Radiators, Australopithecines, Thermal Stress and Baboons
By: Dr. A. E. Johnson, Jr., University of Great Falls
The "radiator theory" correlates increasingly complex cranial emissary foramina with the need for heat dissipation in early hominids, especially those classified as incipiently Homo. This paper examines mastoid and parietal emissary foramina in savannah and forest baboons as an analog to the radiator theory.
No significant differences were found regarding the presence of these foramina in habitat specific baboons suggesting that thermolytic stress alone may not have been the selective agent producing increasingly complex cranial venous systems in early hominids.
Hominids, Heat Loss, Baboon Parallels
Much has been made in recent years of the so-called “radiator theory”
which posits that among the anatomical features which separate robust and
gracile australopithecines include not only the enlarged occipital marginal
sinus found in the former and not in the latter, but a smaller occipital
marginal sinus supplemented by the beginnings of a network of cranial emissary
foramina found in the latter but not in the former (Falk, 1986).
As currently configured, the radiator theory correlates these anatomical
structures in australopithecines with the need to dissipate heat implying
that graciles who may have been refining bipedal locomotion in a heat stressed
savannah environment needed a more efficient mechanism to lose heat
than did the robusts. If graciles were primarily savannah dwellers
while the robust forms remained in the forest or on the forest fringe as
has been theorized (Falk, 1992), it seems reasonable to suggest that
they were subjected to more severe thermal stress and would have,
unlike the robust forms, developed a more efficient mechanism for cooling
the body and maintaining a constant temperature of the brain. Tangentially,
(Falk, 1992) has used the existence of the enlarged occipital marginal
sinus and the near absence of cranial emissary foramina in the robust australopiths,
including the early Afar material, to suggest that these forms are likely
not the ancestor of Homo. She also states that the reduced occipital
marginal sinus and the “beginning of a trend for an increase in the frequency
of mastoid emissary veins in graciles” (Falk, 1986:323) suggests
an ancestral-descendent relationship between the graciles and subsequent
forms of Homo including modern sapiens, a relationship suggested
by others as well but one that has yet to be confirmed definitively.
Does Falk’s thesis which posits a cause and effect relationship
between thermolytic stress and the need for a more efficient cooling mechanism
for the brain in early hominids have an analog in extant, non-human primates?
Because the initial premise of the radiator theory is linked to habitat,
(forest vs. savannah), it seems appropriate to examine one component of
the radiator theory using baboons as models. The question becomes,
are mastoid and parietal foramina which enhance cooling more prevalent
in savannah dwelling baboons than in forest baboons as the radiator theory
suggest they might be. If they are, Falk’s hypothesis
linking thermal stress and enhanced cooling systems in hominids is further
substantiated. If they are not, Falk’s hypothesis remains intact
but may require that explanations in additional to thermolytic stress alone
be sought to explain the increasingly complex system of cranial emissary
foramina observed in early and subsequent forms of Homo.
Baboons are appropriate models for this comparison for two reasons.
First, while they are not bipedal, they are nonetheless principally orthograde.
Orthodgrade as well as bipedal posture on the savannah would have the effect
of reducing the amount of surface area exposed to ultraviolet radiation
in early hominids as well as in extant, savannah living non-human primates.
Wheeler (1988) has demonstrated that reducing surface area exposure to
ultra violet radiation is important in maintaining a constant temperature
of the brain in a heat stressed environment. Second, baboons,
which have often been used as models of early hominid behavior, are distributed
throughout Africa and are found in a variety of habitats paralleling Falk’s
contention that gracile and robust australopiths may have been adapting
to different habitats in East Africa. In a very broad sense, baboons
can be categorized as savannah or forest dwellers if sufficient care is
taken to delineate what kind of habitat the animals are found in.
Clifford Jolly (pers. comm. 1996) suggests that the best way
to differentiate savannah from forest baboons is to track the animals
to a vegetation map of Africa and group the specimens by habitat, not by
conventional taxonomy. Table 1 delineates how the specimens used
in the study were categorized using museum proveniences and Jolly’s suggestions.
