo sp. indet. from East Rudolf,
Kenya, is 52.8 kg (116 lb). These estimates are consistent even if pongid proportions are assumed. Indices of encephaliIation show that the brain to body weight
ratio in Australapithecus is above the great ape averages but well below Homo
supieris. The Homo sp. indet. represented by the KNM-ER 1470, O.H. 7 and O.H.
13 crania have encephalization indices above Az!stralopztlzecus despite the
greater body weight of'the former.
Although present fossil evidence suggests that bipedalism preceded encephalization in hominid evolution, brain size increase is the hallmark of the last two
million years of human history. During
this time the average brain size doubled.
Body siz,e may also have increased over
the last two million years. Some investigators have estimated that one form of
early hominid may have weighed as little
as 18 kg (40 lb) (Lovejoy and Heiple, '70;
Genet-Varcin, '66, '69; Robinson, '72). It
is important, therefore, to view brain size
increase in relation to body size. However,
body size is difficult to estimate from the
fragmentary fossils. Until recently most
estimates were very subjective and often
based upon a single specimen. This has
led to widely different estimates. For example, predictions of the stature of Australopithecus africanus from South Africa
have ranged fi-om 3.5 feet (Lovejoy and
Heiple, '70) to over 4.5 feet (McHenry, '74).
These differences can be critical when
brain to body weight ratios are calculated.
For example, Jerison ('63) using one set of
body weights found that encephalization i n
the australopithecines was not advanced
over the great apes and was far below Homo
sapiens. On the other hand, Holloway ('73b,
'74) using other body weight estimates
J. P H Y S .
4 5 : 77-84,
and other methods indicated that the
brain to body weight ratio in the australopithecine~was not out of the range of modern human variation. This becomes especially important when evaluating the claim
by R. E. Leakey ('73a, '74) and others
(e.g., Pilbeam and Gould, '74) that the
KNM-ER 1470 cranium from East Rudolf,
Kenya, dating between two and three million years is significantly more encephali7ed than the australopithecines and
should, therefore, be classified as the earliest member of the genus Homo. A question
addressed in this report is how much of
the increased brain siLe of 1470 can be
accounted for by the possibility that body
size was greater than in contemporary
In order to estimate fossil hominid body
weight, 43 human skeletons were measured from the Terry Collection presently
housed in the Division of Physical Anthropology, Smithsonian Institution, Washington, D.C. This collection has the advantage
of having body m eight recorded €or each individual as well as age, stature, somatotype
photograph, cause of death, and many
other important data. The sample consists of 22 males and 21 females between
the ages of 19 and 50 years. Excessively
emaciatcd individuals, such as those dying from emaciating diseases like tuberculosis, were eliminated as well as excessively
obese subjects as judged by the somatotype photographs. For comparative purposes, eight Pan troglodytcs and seven female Pongo pymizeus from the Museum of
Comparative Zoology, Harvard University,
the Smithsonian Institution, and Professor
SchultL’scollection at the Anthropologisches
Institut, Zurich, were measured.
Two regions of the body which are most
frequently represented in the Plio-Pleistocene hominid collections wcre used to estimate body weight: the vertebrae and the
proximal femur. Fossil vertebrae included
are the last lumbar and the last thoracic
of Sts 14 (Austrulopithecus nfrzcnnus) from
Sterkfontein, South Afi-ica (Robinson, ’72)
and of SK 3981 (A. rubustus) from Swartkrans, South Africa (Robinson, ’72). The
fossil proximal femora are those from
Swartkrans (SK 82 and SK 97, A . robustus,
described by Kobinson, ’72) and from East
Rudolf, Kenya (KNM-ER 738 and 1503 classified as Australopithecus sp. indet. and
KNM-ER 1472 and 1481c classified as
Homo sp. indet.), described by Leakey (’71,
’73a,b), Leakey et al. (’72), Walker (‘73),
Wood (’751, and Day et al. (‘75). Brain
size cstimates are those reported by Holloway (’70, ’73a), Leakey (‘74), and Day
et al. (‘75). Specimens referred to Australopithecus ufricanus include Taung, Sts
60, Sts 5, Sts 19, Sts 71 and MLD 37/38.
A. robustus is represented by SK 1585. The
East African australopithecines are O.H.
5, O.H. 24 (following Leakey’s suggestion,
’74), KNM-ER 406, 732 and 1813. The
specimens referred to Homo sp. indet. are
KNM-ER 1470, O.H. 7. and O.H. 13. Olduvai Hominid 16 is not included in this
study because of its fragmentary condition. Species of Australopitlzeczis are not
separated in East Africa because of the
difficulty in classifying the East African
The vertebral measurements include the
sagittal and transverse diameters of the
last thoracic and last lumbar centra on
the cranial surface and at the middle (corresponding to Martin measurements 4, 6,
7, and 9, Martin and Saller, ’57). The
average of the four sagittal diameters is
multiplied by the average of the four trans-
verse diameters forming a n approximation
of the cross-sectional area (actually closcr
to 413 the area) of the centrum. The reason
for using eight measurements is to alleviate the problem of minor shape differences having too great a n effect on the
final result.
