[steps] [music chime] [music plays] [dial (narrated):] the evolutionof flight in birds has long been a great scientific puzzle. fossil evidence from the past 40 years has establishedthat birds descended from theropods, a lineage of two-legged dinosaurs that included specieswith 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 studiesof young birds that are learning to fly. and it all started with a bale of hay. [dial (narrated):] i've beenstudying the mechanics of bird flight for over three decades at the universityof montana's flight lab. i became interestedin an intriguing question that critics of evolutionary theoryhad posed to darwin. [dial:] so, there's a gentleman bythe name of sir george jackson mivart.
he challenged charles darwin whencharles wrote the origin of species. [dial (narrated):] darwin had proposed that structuresevolved through intermediates. so a wing would have evolvedfrom a forelimb in stages. but some of those early stageswere clearly not capable of flight. mivart confronted darwin. [dial:] he said to darwin, "how do you explain, in the evolution of birds from reptiles,the function of half a wing?"
[dial (narrated):] so to answer thatquestion, i looked to a living example: bird chicks with small, immature wings. i wanted to make careful observations of how chicks use their wingsas they learn to fly. [dial:] i was keeping the birdson these shiny floor, clean lab conditions, and trying to have them go up a wallthat was slick, to fly up to their siblings. as soon as a rancher came in once
and said, "what are these birdsdoing on the ground? they hate being on the ground! give them a bale of hay,give them something to get up on!" [dial (narrated):] as soonas i got some hay bales for the chicks, we made an interesting observation. [dial:] i came back one day and my sonwho 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..." [dial (narrated):] to better understandthis behavior, my son terry and i decided to carefully measurehow young birds use their legs and wings
together to travel up rampsof different angles. [dial:] so here is our littleexperimental animal here. [dial (narrated):]my friend julia clarke, an expert on the evolutionary originof birds, joined us on our experiments. the first angle is not steep. [dial:] it's wild. this is not a...[clarke:] not a trained animal. [dial:] this is not a trained animal. [wing flapping]
[clarke:] that was easy. [dial:] just walked, no problem.[clarke:] strolled. [dial:] nonchalant, no wing necessary. [dial (narrated):] next,we try a sharper angle. [clarke:] ok,so this is much steeper than before. [dial:] it is.[clarke:] let's see what happens. [dial:] there we go. [julia:] you are not sure what you see. because it's so fast.
[dial:] and until you see itin slow motion, you don't knowhow beautiful it actually is. [dial (narrated):] this time, the birdused its wings as it ran. [dial:] and every timeit 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 birdflying straight up to this refuge. it was using its wingsto pull it forward onto this log.
[dial (narrated):] and now,a really steep ascent. [dial:] that is goingbe a challenge, right? [clarke:] so,this is now nearly vertical. [dial:] this is vertical.standing tall, vertical like any tree. let's see what the animalcan do to negotiate this. it can fly, but... [clarke:] but it's still using its legs. still using its legs. [dial:] it climbed with its legs,
using its wingsto pull it towards the tree, but not to fly it up the tree. pretty cool. [dial:] this has turned out to be not just representativeof the bird we looked at. but every flight-capable bird that we'velooked at in the 15 years since. dozens and dozens of birds. [dial (narrated):] i have also observedthe same behavior in the wild. and i've even seen young birdsuse their wings
to assist their hind limbsto paddle across a body of water. [clarke:] so ken,how did this change the way we think about how dinosaursget in the air? [dial:] well, we nowknow that there are a lot of dinosaurs, little feathered theropodsthat have little wings. and the explanation for their existenceis really difficult to resolve. and i thinkthat a reasonable explanation is to look at what young birdswith similar wings can do today. birds show us the possibilityof what these dinosaurs could have done.
[dial (narrated):] scientists have longdebated two main possibilities for how flight evolved. dinosaurs could have usedtheir clawed hands to climb up trees and then glide down, with this gliding behavioreventually evolving into flight. or dinosaurs could have runfaster and faster on the ground, flapping their wings, and some speciesthen evolved the ability to fly. but our researchsuggests a third possibility.
[dial:] so every birdthat we've looked at, dozens of different species,do this behavior. they exhibit this behaviorof flap and running. no gliding. they don't jump off of a bale of hayand glide down. and they don't fly up. they flap-run up, and they flap down. [dial (narrated):] juvenile theropodsmight have used their forelimbs similarly. in the adults, the legs alone wereprobably sufficient to escape predators
or to hunt down prey, but in growing theropods,small wings provided an advantage. [dial:] just imaginethe selective pressure at the time of theropod dinosaurs. everything's trying to eat everybody,chasing everybody. if you could've moved upto an elevated refuge, with the use of these little wings, you would have lived to see tomorrow. [dial (narrated):] those young therapodswould have run up to escape predators,
and then flapped back downwhen it was safe. over time small wingsevolved into larger ones, until these feathered dinosaurswere able to take to the air.
