BOOKS: Taking Wing
From ANIMAL PEOPLE, March 1999:
Taking Wing: Archaeopterix and the Evolution of Bird Flight
by Pat Shipman
Touchstone (1230 Ave. of the Americas, New York, NY 10020), 1998. 336 pages, paperback; $15.00.
Pennsylvania State University
anthropologist Pat Shipman in Taking Wing
presents the most comprehensive, fair-minded
overview we’ve seen of the many controversies
surrounding Archaeopteryx and evolution.
As she entertainingly outlines, Archaeopteryx
in the 19th century emerged as the most convincing
fossil evidence for evolution itself. In
the late 20th century, Archaeopteryx is focal
point of a raging battle among theorists over
whether birds evolved from therapod dinosaurs
or much earlier, from a common ancestor
shared with the rest of dinosauria.
The central problem, at present, is
that a r c h a e o p t e r y x appears to represent a
hoatzen-like species of bird with many characteristics
more common to reptiles––but also
seems to have been preceded by some much
more modern birds. Heated issues include not
only who came first, but also how the development
occurred: whether birds learned to fly
by gliding down from heights, or by taking
skipping leaps as they ran at high speed.
While Shipman must be lauded for
her thorough presentation of conventional wisdom,
pro and con each point, she and all the
prominent combatants seem frustratingly
oblivious to the obvious. Even if A r c h a e o p –
teryx represents an early evolutionary stage of
a bird, even one with living direct descendants
like the hoatzen (whose possible descendence
from the Archaeopteryx Shipman considers but
rejects), A r c h a e o p t e r y x does not have to be
t h e proto-bird; neither is it necessary that
birds evolved flight exclusively by either gliding
or leaping, even if particular species did.
As Shipman outlines, advanced
small therapods such as C o m p s o g n a t h u s
apparently met all the physical prerequisites
for evolving flight. Such advanced small therapods
also thrived over much of the earth for
more than 100 million years.
It thus seems probable that conditions
might have favored the development of
flight in multiple times and places. Many
bird-like small therapods might have become
modern birds; some very successful bird-like
therapods might have had far-ranging descendants
who evolved flight at several different
points, even as the ancestor species persisted
for tens of millions of years in more-or-less the
original form and habitat.
Except for an Aristotelian insistence
that evolution must be linear, there is no logical
reason to believe that all modern birds are
descended from a single line. Rather, avian
diversity could be explained by multiple evolutions
of flight. P s i t t a c o s a u r i s, the parrotlike
presumed ancestor of the beaked ceratop –
s i a n s, might have come from a common
ancestor with parrots; the duck-like
hadrosaurs might have come from an ancestor
shared with ducks; the raptor-like Oviraptors
and V e l o c i r a p t o r s might have come from an
ancestor shared with modern hawks; and the
ostrich-like Gallimimus line might have come
from an ancestor shared with modern ratites.
In each instance, flight adaptations
clearly separate the avian line from the branch
of d i n o s a u r i a which shares those features.
Paleontologists thus explain the likenesses of
parrots’ beaks and the beaks of Triceratops et
a l as examples of parallel evolution.
Maybe––but why should the parallel involve
adaptations for eating, a basic function shared
by all species, and not adaptations for flight?
The conventional answer is that flight is such a
specialized function that it is much less likely
to have developed separately in species which,
despite their many differences, also have as
many commonalities as most modern birds.
Yet there is a strong and rather obvious
precedent to the contrary. The adaptation
most analagous to evolving flight is evolving
swimming. In either case, a creature adapted
to life on land must change body characteristics
in multiple ways to become suited to survival
in an environment where travel is possible
not only on a plane but also up and down.
We know from the fossil record that
the toothed whales, baleen whales, polar
bears, seals and sea lions, minks and otters,
manatees and dugongs, and hippos each took
to the water in a different time and place.
We know too that many of these
creatures came from a family line which
evolved into land-dwelling ungulates as well
as whales. The toothed whales apparently
evolved directly from the wolf-like
Mesonichids; the baleen whales evolved from
a fork in the family tree which included both
cow-like grazing and semi-aquatic behavior.
The recently catalogued extraordinary ability
of moose to dive deep and remain underwater
may provide a clue to how the fork occurred.
Hippos and sirenians such as manatees
and dugongs developed––separately–
from a branch of the pig line. While sirenians
were the earliest marine mammals still extant,
hippos came relatively late. They nonetheless
evolved whale-like cousins who apparently are
no longer with us because climatic change first
severed their North African habitat from the
ocean and then raised it high and dry.
Polar bears, seals, sea lions, minks,
and otters had a common ancestor in the miasis
cat––shared with all the cats, dogs, raccoons,
and others of the order carnivoria.
If mammals could evolve aquatic
habits at least nine times in 60 million years,
separately developing many parallel adaptations
to aid swimming, why couldn’t d i n o –
sauria have evolved flight at least as often?
Perhaps, one might respond, for the
same reason that mammals have evolved flight
only twice: in full form, just among bats, and
in partial form, among the flying squirrels.
But suppose we accept that logic, and suppose
also that the number of times an order evolves
flight is related to the number of species in the
order. Even today, 65 million years after the
extinction of all of d i n o s a u r i a except birds,
bird species are twice as numerous as mammal
species; four times more numerous than mammal
species exclusive of bats.
Implied is both that flighted animals
are inherently more inclined toward evolutionary
divergence, and that d i n o s a u r i a c o u l d
have produced flying descendants––birds––
anywhere from four to 16 times.
Considering that birds may have
evolved flight at different times could help
solve another evolutionary problem, too: why
some birds don’t fly.
Conventional wisdom is that the
ostrich, the penguin, the emu, the kakapo,
and others lost their ability to fly because they
variously had no predators, preferred to run,
and/or preferred to swim to catch their food.
Conventional wisdom accordingly
assumes that evolution, albeit linear, is also
capable of working backward, taking away an
evolutionary advantage. Many island bird
species have long had no natural predators,
yet still fly, for multiple reasons. They thus
survive in much greater numbers than kakapos
and others who don’t. Roadrunners run; they
fly as well. If they couldn’t, they’d never
evade Wiley Coyote. Cormorants dive as well
as a penguin, but spotting prey from the air
before they dive and then diving with the
momentum gained by plunging from a height
is far more efficient than just plunging in––
which is why cormorants thrive in some parts
of the world where penguins are in trouble.
Noting that ratites are among the
most primitive of living birds in many
respects, and that kakapos seem to be the most
primitive living parrots, it seems possible that
these and other flightless birds never flew.
They may instead be the closest surviving relatives
of the first fliers in their respective lines,
almost but not quite ready for takeoff.