COMMON DESCENT

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Today we’re talking about all things that flap, fly, swoop, and buzz. Flight is a feat accomplished by few and envied by most. But how did it happen?

In the News:
Dinosaur Footprints in Australia
A new study on the famous dinosaur trackways of Western Australia reveals a surprising diversity of species represented in the prints. [Report]
New Tyrannosaur Had a Scaly & Sensitive Face
A new cousin of Tyrannosaurus has been discovered in Montana, with well-preserved facial features that hint at their appearance and behavior. [Report]
Early Life in the Chicxulub Asteroid Crater
Drill cores reveal that the Chicxulub asteroid impact may have created conditions that allowed microscopic life to thrive, perhaps similar to Earth’s earliest organisms. [Report]
A Beautifully Preserved Ancient Bird
A new specimen of an early bird species preserves remarkable detail, providing clues to wing evolution, reproduction, and appearance early on in bird history. [Press Release]

Flight in the Animal Kingdom

Flight is one of the most extreme forms of movement that evolution has ever designed. With many benefits, and almost as many costs, flight yields high rewards for those animals that can do it.

Only four groups of animals have developed powered flight: insects, pterosaurs, birds, and bats. These lineages are some of the most diverse groups of animals on the planet, past and present. The ability to fly gives them access to homes and food out of reach of other creatures, and an excellent method of escape and transportation.

Learning to Fly

Each group has come up with a different answer to the question of flight. Insects – the first animals, and only invertebrates to evolve flight – developed a total of four wings from extra body appendages.

Insects have two different strategies for moving their wings: Direct flight (left), where muscles move the wings directly, and Indirect flight (right), where muscles contort the body. From Bugboy52 and Siga from Wikimedia.

As for vertebrates: all three flying groups transformed their forelimbs into wings, but each accomplished this in a unique way. Birds reduced their arm bones and covered them with aerodynamic feathers, and bats and pterosaurs both elongated their arms and fingers to support membranes known as patagia, which form an aerodynamic surface.

All three groups of flying vertebrates have similar body plans as well, with powerful chest muscles to flap their wings and light-weight sturdy bodies. How they get into the air differs, however. Birds can leap with their powerful legs, and bats often drop from high places to glide or launch themselves into the air with their strong arms.

Pterosaurs are more mysterious, being extinct. In the past, some researchers had suggested that they must have taken off only from high places such as cliffs, but new models are showing how they might have taken off from the ground using all four limbs, similar to some bats today.

Evolving flight

It’s no simple task to take to the sky. Though we are used to seeing flying animals today, the evolution of flight is full of interesting questions and mysteries. Flying animals tend to be delicate, and once flight is achieved, diversification happens quickly over a short time period. This means, unfortunately, that the fossil record doesn’t provide many answers to questions of how flying animals got their start.

For birds, the fossil record provides beautiful sequences showing how their wings and feathers and bird-like bodies evolved among dinosaurian ancestors. Exactly how they got into the sky, however, is an open question.

Typically, there are two schools of thought on this: did they start in the trees as gliders that eventually developed flapping flight (“trees-down”) or did their wings aid them in running and climbing until these movements allowed them to take flight (“ground-up”)? This is difficult to answer, but we can gather clues from fossils, as well as the behaviors of modern-day birds.

Modern birds often flap their wings to create a downward force to aid in climbing, providing
strong support for the ground-up hypothesis of bird flight evolution.

As for bats, pterosaurs, and flighted insects, their origins are much more mysterious; the earliest known fossils of each of these groups are already fully flighted. These groups may have started as gliders (such as many of the close mammal relatives of bats), or ground up from running ancestors (which has been suggested for pterosaurs), or through different means entirely (insects have inspired many hypotheses).

Hypotheses for each exist and are constantly being updated as we make new discoveries. But for now the secrets of their journeys to the sky remain hidden.

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If you enjoyed this topic and want more like it, check out these related episodes:

• Episode 99 – Evolution of Insects
• Episode 79 – Pterosaurs
• Episode 37 – Evolution of Birds
• Episode 59 – Bats
• Episode 148 – Gliding

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