Concept Boat competition
|Discussions about the origins of wings and powered flight
in animals such as insects, pterosaurs, bats, and birds have centred around two possible
ways of becoming airborne:
- either moving rapidly across the surface and then lifting into the air -
the cursorial explanation
- or descending through the air from a tree or another high vantage point
- the arboreal explanation.
Insect flight evolved about 330 million years ago. There is genetic
evidence that wings evolved from articulated gill plates on the limbs of aquatic
ancestors, rather than being novel outgrowths from the body wall (Carroll, Weatherbee, and
Langeland, 1995; Averof and Cohen, 1997). Perhaps surprisingly, some
originally winged insect lineages such as the stick insect have subsequently
become wingless during evolution, although they appear to retain the
potential to re-acquire wings at a later time (Whiting, Bradler, and
the first windsurfers!
Modern stoneflies walk on the surface of water, and raise their
rudimentary wings if they feel a puff of air. They then get propelled across the water by
the breeze (Marden and Kramer, 1995). Some species even stand on their hind limbs and flap
their wings as they sail (Kramer and Marden, 1997). Sailing like this may have had
sufficient potential to drive the evolution of insect wings in the past.
Averof, M., and Cohen, S.M. (1997) Evolutionary origin of insect wings from ancestral
gills. Nature, 385, 627-630.
Brodsky, A.K. (1994) The evolution of insect flight. Oxford: Oxford
Carroll, S.B., Weatherbee, S.D., and Langeland, J.A. (1995) Homeotic genes and the
regulation and evolution of insect wing number. Nature, 375,
Kingsolver, J.G. (1985) Butterfly engineering. Scientific American (August), -97.
Kramer, M.G.,and Marden, J.H. (1997) Almost airborne. Nature, 385,
Marden, J.H., and Kramer, M.G. (1995) Locomotor performance of insects with rudimentary
wings. Nature, 377, 332-334.
Whiting, M.F., Bradler, S., and Maxwell, T. (2003) Loss and recovery of
wings in stick insects. Nature, 421, 264-267.
The evidence points to the evolution of birds from land-based two-legged
carnivorous dinosaurs (theropods) some 150 million years ago.
dinosaurs were large or medium sized animals, but recently the fossil
remains of a much smaller dinosaur - Microraptor - have been
described by Xu, Zhou, and Wang (2000). This species has several features
that may place it on the evolutionary pathway to birds, including the
possession of feathers. Thus, the emergence of feathers probably predated
the emergence of flight. Similarly feathered dinosaur fossils have also been found in Madagascar
(For a discussion of the possible evolution of feathers, click here.)
||Fossils of a remarkable four-winged creature that lived
124-128 million years ago have been described by Xu et al (2003). Microraptor
gui was probably capable of gliding flight, living among trees. It
carried modern-looking asymmetrical flight feathers on both pairs of
A recent finding
of the fossil remains of Apsaravis has given insight into the
emergence of the flight mechanism, particularly the mechanism by which the
hand region of the upper limb is automatically extended as required in the
transition from upstroke to downstroke (Norell and Clarke, 2001).
We can be more
confident about Archaeopteryx as a direct ancestor of modern birds.
Seven fossilised specimens
of Archaeopteryx have been found in Germany. They have feathered wings and tails,
but there is still some doubt about how capable they were of flapping wing
flight given the relatively modest development of the flight muscles and unspecialised
wrist bones (Speakman and Thomson, 1994). Archaeopteryx lacked the supracoracoid
muscle of modern birds, but the pectoral muscles may have been adequate for
powered flight. Archaeopteryx had a full set of teeth and a long bony
tail, unlike modern birds.
Birds have flight adaptations that are similar to those
of pterosaurs: light, hollow bones, keeled sternum for attachment of
flight muscles, and short and stout humeri. A difference is that in birds
the clavicles are fused to form the furcula (wishbone) which helps to
stabilise the shoulder during the wing beat. Another difference is that
much of the bird's wing is supported by the radius, ulna, and
carpometacarpus rather than by an elongated 4th digit as was the case in
Modern birds grow rapidly, reaching full size in about 1
year and starting to fly when growth is nearly completed. Dinosaurs also
grew to adult size relatively quickly, especially compared with the growth
of lizards which tends to be slower and more prolonged. It has been
proposed that a key feature of bird evolution has been acceleration and
curtailment of the growth phase inherited from their dinosaur ancestry (Chinsamy
and Elzanowski, 2001; Padian, Ricqles, and Horner, 2001; Erickson, Rogers,
and Yerby, 2001).