The femoral mcasureinents include the
vertical diameter of the head (Martin measurement 18, Martin and Saller, ’57), the
distance from the center of the head to the
most lateral surface of the greater trochanter taken perpendicular to the shaft
axis, and the transverse and anteroposterior
diameters of the shaft just below the lesser
trochanter (Martin measurements 9 and
10, Martin and Saller, ’57). The two shaft
diameters were multiplied together to give
a n approximate measure of cross-sectional
area (again closer to 4/3 the area). These
measurements vary allometrically with
body weights. To overcomes this problem,
each measurement is raised to the exponent
which describes its relationship to body
weight so as to approximate isometry.
These four measurements are used to compensate partially for the fact that the australopithecine femora have relatively smaller femoral heads, longer femoral necks,
and more variable shaft diameters than
do modern humans.
Least squares regression formulae are
calculated predicting body weight from
vertebral and femoral size. This technique
assumes that errors of measurement are
not present in the dependent variable which
is not the case here, but tests showed that
very similar predictions result using other
techniques such as Bartlett’s “best fit”
which do not make this assumption (Simpson et al., ’60).
Four indices of encephalization are calculated. The constant of cephalization
(CC) was developed by Hemmer (‘71) from
Snell’s (1891) and Dubois’ (1897) index
of cephalization. It is the antilog of the
9-intercept value in the equation
l o g y = 0.23 log x
+ log k
where y is the brain size in cubic centimeters, x is the body weight in grams, and
h is the constant of cephalization. The constant, 0.23, is used because it best describes the allometric relationship between
brain and body size i n closely related
groups of primates. Von Bonin (’37) and
Jerison ('55, '61) showed that 0.67 fits
mammals in general better than the lower
The index of progression (IPj was derived by Bauchot and Stephan ('66, '691,
Stephan and Andy ('69) and Stephan ('72).
It is the ratio of actual brain size to brain
size predicted on the basis of body weight.
The latter prediction is based upon the
where the convalue 0.0429 (body wt,)o.ri3
stants were derived empirically from the
brain to body weight ratio in basal ("prirnitive") insectivores. A similar formula gives
the encephalization quotient (EQ) which
Jerison ('70, '73) developed. It is also the
ratio of actual brain size to predicted brain
siLe although the predicted brain size is the
quantity 0.12 (body wt.j2 :{ which was derived as a n average for living mammals.
The extra neurons (Ncj formula was also
developed by Jerison ('63, '73). It is the difference between actual brain size and predicted brain size multiplied by a constant
which gives a n estimate of the actual number of neurons associated with improved
adaptive capacities. The formula can be
expressed as
8 X 107 (E213 - P
where E is the brain size and P is the body
this equation is given elsewhere (McHenry,
'75). There are many other ways to relate
vertebral size to body weight and each
formula gives somewhat different answers.
This particular equation if used because
it has a high correlation coefficient and
because it includes a large number of variables (to minimize the effects of niinor
shape differences in the fossils). The predicted body weight for Sts 14 is 27.6 kg
(61 lbj and for SK 3981 is 36.1 kg (79 Ib).
It can be assumed that these specimens are
both small for their taxa [A. clfricnrzus and
A. robustus, respectively) since other individuals from the same sites are larger
(McHenry, '74). From Sterkfontein there
is Sts 73, a vertebral centrurn which Robinson ('72) believes is a n upper lumbar,
but could be a lower thoracic since it has
a feature which may be a rib facet. Whichever vertebra it is, its size is greater than
any of the vertebra of Sts 14 or S K 3981.
A modern human with a last thoracic as
large as Sts 73 would be expected to weigh
43.0 kg (95 lb). From Swartkrans there are
the two proximal femora, SK 82 and S K
97, which appear to have belonged to
larger individuals as will be shown below.
Body weight can be predicted from the
size of the proximal femur by the equation
Body wt = 0.028 (head)','*
0.013 (shaft
The formula
Body w t = 12.16 (vertebral
relates body weight and average vertebral
cross-sectional area in the human sample
such that r = 0.69. A scatter diagram for
+ 0.018 (neck)1."3 +
- 12.07.