Methods and Materials
For the purposes of this study, 67 adult baboon specimens now housed
in the Division of Mammals at the National Museum of Natural History at
the Smithsonian Institution were examined and scored as to the presence
or absence of patent mastoid and parietal foramina. Of the specimens
examined, Table 2 details, among other things, how many specimens within
the established categories evidenced at least one mastoid or parietal emissary
foramen.
Results and Discussion
The data from Tables 1 and 2 suggest that 60% (23 of 39 specimens)
of the sample identified as savannah baboons evidence at least one patent
parietal or mastoid emissary foramen. Among the forest dwelling baboons
the comparable figure is 64% (18 of 28). Initially, these comparative
percentages suggests that savannah and forest baboons do not differ markedly
in the presence or absence of the foramina under consideration. The
apparent absence of any substantial differences between the two groups
is further demonstrated by the results of a chi-square analysis.
Table 2 illustrates the 2 x 2 contingency table used in the analysis.
This table is without fixed margins and represents a “Double Dichotomy”
design as recommended by Zar (1996:490).
The analysis results in a calculated Xc2 of 0.10. The critical
Xc2, a=.05, df=1 necessary for the rejection of the null hypothesis of
independence is 3.84. Therefore, the null hypothesis that habitat
is independent of the presence of mastoid and parietal foramina in baboons
is retained as tenable given the particular qualifiers of this study.
If thermolytic stress selected for increasingly complex foramina systems
as cooling mechanisms in early hominids, it apparently does not do so in
baboons at least when mastoid and parietal foramina are used for
the comparison.
Accepting the null hypothesis of independence between habitat and differential
presence of foramina in baboons does not of course preclude the possibility
that such an association could in fact have characterized early hominids
as the radiator theory suggests. It remains plausible that the emissary
foramina which have their beginnings in the gracile australopiths might
well have been associated with the need to dissipate heat. But
because no such association seems to characterize baboons, perhaps the
thermolytic stress to which graciles and subsequent forms of Homo were
subjected came not only from life on the savannah itself but from new behaviors
which themselves would have generated excess heat adding to the thermolytic
press of their environment. It has often been speculated, Wolpoff
(1980, 1996) for example, that early savannah dwelling hominids Australopithecus/Homo
must have developed adaptations to heat stressed savannah life including,
sweating, carrying water, etc. Perhaps the increasingly complex
system of cranial foramina for cooling attributed to early Homo had as
much to do with newly acquired heat producing behaviors as it did with
newly acquired habitat. The combination therefore of habitat as well
as habit may have worked in tandem to increase the heat stress on early
hominids thereby selecting for increasingly elaborate foramina to maintain
body temperature and cool the brain in late graciles and early Homo.
Baboons, whether savannah or forest dwellers apparently, do not need a
sophisticated venous system for cranial cooling because their behaviors
seem to be primarily foraging. Such behavior in and of itself
does not generate excessive heat. Whether or not behavioral
differences can be demonstrated for gracile and robust hominids that might
have necessitated the need for an enhanced cooling mechanism in one form
but not in the other remains to be determined however.
Falk, D. (1986) Evolution of cranial blood drainage in hominids: Enlarged
occipital/marginal sinuses and
emissary foramina. Am. J. Phys. Anthropol. 70, 311-324.
Falk, D. (1992) Braindance: New Discoveries About Human Origins and
Brain Evolution. New York:
Henry Holt.
Falk, D. (1990) Brain evolution in Homo: The “radiator” theory (with
peer commentary). Behav. and Brain
Sci. 13, 333-381.
Wheeler, P. (1988) Stand tall and stay cool. New Scientist. 118, 62-65.
Wolpoff, M. (1980) Paleoanthropology. New York: Alfred A. Knopf.
Wolpoff, M. (1996) Human Evolution. New York: McGraw Hill.
Zar, J. (1996) Biostatistical Analysis. Upper Saddle River, New Jersey:
Prentice Hall
Dr. Al Johnson is the Dean of the graduate program at the University of Great Falls in Great Falls, Montana. He holds a Ph.D. in education, an M.A. in anthropology, and a second M.A. in education. His teaching areas include paleoanthropology and statistics. He has published in the Journal of Human Evolution, the Journal of Dental Research, the Journal of American Indian Culture and Research, the Lewis and Clark Law Review, and in Research in Higher Education. He has done research at the Smithsonian Institute and at the Cleveland Museum of Natural History.