In today’s Academic Minute, Dr. Hans Larsson of McGill University discusses the evolutionary pressures that influenced the development of flight in birds.
Hans Larsson holds the Canada Research Chair in Macroevolution at McGill University in Montreal, Canada, where his lab conducts research around the theme of macroevolution. More specifically, Larsson’s lab identifies large scale patterns of evolution by examining the evolution of skeletal anatomy and bone microstructure of vertebrate animals across time and diverse ecosystems. He holds a Ph.D. from the University of Chicago.
Dr. Hans Larsson – Evolution of Birds
How did birds begin to fly? If you’ve ever wondered about the origins of bird flight, you’re not alone. The question has confounded scientists for over a century. Evolution of flight is extremely rare, only present elsewhere in insects, bats, and the extinct flying reptiles called pterosaurs. The origin of birds, however, has the best fossil record and has generated much debate. Two theories have emerged as leading possibilities: birds began to fly "from the trees down" or "from the ground up"?
The “trees-down theory” assumes bird ancestors lived in trees and began flight by gliding. In this scenario, birds first took to the air by jumping from trees and gliding to the ground. Many living animals do this today, like gliding squirrels, lizards, snakes, and even frogs. The “ground-up theory” has bird ancestors living on the ground that started to flap, and later fly, to either run faster, hop further, or climb steep tree trunks.
We tested both hypotheses by asking the question: “Are there any arboreal adaptations at the origin of birds?” We examined the arm and leg skeletal anatomy of fossils for clues about the origin of birds and compared those details to a broad selection of living arboreal and terrestrial birds and mammals.
Living arboreal animals have sophisticated anatomies, like mobile ankles and shortened legs for scampering throughout trees. We found conclusive evidence that bird ancestors were not arboreal, rejecting the “trees down” hypothesis. In fact, the limb anatomies at the origin of birds, including Archaeopteryx, is most like those of running mammals and birds today, like horses and ostriches.
Better understanding of the ecology of the origin of bird flight helps refine evolutionary hypotheses for their unique flight mechanics.
Production support for the Academic Minute comes from Newman’s Own, giving all profits to charity and pursuing the common good for over 30 years, and from Mount Holyoke College.