From Scales to Feathers

One of the common misconceptions about evolution is that it’s a directed process—one that moves from point A to point B with a specific goal in mind. By this reasoning, dinosaurs evolved feathers for the specific purpose of flight. From these dinosaurs, we get birds. End of story.

But evolution is hit-or-miss proposition, and there’s much more miss than hit. The majority of genetic mutations have no effect whatsoever, but a very small percentage result in an adaptive improvement every few million years or so. The genetic accident that caused the first two-legged dinosaurs to sprout feathers 120 million or 130 million years ago provided one such improvement. Because feathers (like fur) provide good insulation, feathered dinosaurs in chilly climates had an adaptive advantage over their lizard-skinned cousins. These fluffy dinos lived longer and had more kids, and eventually in some parts of the world feathers became the norm rather than the exception.

As with so much in evolution, the true potential of feathers revealed itself very slowly. When they first appeared 100 million years ago, feathers were more like scraggly strands of hair than the majestic plumage found on birds today. The same process of slight, incremental genetic mutation that gave rise to feathers in the first place went to work in causing these feathers to grow to larger sizes. In addition to keeping dinosaurs warmer (which increased their chances for survival and allowed them to produce more offspring), this development also conferred an aerodynamic advantage.

Paleontologists believe the advantage of aerodynamic lift would have revealed itself during a crucial episode in a dinosaur’s day. For example, as the dinosaur ran to catch prey or evade a predator, its feathers would supply a slight lift that enabled it to cover more ground in a shorter amount of time. The bigger the feathers, the greater the lift. Since eating regularly and not winding up as a T. Rex’s lunch have a clear evolutionary benefit, the trend was toward larger and larger feathers.

At the same time that feathers were growing longer, some dinosaurs were growing smaller. Because less weight allowed for more lift, natural selection favored a trend to lighter bones and smaller frames. The earliest birds-to-be were most likely animals about the size of turkeys, flapping their feathered arms furiously as they hopped, skittered, and jumped across the Cretaceous landscape. Over the course of millions of years, all the necessary ingredients of powered flight—low body weight, long feathers, and high running speeds—converged, and the first birds, in the modern sense, took to the air.

An alternate theory to which a few paleontologists subscribe suggests that some early feathered dinosaurs learned to live in trees and would glide from tall branch to tall branch, much like flying squirrels. This theory was lent some credibility by the recent discovery of a feathered, four-winged dinosaur dubbed Microraptor. However, the persistence of the flying squirrel today is seen by most paleontologists as evidence against this gliding theory.

While the evolution of feathers led to modern birds, it’s important to realize that feathers aren’t a necessary precondition to flight (see: bat). Flying reptiles called pterosaurs, which lived around the same time as dinosaurs, achieved featherless flight by means of long flaps of skin anchored to their middle fingers. (To further complicate matters, it’s possible that some pterosaurs had feathers as well, but it’s clear that their broad, flat wings provided most of the lift. Evolution is a tricky business.) Some of these reptiles reached truly astonishing sizes: For example, Quetzalcoatlus, with a wingspan of over 25 feet, was the largest creature ever to take to the air.

Many laypeople believe, incorrectly, that pterosaurs were the creatures that evolved into modern birds. The fact is, these flying reptiles may have been rendered extinct at the end of the Cretaceous by true birds, which were better adapted to an avian lifestyle thanks to their small size and better maneuverability. It just goes to show that evolution doesn’t always follow a linear path; sometimes, an unexpected solution to the problem of flight (i.e., feathers) can spell the end of another promising solution (i.e., the lizard-skinned wings of ptersoaurs).