This formula is derived from the human
sample for which the multiple correlation
coefficient is 0.68. Figure 1 displays this regression line and the scatter of points
around it. The weights predicted are
Sts 14
Sts 73
S K 3981
SK 82
SK 97
KNM-ER 738
KNM-EK 1503
KNM E R 1472
KNM-ER 1481c
Austruloprthecus afrzcunus
A u $ t r a l o p z t k e c u s nfricanus
Aiistrn lo pi t heczr 3 robzr $ f i r s
A u ~ t ~ r r l o p r t k e r ruos b u s t u s
Auqtrriloprtkec U S robustus
A z i ~ t m l o p r t l i e cU S sp indet
A u s t n r l o p i t k e c u s s p indet
Homo sp indet
H o m o sp indet
Mean for Au\trolop7thecus = 4.1 2 k g (95 lb)
Mean for f?omo sp indet = 52 8 k g (116lb)
0 8 9
Enc e phrr I rzn t iu n i nil i c e s
of pro-
of cephaIization p
gression s
quotient 4
Weight i s cxprcssed in kilograms but grams a w used in t h c calculations
Hrmmer, '71.
Stephan. '72
Jer1si)n. ' 7 0 . '71, ' 7 3 .
5 Jerison. '63.
ii Brain size a n d body weight d a t a from Pllbearrl and Gjuld ('74).
shown in table 1. These estimates appear
to be reasonable since very similar predictions are derived by using other measurements on the fossils such as total
length of the femur. Although the formula
is based on modern human proportions,
figure 1 shows that the pongids lie very
close to the regression line, suggesting that
the line is valid for hominoids generally.
The mean body weight for the East African fossils classified as Austrnlopzthecus
is estimated to be 46.5 kg (102 lb). The
two fossil femora classified as Homo sp.
indet. have a predicted average weight
of 52.8 kg (116 lb). A value of 44.4 kg
(98 Ib) is suggested for the South African
A robustus, an estimate midway between
the highest and lowest predictions for
that taxon. A mean body weight for the
South African A. u f r i c c ~ n ~is~ predicted
be 35.3 kg (78 lb) based upon the Sts 14
and 73 vertebrae. The average predicted
body weight for all of the specimens referred to Australopithecus is 43.2 kg
(95 lb).
Indices of encephalization are shown i n
table 2. These are based upon Holloway's
('70, '73a) derivations of endocranial volFig. 1 Scatter-plot a n d regression line relating
femoral size to body weight by the tormula given
in the text. Only the h u m a n s were used i n the
calculation of the regression line. The pongids
were plotted for reference only. The units o n the
abscissa are the sums of t h e three allornetrically
adjusted measurements of the proximal end of the
femur described i n the text.
umes which average 442 cc for the South
African A . africunus, 530 cc for A. robustus, and 534 cc for the East African fossils assigned to the genus Austrtilopithecus.
The Homo sp. indet. endocranial volumes
average 704 cc. The four indices given in
table 2 show similar results: the australopithecines are well above the pongids in
encephalization and Homo sp. indet. is
moderately above the australopithecines.
These body weight estimates can be compared to some earlier predictions based
on fewer specimens and more subjective techniques. Genet-Varcin ('66, '69) estimates 18-22 kg for A. afiicanus and 40 kg
for A. robustus. Lovejoy and Heiple ('70) also give values from 18-23 kg for A. ufricu~zus
based on their reconstruction of the Sts
14 femur. Robinson ('72) proposes that Sts
14 weighed between 18 and 27 kg but the
robust form of early hominid was 68-91
kg. Wolpoff ('73) gives a larger figure for
A. africanus (37.3 kg) based on many
more specimens and a more empirical
technique. McHenry ('74) reports 35 kg
and 42 kg for A. ufiicnnus and A. robustits,
respectively. Finally, Pilbeam and Gould
('74) submit 2 2 4 0 kg for A. africnTzus,
40.5 kg for A. robustus, 47.5 kg for A.
boisei, and 43 kg for Homo fzabilis.
Despite their different coefficients and
assumptions, all of the encephalization
indices show the same basic pattern: the
australopithecines are above the pongids
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I thank C. K. Brain and the staff of
the Transvaal Museum, Pretoria, South
Africa, and K. E. F. Leakey and the staff
of the National Museum of Kenya, Nairobi, for permission to study the original
fossil material; A. Schultz for pongid body
weights and permission to use his material
at the Anthropologisches Institut, Zurich ;
J. Biegert, B. Lawrence, C. Mack, and R.
Thorington for permission to the the material in their charge; J. L. Angel and D.
.J. Ortner, Division of Physical Anthropology, Smithsonian Institution. for permission to use thc Terry Collection; R.
S. Corruccini for advice; L. J. McHenry for
assistance. Partial funding was provided by
the Committee on Research, University of
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encephalization, weight, body, early, hominis
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