The Panglossian Paradigm revisited : The role of non adaptive mechanisms in hominid brain and body size evolution
Date
2009-01-21T08:06:39Z
Authors
Spocter, Muhammad Aadil
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Abstract
The largely dominant adaptationist argument is currently used as the
framework within which hominid brain evolution is explained; however these
adaptationist explanations are inherently problematic and only suffice to ‘clutter’ our
knowledge of the possible causes of hominid brain evolution. This study addresses the
caveats observed in the fossil record and aims to assess the relative influence of
structural laws of form, phylogenetic constraints, and adaptive factors during the
course of primate and hominid brain evolution. A combination of methods such as
variance partitioning, phylogenetic regression procedures and path analysis indicate
that constraints have played a critical role in the scaling attributes of the primate and
hominid brain. In particular, developmental constraints governing the scaling
attributes of the skull and body are shown to explain up to 50 % of the variation in
body mass whereas phylogenetic constraints are purported to have played a lesser role
(i.e. 0.8 -3.6 %). In addition, the scaling attributes of neural and non-neural
components of the cranial vault suggest a highly constrained suite of traits and
suggest that as much as 96 % of the variation in both brain mass and residual
endocranial space may be explained by correlated scaling with the cranial vault.
Constraints are observed to be far more pliable than traditionally thought – a feature
highlighted by intraspecific analyses of scaling attributes in humans. Low regression
coefficients typical reported for intraspecific curves are shown to arise during
development as greater variation in body parameters is allowed with advancing age.
Grade shifts in the scaling of brain and body size for primates and other mammalian
orders is also emphasised by this current study and it is argued that correlated changes
between the brain and body size may not necessarily impact upon the ‘complexity’ of
the neural system as the functional integrity may be maintained via higher output
states initiated at certain levels of organisation such as at the level of the cortical area.
Although constraints should rightfully be given greater coverage in explanations
concerning hominid brain expansion, it is only through implementation of research
protocols that take a pluralistic approach to an understanding of the role of both
constraints and adaptation in the formation of the brain that our interpretation of the
likely mechanism for hominid brain expansion may be understood.
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Keywords
hominid, brain evolution, body size, constraints