Some of the earliest birds may have taken to the skies using a set of four wings instead of two.
Birds started to appear around 150 million years ago and were descended from therapods, a group of dinosaurs that included Tyrannosaurus Rex. It isn't clear exactly how and why the ability to fly appeared however it is generally thought that the first wings would have been used for gliding between the trees before they developed full flying capabilities.
One major difference in some of the earlier bird species was that, based on fossil evidence, these earlier fliers had large feathers on both their fore and hind limbs suggesting that they flew using two sets of wings. Over the next few million years however these additional wings would have slowly been lost, eventually leaving just the one set that we see in modern birds today.
"Roughly 150 million years ago, birds began to evolve."
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Source: Smithsonian Magazine
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