The Origin of Flight–What Use is Half a Wing? | HHMI BioInteractive Video

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KEN DIAL: The evolution
of flight in birds has long been a great
scientific puzzle. Fossil evidence from
the past 40 years has established that birds
descended from theropods– a lineage of two
legged dinosaurs that included species with
feathers on their bodies and arms. But those early
animals could not fly. So how did birds take flight? A new hypothesis came from
my studies of young birds that are learning to fly. And it all started
with a bale of hay. I’ve been studying the mechanics
of bird flight for over three decades at the University
of Montana’s Flight Lab. I became interested in
an intriguing question that critics of evolutionary
theory had posed to Darwin. So there’s a gentleman by the
name of Sir George Jackson Mivart. He challenged Charles
Darwin when Charles wrote “The Origin of Species.” Darwin had proposed
that structures evolve through intermediates. So a wing would have evolved
from four-limb in stages. But some of those early stages
were clearly not capable of flight. Mivart confronted Darwin. He said to Darwin,
how do you explain in the evolution of
birds from reptiles, the function of half a wing? So to answer that question
I look to a living example. Bird chicks with
small, immature wings. I wanted to make
careful observations of how chicks use their wings. As they learn to fly. I was keeping the birds
on these shiny floor clean lab conditions
and trying to have them go up a wall that was slick
to fly up to their siblings. As soon as a rancher
came in once and said, what are these birds
doing on the ground? They hate being on the ground. Giving them a bale of hay, give
him something to get up on. As soon as I got some
hay bales for the chicks, we made an interesting
observation. I came back one day, and my
son who had been helping me, I asked him how are they doing? How was the data today? And he says, it was horrible. I say, why? He says, they were cheating. And that moment– a
watershed moment in my life. What do you mean
they were cheating? They ran straight up vertically. I said, that’s impossible. To better understand
this behavior, my son Terry and I decided
to carefully measure how young birds use their legs
and wings together to travel up ramps of different angles. But here’s our little
experimental animal here and– My friend Julia Clark, an expert
on the evolutionary origin of birds, joined us
on our experiments. The first angle is not steep. It’s wild. JULIA CLARK: Right. KEN DIAL: This is not a– JULIA CLARK: Not
a trained animal. KEN DIAL: This is
not a trained animal. JULIA CLARK: That was easy. KEN DIAL: Just walked. No problem. JULIA CLARK: Strolled. KEN DIAL: Nonchalant. No wing necessary. Next, we try a sharper angle. JULIA CLARK: OK. So this is much
deeper than before. KEN DIAL: It is. JULIA CLARK: Let’s
see what happens. KEN DIAL: There we go. JULIA CLARK: You’re
not sure what you see. KEN DIAL: Exactly. JULIA CLARK: Because
it’s so fast. KEN DIAL: And until you
see it in slow motion, you don’t know how
beautiful it actually is. This time the bird used
its wings as it ran. Every time, it felt like it
was falling backwards it used its wings, not to pull it up. And this is what we discovered. It wasn’t to lift it up and take
it like a bird flying straight up to this refuge. It was using its wings to pull
it forward on to this log. And now a really steep ascent. That is going to be
a challenge, right. JULIA CLARK: So this
is now nearly vertical. KEN DIAL: This is vertical. Standing tall,
vertical like any tree. Let’s see what the animal
can do to negotiate this. It can fly, but– JULIA CLARK: But it’s
still using its legs. Still using his legs. KEN DIAL: It climbed
with its legs, using its wings to pull
it towards the tree. But not to fly it up this. Pretty cool. This has turned out to be not
just representative of the bird we looked at, but every
flight capable bird that we’ve looked at
in the 15 years since. Dozens and dozens of birds. I have also observed the
same behavior in the wild, and I’ve even seen young
birds use their wings to assist their hind limbs to
paddle across a body of water. JULIA CLARK: So, Ken, how
did this change the way we think about how
dinosaurs get in the air. KEN DIAL: Well, we
now know that there are a lot of dinosaurs–
little feathered theropods that have little wings. And the explanation
for their existences is really difficult to resolve. And I think that a
reasonable explanation is to look at what the young
birds with similar wings can do today. Birds show us the possibility
of what these dinosaurs could have done. Scientists have long debated
two main possibilities for how flight evolved. Dinosaurs could have
used their clawed hands to climb up trees
and then glide down. With this gliding behavior
eventually evolving into flight. Or dinosaurs could have
run faster and faster on the ground,
flapping their wings and some species then
evolved the ability to fly. But our research suggests
a third possibility. So every bird that we’ve
looked at– dozens of different species– do this behavior. They exhibit this behavior
of flap and running. No gliding. They don’t jump off of a
bale of hay and glide down and they don’t fly up. They flap run up
and they flap down. Juvenile theropods might
have used their four limbs similarly. In the adults the legs
alone were probably sufficient to escape predators
or to hunt down prey. But in growing therapods, small
wings provided an advantage. Just imagine the
selective pressure at the time of
therapy dinosaurs. Everything’s trying to eat
everybody– chasing everybody. If you could have moved
up to an elevated refuge with the use of
these little wings you would have lived
to see tomorrow. Those young theropods would
have run up to escape predators, and then flapped back
down when it was safe. Over time, small wings
evolved into larger ones, until these feathered dinosaurs
were able to take to the air.

 

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