EARLY birds didn鈥檛 learn to fly by jumping out of trees, or by running along
the ground chasing flying insects, as most palaeontologists would argue.
According to a study that鈥檚 taken another look at the fossil record, they
developed wings to jump onto prey from ledges and rocks.
Most experts are split into two camps over the origins of bird flight
(This Week, 8 May, p 10).
Some believe that small reptiles took to climbing trees and
developed prototype wings and feathers that helped them glide down back to the
ground. Others insist that fast-running dinosaurs developed feathers for
insulation, then used them to take ever greater leaps into the air to snatch
flying insects.
But now Adrian Thomas of Oxford University and his colleagues have
re-examined the fossil record of early birds, including the most famous,
Archaeopteryx. They now believe flight evolved in a pouncing protobird or
鈥減roavis鈥� that used its feathers to control and balance its body during a
feet-first, predatory leap. This pounce turned into a swoop, which finally led
to true flapping flight (Proceedings of the Royal Society B, vol 266, p
1259).
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Thomas points out that fossils of Caudipteryx, a primitive feathered
cousin of Archaeopteryx, have symmetrical feathers, which could provide
enough drag to stabilise a short fall. But the ground-up and tree-down theories
require the first birds to have evolved asymmetrical feathers, which can
generate lift.
Both popular theories also predict that early birds became lighter as their
wings evolved. But fossils show that birds already had functional wings before
they began to shed excess weight from their tails and skulls. A pouncing proavis
could remain heavy well into the swooping phase of its development, say the
researchers.
The pouncing proavis model also explains why Archaeopteryx has fully
feathered wings attached to an unremarkable reptile skeleton. The body did not
seem to have adapted for flight. 鈥淚t鈥檚 very odd, and you have to work hard to
see how the other theories could fit this,鈥� says Thomas. But in the pouncing
theory, an advanced wing could easily appear alongside a simple body. 鈥淏irds
evolved from predators that specialised in ambush from elevated sites, using
their raptorial hindlimbs in a leaping attack,鈥� Thomas concludes.
Jeremy Rayner, a bird aerodynamics expert from the University of Bristol,
says it鈥檚 an interesting idea, but he鈥檚 not convinced. For instance,
Caudipteryx鈥檚 feathers are soft and floppy like down, and it鈥檚 difficult to
see how they could have created enough drag, he says.