вход по аккаунту


Asymmetry from molecules to mind. Review of The Evolution of Lateral Asymmetries Language Tool Use and Intellect J. Bradshaw and L. Rogers. New York Academic Press 1993 xii + 463 pp $72

код для вставкиСкачать
American Journal of Primatology 33:323-327 (1994)
Asymmetry From Molecules to Mind
Review of The Evolution of Lateral Asymmetries, Language, Tool Use, and Intellect, J.
Bradshaw and L. Rogers. New York, Academic Press, 1993,xii + 463 pp, $72.00.
This book seeks to integrate information about asymmetries across the animal
kingdom, particularly with reference to the question of how the asymmetries
present in humans are related to the characteristically human capacities of language, tool use, and intellect. The authors consider the question of asymmetries in
human behavior as “one of the three great mysteries”, the other two being the
origins of matter and life. They suggest that the unique characteristics of humans
(consciousness,praxis, tool use, language, and art) are very recent, and that all can
be related to cerebral asymmetry. Asymmetry thus looms large as a uniting concept which may allow integration of findings and thinking about the human condition from diverse fields of inquiry. The authors themselves come from quite
different scientific backgrounds (Rogers in comparative behavioral and developmental psychobiology;Bradshaw in human evolution and cognition); the strengths
of each author are showcased in turn in the various chapters addressing these
differing perspectives on lateral asymmetry. Integrating the enormous corpus of
literature and the broad sweep of topics they consider is a daunting task, and they
manage it rather well, on the whole.
Lateral asymmetry is receiving increasing interest from primatologists [e.g.,
the 1993 volume edited by Ward & Hopkinsl, in line with its increasing visibility
in human cognitive neuroscience [e.g., Geschwind & Galaburda, 1987; Corballis,
19911. Comparative interests in behavioral asymmetry were given a provocative
theoretical boost by MacNeilage, Studdert-Kennedy, and Lindblom [19871, in their
well-known article posing postural origins for the evolution of neuromotor asymmetries in primates. This volume provides an even larger evolutionary view of the
evolution of laterality than that proposed by MacNeilage et al. [19871: It looks
beyond the order Primates, into molecular and organismic sources of lateral asymmetry across orders. In so doing, it provides a challenge to primatologists to clarify
what aspects of asymmetry, if any, are unique to primates, and how these arise,
developmentally and phylogenetically.
The book opens with discussion of the sources of biological asymmetries in
carbon molecules, organic molecules, RNA and amino acids, and cytoplasm. As the
authors make very clear in the early chapters of the book, structural asymmetry is
a condition of life; what begs explanation is not the presence of asymmetry, but
rather its developmental patterns and consequences. Thus a good deal of space is
given over to presenting available data on the development of asymmetries across
animal phyla. To one not versed in the arcane literature of developmental morphological asymmetries in animals, the history of claw development in lobsters, or
of body flattening in flatfish, class Osteichthyes, made fascinating reading.
But the main focus of these authors is not on visible structural asymmetries,
ubiquitous as they may be. Rather, the main focus is on behavioral asymmetries,
which for the most part are mediated by the central nervous system. Thus, the
0 1994 Wiley-Liss, Inc.
324 I Fragaszy
discussion quickly turns to neural asymmetries. Some of the most exciting findings
regarding behavioral and neural asymmetries come from Rogers’ and colleagues’
work with chickens. The chicken turns out to be a stellar experimental system in
which to investigate the development of behavioral asymmetries, particularly
those mediated by vision, and their neural structural correlates. Visual pathways
in birds, as in some mammals with laterally placed eyes, cross over almost completely a t the optic chiasm, so that monocular vision restricts visual input to one
hemisphere of the brain. Moreover, unlike all mammals, birds have relatively few
commissures connecting the two halves of the forebrain. Birds (except owls, with
forward-facing eyes) frequently move the eyes independently to scan the environment, and the two eyes can focus independently. Functionally, birds have two
independent eyes, and two independent visual systems that receive information
from these eyes. If unihemispheric processing dominates and visual field information is essentially monocular, asymmetries in control of various behaviors are
inevitable, to prevent conflict between the hemispheres. In short, the nature of the
visual system set the stage for the evolution, retention, and/or elaboration of laterized brain functions in birds.
Developmentally, structural and chemical asymmetries in the visual system
have been shown to emerge in the chick from the organizing effects of light on the
eye that is facing toward the shell, and thus is stimulated by light before hatching.
