I found this picture on Facebook last semester immediately after taking an ornithology exam that covered the concept shown, and decided to share some of what I learned here, too. The information might be a bit more esoteric than what I normally write about, but I tried to make it as accessible as possible by including informative links.

Image via The Scientist

This image depicts the cursorial hypothesis of avian evolution, which states that birds are descendants of theropod (bipedal) dinosaurs, and that flight developed from the "ground up" through leaping after and/or "netting" insects. The alternative hypothesis is known as the pseudosuchian thecodont hypothesis, which contends that modern birds are directly descended from arboreal (tree-dwelling) archosaurs, and that flight developed from the "trees down" — first by parachuting, and then gliding between trees.

Despite the latter hypothesis sounding more logical and intuitive, the former is becoming increasingly popular. The rise in popularity can be attributed to the existence of an 80 million year gap between the pseudosuchians and Archaeopteryx (until very recently the oldest known intermediary between reptiles and birds) in which no intermediate fossils linking pseudosuchians to modern birds have been found. The recent discovery of Aurornis xui seems to further discredit the hypothesis, as it is a theropod 10 million years older than Archaeopteryx, which (in my opinion) makes it that much less likely that birds are the direct descendants of pseudosuchians.