MadSci Network: Evolution

Re: How, and from what, did birds evolve?

Date: Fri Jul 16 11:39:10 1999
Posted By: Andrea Bixler, staff (postdoctoral associate), biology, UM-St. Louis
Area of science: Evolution
ID: 930443588.Ev

The second part of your question is easier than the first part, so I'll 
start there.  From what did birds evolve?  From reptiles.  In fact, some 
scientists would argue that there is not a separate group of animals called 
birds; instead there are just a couple different types of reptiles (well, 
more than a couple--there are turtles, snakes, lizards and birds)! The 
fossil that first got people thinking in this direction was an 
Archaeopteryx, which means "ancient wing."  As the name implies, this 
fossil was of an animal with wings, but it was awfully reptilian-looking  
too.  As you might imagine, scientists were really excited about the 
discovery of this fossil!  You can probably find a picture of it in any 
encyclopedia, or you can see drawings of it at  http://www.  This site also 
has a lot of information about the fossil, some of which is very technical 
but some of which is easy to understand.  Archaeopteryx has some features 
that are reptilian, like the fact that it has teeth and no beak, and some 
features that are very bird-like--feathers and an opposable big toe.  Seven 
fossils of Archaeopteryx have been found, and fossils of other species that 
seem to be part-bird, part-dinosaur also support the theory that birds 
evolved from reptiles.  This probably occurred in the Jurassic Period, 
around 200 million years ago.

Now, how did birds evolve?  I assume you want to know how they evolved to 
fly.  Well, that, as I said, is a little harder to answer.  First of 
all, the ancestor of the bird had to have wings.  It is thought that 
wings evolved first for some other reason, such as to aid the animal 
in leaping up in the air or in gliding from a tree to the ground (like 
a flying squirrel might do).  Once the animal had wings, there would 
be many advantages in evolving flight, such as increased ability to 
escape from predators, or ability to catch prey, or even freeing the 
hind legs for fighting (have you ever seen birds fight?  a lot of 
times they fly up in the air a little ways and scratch at each other with 
their hind feet).  There are two main ideas about how flight would have 
evolved once the ancestors of birds had wings.  The two ideas are referred 
to as the "ground up" and "trees down" ideas.  The ground up idea suggests 
that the birds' ancestors lived on the ground and because wings were so 
advantageous in increasing their leaping and running ability, they evolved 
stronger chest muscles and larger wings and began leaping more and farther 
off the ground until they were flying.  The trees down idea suggests that 
the ancestors of birds were living in trees, and derived such an advantage 
from being able to use their wings to glide between tree branches and from 
trees to the ground that they evolved stronger chest muscles and larger 
wings so that they could extend the length of their gliding flights and 
eventually were able to fly long distances.  So, the short answer is, 
scientists are still trying to figure out exactly how flight evolved, but 
there are some really interesting ideas about how it could have happened.

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