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Darren Naish's "Birds of a Feather" article in the
"Fortean Times" is excellent (haven't seen the pics yet, though
they're prob. good too) - the kind of standard that "Sci Am" should
have been aiming for!
He touches on the issue of re-emergence of
long-lost features:
" . . .the phorusrhacoids (a group of extinct, flightless, predatory
birds) re-evolved large, clawed hands with mobile fingers. They doubtless used
these hands in subduing their mammalian prey and evolved their hands from
typical avian wings. Phorusrhacoids also developed large, recurved claws on
their second toes . . ."
One may wonder whether the genetic information
to redisplay an old feature can still be there after tens of millions of years -
after all, unexpressed genetic material undergoes much faster mutation than
expressed stuff which is subject to evolutionary honing. However some lost
features ARE protected genetically since they are expressed elsewhere.
Thus the genes that manufactured Archaeopteryx's hand claws were almost
certainly the same ones that manufactured its - and modern birds' - feet
(subject to a few minor adjustments). (Sometimes similar effects are
caused by a duplicated set of genes, admittedly, but re-use of the same genes is
surprisingly common, and can be checked for in living species.
Incidentally, birds are very efficient with their genomes which are only about
one third the length of mammals'.)
However, when the manufacturing genes are totally unexpressed
anywhere, then they really ARE subject to rapid degradation.
It might be worth bearing this distinction in mind when trying
to guess whether a feature may have been re-evolved. For example long
tails and teeth are not expressed anywhere in birds now, although a tail
vertebra is produced by adapting the basic vertebra process. With
furculae, invisible cartilagenous forms might have proved useful as
bumpers.
Are any examples of normal-winged birds with throw-back clawed
hands ever found? And why did the phorusrhacoids lose out to the early
carnivorous mammals when a similar design survived for so long
before?
JJ
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