PLAIN LANGUAGE SUMMARY:
We
can deduce that extinct animals had particular soft tissues or behaviors via
extant phylogenetic bracketing (EPB,
Witmer 1995). Application of EPB
entails the recognition of two extant lineages that diverged earlier and later,
respectively, from the extinct organisms' lineage (Figure 1). Parsimony indicates that a
character or character-state shared by each of two lineages existed in their
most recent common ancestor (MRCA). Thus, unless subsequently lost in particular
lineages, the character or state was also present in all other descendants of
the MRCA including those whose extinction left no direct evidence of the
character. The EPB method can be applied using DNA sequences as proxies for the
proteins that those sequences encoded. Results of the search for genes that code
for opsins - the proteins that signal the detection of light in retinal
photoreceptors - suggest that the MRCA of the two major bony fish radiations had
four different opsin genes (Bowmaker 1998). Many
tetrapods retained all four genes whereas the basal eutherian mammals apparently
retained only two (Jacobs 1993). By newly deriving a
third opsin, our ancestors endowed us with a color vision system approaching,
but not equaling, the system probably retained by most Mesozoic tetrapods. Many
animals also have oil droplets screening the light-sensitive parts of their
photoreceptors (Walls 1942). Again eutherian mammals are aberrant
in showing no trace of these structures which (in animals that have them)
probably sharpen the spectral tuning of photoreceptors and thus enhance
perceived color contrasts (Bowmaker
et al. 1997). Eutherian mammals also lack double cones and the mosaics of
these structures found in a variety of other vertebrates (Walls 1942). Double cones and their mosaics have
been known for roughly 100 years, but no compelling theory explains the utility
of such anatomical arrangements. Comparative anatomy and EPB allow reasonable
inferences about which extinct animals had double cones and mosaics, but these
inferences do not help us to understand these animals because we do not know how
double cones are used by living animals.
Much of our perceptual world is built upon what our eyes report to us. Indeed, much of the primate cerebral cortex is devoted to deciphering signals from the retina (Felleman and Van Essen 1991). However, the primate visual system evolved from one that was secondarily simplified from a more complex state (as indicated by the apparent loss of oil droplets, double cones, and some opsins). Hence our visual worlds are probably very different from those of animals that never underwent this simplification. Psychologists have recognized for some time that what we perceive is a reconstruction of the world around us. This reconstruction strongly depends upon the characteristics of our sense organs. Biologists are only beginning to appreciate the significance of this insight when studying the behavior of other animals (Bennett, et al. 1994). Hopefully, paleobiologists will not lag behind neobiologists in reaching this understanding.
M.P.Rowe. Department of Psychology, University of California, Santa Barbara, CA, 93106, USA.