Typically, the right eye faces the shell, and this population-wide positional bias,
leading to differential stimulation between the two eyes, starts a cascading series
of differentiating events in the nervous system. Behavioral lateralization is evident in many visually guided behaviors in chickens, such as rate of discrimination
learning (better when the right eye is used), or detection of novelty (better when
the left eye is used), copulatory behavior, and attack behavior (which occur more
readily when the left eye is used). This is psychobiological developmental sleuthing
a t its best.
One lesson to be learned from birds about asymmetry is that lateralization can
take many forms in addition to simple dominance of one hemisphere over another
in some particular function. It can take the form of complementary specialization
or differential use for the same function, or the form of different neural sites,
neurochemical changes, or integration of pathways. A second lesson is that the
emergence of asymmetry is epigenetic: i t is a consequence of a dynamic developmental process in which environmental conditions influence cellular metabolism.
Lateral asymmetries in mammals have been most studied in rodents. Laterality in rats and mice is randomly distibuted (i.e., there is no population bias
toward one side) for paw preference in reaching, direction of circling, and tail
posture. This form of lateralization depends on laterality of neurotransmission,
particularly that mediated by dopamine, in the nigrostriatum (not the cortex). For
other characteristics, a population-wide bias is evident. The right side of the body
(left hemisphere of the brain) is more involved in sequential processing and processing of species-specific vocalizations; the left side of the body (right side of the
brain) for control of spatial behavior, affective behavior, and parallel processing.
These asymmetries depend upon processing differences in the cortex. Control of
sexual behavior is also lateralized in the population, and this is associated with
asymmetries of neurotransmission in the hypothalamus. These patterns are similar in broad measures to those seen in humans, so rats and mice do make respectable models of behavioral asymmetries in humans. Therefore, we may uncover in
rodents, aspects of laterality that we did not suspect existed in primates, and then
find them in primates. This seems to be underway with brain modulation of the
immune system.
Asymmetry From Molecules to Mind I 325
Developmentally, asymmetry in rodents is influenced by sex hormones and
experience (such as handling). The particular roles and extent of influence of the
two hemispheres on each other appears to be influenced by the size of the corpus
callosum, and the size of the corpus callosum has been shown to be sensitive to
various developmental experiences. It has been shown that various stresses can
induce a population-wide bias for the left eye to open before the right, which may
trigger a similar cascade of developmental processes as were illustrated in chickens. Moreover, the form and extent of lateralization in the adult brain is not
static-it can change. This has been shown to be true in primates, as well. As
recent work cited in this volume on the topographic representation of body parts in
primates illustrates, the brain is really more like software than hardware.
Asymmetries in primates have been most studied in terms of motor activity
(manual preferences), cognitive processes, and brain morphology. The debate in
the primate community is whether there are population-wide biases in any aspect
of functioning like those observed in humans. Much attention has been devoted to
hand preferences, drawing upon the obvious human population bias for righthandedness, and upon MacNeilage et al.’s [19871 theory linking posture and lateral biases in prehension in primate evolution. Unfortunately for this effort, the
literature on hand preferences in nonhuman primates is worse than messy for
virtually every aspect of design, including numbers of subjects, nature of tasks,
scoring procedures, and statistical methods [see Hopkins & Morris, 1993 for a
recent review of this problem]. Although it is easy to find individual lateral biases
in primates for many actions, it is extraordinarily difficult to draw clear conclusions about the origins or implications of these biases. At. present, several studies
by Jeannette Ward’s group reviewed in this volume, suggest that prosimians display a population bias for left-hand reaching for prey capture, and that both age
and sex influence the degree of lateral bias. Otherwise, there are controversial
claims for the presence of a right-handed bias for reaching in the great apes, and
no general findings for monkeys.
Better luck has been had with the search for cognitive asymmetries. Perceptual processes have been one focus of this search, and in general humans and
nonhuman primates appear similar in these studies. A right-hemisphere advantage has been demonstrated in a few studies of tactile discrimination and visually
monitored spatial adjustments. Visual discrimination of faces appears to be mediated prinicipally by the right hemisphere, as in humans. Species-specific vocalizations are better discriminated by the left hemisphere. Work on language-proficient
apes has demonstrated left-hemisphere advantage in processing “meaningful” visual stimuli, and a right hemisphere advantage in processing “nonsense” stimuli-a finding parallel to that with humans.
Two chapters deal with human evolution. one concerning early human evolution and migrations, and the other concerning the rise of tool-use, art, and culture
in prehistoric peoples. These were admirably comprehensive in coverage, including
careful attention to the reports on lateral asymmetries in the apes, and attention
to reports on tool-using and other evidence of cognitive complexity in nonhuman
primates. The authors’ portrayal of the firmness of the conclusions drawn from the
reports of behavior in apes and other nonhuman primates is unsettling, however.