Generally the arboreal hypothesis for the origin of flight in birds has
been the more popular, but a recent paper by Burgers and Chiappe (1999) suggests that the
apparent gap between the running speed of Archaeopteryx (2 metres per second) and
required take-off speed (6 metres per second) could have been made up by the thrust
produced by flapping its wings - a cursorial origin of flight. They point out that the
structures and functions necessary for wing-generated thrust were already present in the
flightless ancestors of birds.
Both the arboreal and cursorial hypotheses for the
origin of bird flight have explanatory gaps. For example, gliding
tree-dwellers of the present day such as the flying squirrels and lemurs
make no effort to prolong their flight by flapping their appendages,
raising the question of why tree-living ancestors of the birds may have
done so. With regard to the cursorial hypothesis, it is necessary to
suggest an explanation as to why natural selection would have favoured the
development of protowings in running ancestors of birds. Dial observed
that some predominantly ground-living species of extant birds routinely
run up tree-trunks and other inclined surfaces to reach safety, and beat
their wings to improve traction as they do so (work described by Wong,
2002). This wing-flapping behaviour was also observed in juveniles of
these species even before they were able to fly. Thus Dial proposes that
the use of wing-beating during inclined running might have provided the
necessary incentive for the evolution of wings in ancestors of the birds.
Sceptics dismiss a dinosaur origin for birds and suggest instead that
the ancestors of modern birds diverged from reptiles before the dinosaurs appeared.
Burgers, P., and Chiappe, L.M. (1999) The wings of Archaeopteryx as a primary
thrust generator. Nature, 399, 60-62.
Chiappe, L.M. (1995) The first 85 million years of avian evolution. Nature, 378,
Chinsamy, A., and Elzanowski, A. (2001) Evolution of
growth pattern in birds. Nature, 412, 402-403 (26 Jul).
Erickson, G.M., Rogers, K.C., and Yerby, S.A. (2001)
Dinosaurian growth patterns and rapid avian growth rates. Nature, 412,
429-432 (26 Jul).
Norell, M.A., and Clarke, J.A. (2001) Fossil that fills
a critical gap in avian evolution. Nature, 409, 181-184.
Padian, K., Ricqles, A.J. de, and Horner, J.R (2001)
Dinosaurian growth rates and bird origins. Nature, 412,
405-408 (26 Jul).
Speakman, J.R., and Thomson, S.C. (1994) Flight capabilities of Archaeopteryx. Nature,
Unwin, D.M. (1998) Feathers, filaments, and theropod dinosaurs. Nature, 391,
Wong, K. (2002) Taking wing. Scientific American,
Xu, X., Zhou, Z., and Wang, X. (2000) The smallest known
theropod dinosaur. Nature, 408, 705-708.
Xu, X., Zhou, Z., Wang, X., Kuang, X., Zhang, F., and Du,
X. (2003) Four-winged dinosaurs from China. Nature, 421,
335-340 (23 Jan).
The first bats appeared about 50 to 60 million years ago,
which means that they have been evolving for less than half the time that
birds have been evolving. The fossil record for bats is
rather patchy, probably due to the delicate nature of the bat skeleton and
because the early bats lived in tropical forests where post-mortem preservation was poor.
It is not yet clear whether the microchiropteran and megachiropteran taxa
are derived from a common bat-like ancestor or whether they evolved
separately from earlier mammalian forms. So far, 27 genera of fossil bats have been found.
It seems probable that the earliest bats were gliders, and that powered
flight emerged later.
It is clear that pterosaurs were able to fly, but to what extent is
still not clear. The hindlimbs formed an important part of the flight apparatus in some
species, supporting the flight membranes and probably assisting during tight maneuvres and
braking (Unwin and Bakhurina, 1994).
Unwin, D.M., and Bakhurina, N.N. (1994) Sordes pilosus and the nature of the
pterosaur flight apparatus. Nature, 371, 62-64.
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