To take flight, first strengthen your legs: It sounds like a self-help proverb, but it could explain how birds first took wing.

Until now, most explanations of the evolution of flight have assumed that going airborne was an end in itself, driven by the need of some early dinosaur to glide down from trees or up off the ground.

But flight could have instead been an incidental benefit of beefier muscles needed to compensate for losing a heat-generating protein.

“Flight is seen as the hallmark of bird evolution,” said developmental biologist Stuart Newman of the New York Medical College. “But you can make the argument that the particular form bird skeletons took that opened the way for flight was a side effect.”

Newman’s research shows that all birds and reptiles lack a single gene that codes for a protein called UCP1 or, with a nod to its function, thermogenin. It’s an essential part of the metabolic reaction that burns brown fat, helping bodies self-regulate internal temperature and generate heat without shivering.

Thermogenin’s absence from birds and reptiles hints at its loss in some early common ancestor, with the thermogenin-retaining relative later giving rise to mammals. But whereas reptiles became cold-blooded, basking in sunshine when needed, birds stayed warm-blooded.

Image: Markiza/Flickr

As Newman describes in a September Bioessays paper, the key to their warmth is muscles. Muscles are powerful generators of heat, which is a byproduct of the chemical reaction that makes them contract. Bird muscles also have further heat-generating adaptations. And birds are, in a word, jacked.

In ounce-for-ounce comparison, mammals and reptiles are scrawny weaklings next to birds. And it’s not just avian breast muscles that are pumped, as would be expected in flyers, but their legs too.

“My hypothesis is that birds basically salvaged their existence by developing very large skeletal muscles,” said Newman.

Once heavily muscled, he believes proto-birds would naturally have gravitated towards bipedalism, which isn’t a particularly challenging transition. Indeed, walking on two legs was widespread in dinosaurs.

Bipedality releases upper limbs, both literally and in evolutionary terms, allowing them to accumulate large mutations with relatively little risk. Combine that with powerful breast muscles, and wings would soon follow.

Testing Newman’s hypothesis may not be possible, as it would require comparing early bird and dinosaur skeletons and genes, and DNA is lost in the fossil record. But that flight could plausibly have been a fortunate side effect of some unrelated adaptation, rather than the original driver of bird development, is a useful evolutionary lesson.

Newman also suggests people at least reconsider the phenomenon of flightlessness in birds, which is generally portrayed in terms of loss.

“It’s almost universally accepted that all flightless birds come from flighted ancestors,” said Newman. “That might be true — but maybe it’s flying birds that have flightless ancestors. Maybe flightless birds were the leading edge.”

Top image: Lip Kee Yap/Flickr

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Citation:”Thermogenesis, muscle hyperplasia, and the origin of birds.” By Stuart Newman. Bioessays, Vol. 33 No. 9, September 2011.