The Earth's atmosphere is an incredibly dynamic environment.

It's made of mostly nitrogen and oxygen gases, which are, of course, invisible to the human eye.

Even though we can't always see what's going on in the atmosphere, we know air masses are in constant motion.

We feel some of these movements as wind.

In the most extreme cases, we get hurricanes and tornadoes. There are also colossal, undulating rivers of air like the jet stream.

And warm air rises high into the sky as billowing columns we call thermals. In other places, masses of air sink back down

to Earth, piling up to form high-pressure areas. There's a lot of activity in the atmosphere.

It's a vast and wild place. Several groups of animals have independently evolved the ability to fly, to brave this aerial frontier.

First it was the insects, then the flying reptiles known as pterosaurs, then birds, and finally bats.

These animals all fly under their own power. Well, in the case of pterosaurs, they flew under their own power, since those guys are all extinct.

And I guess you could say that one primate, in the form of Homo sapiens, managed to get into the sky as well.

But we cheated and took a shortcut, didn't we?

We got up there using science and technology, through the process of cultural rather than biological

evolution.

Flight has evolved more than once in the history of life because, presumably, it offers

some major advantages.

If nothing else, flying is often the most energy-efficient way

for an animal to get from point A to B. To cover one kilometer, for example, a bird will use up only

1% of the calories that a mouse of the same size would burn running that same distance. However, and this is a big, however,

flying requires an enormous amount of energy, in general, if you look at it moment by moment.

Getting off the ground is hard and takes loads of energy. Staying airborne is hard too.

Each group of flying animals has amazing adaptations to deal with these challenges. After millions of years of evolution, they're so well

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