|MadSci Network: Evolution|
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. talkorigins.org/faqs/archaeopteryx/info.html. 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.
Try the links in the MadSci Library for more information on Evolution.