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Figure 24 A pair of hoatzins, Opisthocomus hoazin , peer from their perch above the
Pistia -covered upper Amazon lake. Hoatzins eat leaves, including water lettuce leaves, almost
exclusively.
But were they? Claws have reappeared in the hoatzin lineage. Have the
hoatzins resurrected an ancient developmental pathway that has lain hidden
and inactivated in bird genomes for 150 million years? Are they survivors
of a bird lineage that retained the claws during all this time? Or have they
evolved a brand new developmental pathway? We are not yet able to distin-
guish among these possibilities, though the last possibility is the least likely
one. There is growing evidence that it is far easier to restore the genetic func-
tions of an old suppressed developmental pathway than it is to evolve a new
pathway from scratch.
We do not know whether some of the hoatzin's near relatives might also
have had wing claws, because all of these near relatives seem to have been
lost through extinctions. Argument continues about whether hoatzins are
more closely related to the doves, the cuckoos, or the turacos, such as the
 
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