Origin of Flight
The theories on the origin of flight has been debated for
over a century.
It is one of the most challenging aspects of evolutionary
biology. There
are two major conflicting theories: (1) The "Ground
Up" or cursorial
theory and (2) "Trees Down" or arboreal
theory. Each theory
relies upon speculation about the paleoecology and
functional
adaptation of the proavians.
The
Cursorial Theory
The
Arboreal Theory
The
Insect Net
Criticisms
of the Arboreal Theory
Criticisms
of the Insect Net
The
Fluttering Model
Criticisms
of the Fluffering Model
Other
Criticisms
This theory proposes that flight evolved in running bipeds
through a series
of short jumps. As the jumps became more and more
extended, the wings
were used for balance and propulsion eventually resulting in
flight without
a gliding intermediate. There are two models that
attempt to explain this
theory.
The
Insect Net
It is proposed that
obligatory bipedalism allowed free movement of the
forearms, making them available for catching prey. As
the feathers enlarged
over time it would make them a better tool for batting and
catching insects, by
chasing and swatting them with their wings. As the
forelimb enlarged, the
motion would turn from swatting into actual flapping
flight.
The motion used to catch insects is different from the
motion of a flight stroke.
Wings and feathers would be damaged for flight if used for
batting prey.
To catch prey, air holes would need to be present (like a
fly swatter) to let
air pass. Archaeopteryx's wings did not have air
holes, thus is would be
unable to catch insects in this manner.
Instability and loss of balance with "insect
nets".
It is proposed that the proavians used their jaws to catch
prey but employed
their wings as bilateral stabilizers during a jump into the
air. It is suggested that
the rudimentary wings were effective for balance while
running, jumping, and
turning, until capable to take off at high speed. The extension
of the forelimbs
would cause minute increments of lift, enabling them to jump
further and capture
more prey. The motion of the forelimbs for
stablilization would mimic the flight
stroke of a bird. The increase in airfoils would
allow greater and greater lift
leading to powered flight.
Outstretched wings would increase drag and slow down
locomotion.
There is question as to why selection would favor flapping
wings for
a terrestrial foraging animal.
Flight at low speed and close to the ground is more
sophisticated and complex
than flight at high speed.
Foraging in flight implies instability and a great degree of
maneuverability
requiring very fine coordination. It is unlikely a
fine control developed in
jumping proavians with a long stabilizing tail.
Other Criticisms to the Cursorial Theory
The theory works against gravity.
It is energetically too expensive.
Does not address the transitional form between the preflight
stage and
active flapping flight.
Does not explain the origin of feathers, endothermy,
brain enlargement,
and three-dimensional perceptual control.
The Arboreal Theory
This theory proposes that a climbing proavian started to
glide between
trees eventually leading to wing strokes as the precusor to
flight.
The rudimentary feathers that developed from scales on
arboreal reptilian birds
acted like a parachute slowing the descent as the animals
leaped from branch
to branch, hence controlling their jumps and
falls.
It is suggested that proavians were ground foragers and used
trees to nest, hide,
and roost. The proavians would climb trees through the
use of manual and pedal
claws that are evident in the fossil record. As
the jumps became a greater
distance, an unspecialized glide with a steep angle would develop.
Gliding would
increase distance, maneuverability, and slow down
landing. Once gliding became
specialized then flapping would begin. During landing
they would flap their wings
to slow the decent. This motion would eventually give
rise to active flight.
Proponents of the cursorial theory claim that gliders were
evolutionary
dead end.
Climbing would damage feathers (this has been disregarded
since the
extant Hoatzin chick can climb trees without damaging their
wings.