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When Rutherford and Lindquist picked some of these abnormal fl ies and
bred them, they found that within a few generations all the progeny were
abnormal. Even the progeny fl ies that had two functioning Hsp90 genes
continued to show abnormalities.
What had happened? When Rutherford and Lindquist saw funny-looking
fl ies among the progeny of their crosses, they were fi nding the fl ies that had
the least robust developmental pathways and that were therefore most likely
to be sensitive to low chaperonin levels. After they had selected and bred
these fl ies for several generations, they ended up with lines of fl ies with
developmental pathways that always tended to be easily disturbed. The fl ies'
development was abnormal even when chaperonin molecules were present
in their usual numbers and were doing their best to maintain discipline.
Rutherford and Lindquist's developmentally disturbed fruit fl ies had so
many things wrong with them that they could never have survived outside of
the laboratory. Such organisms would probably not have survived the kinds
of large developmental disturbances that might have sent Precambrian crea-
tures of on new evolutionary paths. But perhaps the criteria for survival are
less strict for animals smaller and simpler than fruit fl ies. Small life forms
that consist of only a few cells, such as the early animals that lived in the Pre-
cambrian, might have a better chance of surviving drastic body-plan modifi -
cation than large complicated organisms such as present-day fruit fl ies.
What if we could perform experiments like those of Rutherford and
Lindquist on organisms simpler than fruit fl ies? How drastically could we
modify such simple organisms and still leave them able to survive and even
thrive?
There are signs in present-day animals that drastic modifi cations of their
remote ancestors' body plans did indeed take place. Early in the nineteenth
century the French anatomist Geof roy Saint-Hilaire came to a remarkable
conclusion about a major pair of branches in the tree of life. Vertebrates, he
declared, are simply upside-down arthropods (or vice versa). At some point
early in our history we (or they) fl ipped over, so that our spinal cords form
along our backs and those of the arthropods form along their bellies.
However, our faces and the faces of arthropods do not show this rotation.
Both groups of animals have their eyes above and their mouths below. So it
 
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