This is clearly not the literature in which they feel at home. Rather than drawing
upon the comparative literature critically, as they do in the sections on studies in
other taxa, human evolution, and language, they generally have accepted the
conclusions of the authors of the original reports they cite, in their broadest interpretation. Consequently, they present too tidy a picture of cognitive and behavioral
continuity across taxa of nonhuman primates.
326 I Fragaszy
But keeping the overall scope of the book in mind, this is a picky criticism from
a primatologist sensitized to the interpretation of these kinds of observational
data. In the main, these chapters did not deal with the comparative data; they
dealt with the emergence of uniquely human activities of using fire, burial rituals,
and art. A dextral bias is already evident in the manufacture of tools in the
Oldowan period, and becomes more widespread in more modern peoples. As the
authors note, (a) tool-using behaviors are not uniquely human, and (b) humanmade tools remained very simple in design and manufacture for a very long time,
even while brain size was changing rather dramatically. They conclude that selective advantage afforded by tool-using behaviors was certainly not the only influence on human evolution during this era, and in fact probably not even an
important one. The emergence of more advanced tools appears to have happened
rather suddenly, in what ;he authors term the “cultural explosion’’ of the upper
Paleolithic period in Europe, which they suggest happened when anatomically
modern humans met up with, and eventually outlasted, the extant Neanderthal
In a concluding chapter, an integrated view of the nature and evolution of
language and speech is attempted. The neurological substrates of language partly
involve those cortical areas of the parietal lobe in the left hemisphere that in the
right hemisphere are given over to spatial processing and that support visuoperceptual pattern recognition. The authors argue that language cannot be linked too
tightly to art through common evolutionary origins or neural mechanisms of symbolism, categorization, and referencing. Similarly, it cannot be linked too tightly to
tool construction and use, or other behaviors that involve sequential and syntactic
elements. In short they argue that the popular scenario of upright posture freeing
the hands, leading to tool-use, and subsequently leading to language, should be
discarded. Gestures appear phylogenetically either independently of tool-use, or
well before-shared gestures are quite apparent in chimpanzees and humans, but
shared tool-use is much less apparent. The authors conceive of the evolution of
intelligence and encephalization in terms of mosaic changes in tool-use and technology, social interchange and society, consciousness and understanding, and intellect. Thus, there is no single causative factor responsible for the emergence of
The authors conclude, very humbly after 387 pages of text on the subject, that
“It is easy to overemphasize the role of cerebral asymmetry and behavioral lateralities in human evolution . . . Cerebral asymmetry is just one dichotomy among,
for example, cortical-subcortical, anterior-posterior, and so on. However, it is a
neat and clear division, . . . and the functions so divided do seem to address some
so-called uniquely human characteristics . . . On the one hand, we have tried to
show that these characteristics may not be nearly so uniquely human after all; on
the other hand, we have also tried to show that . . . neither are we so unique as
previously thought in being overtly o r covertly asymmetrical. . . . Behavioral
asymmetries are common in all genera, being evident even half a billion years ago.
We have not crossed a Rubicon, linguistic or otherwise; we are merely rather
further out on a number of evolutionary limbs . . .” (pp. 387-388).
Given that they started the volume by claiming human asymmetries to be one
of the three “great mysteries of life”, it seems that the authors could find life rather
dull from here on, unless, of course, they recognize the remaining fundamental
questions about asymmetries (of development, of integration, of function) as holding equivalent interest. We still have a long way to go before we fully understand
asymmetries in our order. We would be wise to consider the ubiquity of the phe-
Asymmetry From Molecules to Mind I 327
nomena across orders, as these authors have ably done, before investing too m u c h
effort in creating primate-specific explanations for the varied asymmetries found
in our own order.
Dorothy M. Fragaszy
Department of Psychology
University of Georgia
Athens, Georgia
Corballis, M. C. THE LOPSIDED APE. New
York, Oxford University Press, 1991.
PATHOLOGY. Cambridge, Bradford/MIT
Press, 1987.
Hopkins, W. D.; Morris, R. Handedness in
great apes: A review of findings. INTERNATIONAL JOURNAL OF PRIMATOLOGY 14:l-25, 1993.
MacNeilage, P. F.; Studdert-Kennedy, M. G.;
Lindblom, B. Primate handedness reconsidered. BEHAVIORAL AND BRAIN SCIENCES 101247-303,1987.
EVIDENCE OF PRIMATE ASYMMETRIES. New York, Springer Verlag, 1993.
Без категории
Размер файла
376 Кб
xii, asymmetric, 463, mind, evolution, molecules, tool, academic, new, intellect, roger, 1993, york, language, lateral, bradshaw, use, pres, review
Пожаловаться на содержимое документа