“Which came first, the chicken or the egg?” Evolutionarily speaking, it’s a yawn of a conundrum. We know it was the egg, which evolved (with shell to enable a terrestrial lifestyle) some 300 million years ago, long before a chicken first clucked across a patch of open ground.

In between the origin of the egg and the domestication of the chicken, however, there are plenty of other interesting features to consider. Take the feather. There were hints of a revolution 150 years ago when part-dinosaur, part-bird archaeopteryx was discovered. Recently, discoveries in China have pulled back the curtain to reveal a varied cast of feathered dinosaurs, and we've found it wasn't just the direct ancestors of birds that were sporting down coats.

These discoveries have made the question of evolutionary origins even more interesting. At one point, you could have wondered whether feathers—which are basically made of the same stuff as scales— arose directly to aid flight or had been co-opted for the purpose from some other function. The prevalence of feathers and feather-like structures in flightless organisms points to the latter. So when did they first appear, and what were these other functions?

A perspective in Science written by University of Texas at Austin paleontologist Julia Clarke lays out the basics of what we know so far—and where we might look for pieces of the puzzle that are still missing.

The earliest “proto-feathers” were not flap-like scales; they were thin fibers that would have provided little aerodynamic advantage even if they were attached to, say, a tree-hopping glider. When the light, flat, branching structures we would recognize as feathers finally appeared, the organisms donning them still weren’t fliers.

Feathers on modern birds perform a number of duties beyond enabling flight. Obviously, they provide insulation. They can also help birds blend into the background (camouflage) or get noticed (sexual displays). Researchers have recently been able to identify color patterns in the feathers of some fossils, with some showy results. And a study published a few months ago showed the first evidence for differences between sexes in such fossils—with males of a 130 million year old species of bird owning long tail feathers that are likely the result of sexual selection.

So when did feathers first evolve? We still don’t know, but fossil finds keep pushing the date farther into the past. We now know that the earliest proto-feathers may have appeared 100 million years before birds used them to take flight. Many of the theropods—the group of dinosaurs from which birds emerged that includes raptors and the iconic T. rex—had bristles or modern-looking feathers, but they were not alone. A very distantly related Triceratops-like dinosaur has been found with proto-feathers, as have some flying pterosaurs (though their bristles were a little different).

These groups last shared a common ancestor almost 100 million years before these fuzzy organisms lived. So did that common ancestor have bristly proto-feathers, or did these feathers evolve independently two or more times? (Only to be lost again by many lineages.) Unfortunately, the fossil record is not cooperating with our curiosity. The exquisitely preserved specimens from China all come from the Late Jurassic and Early Cretaceous, while the groups diverged back in the Triassic. None of the fossils we've found from the intervening time preserved soft tissue features well enough to tell us what was happening on the feather front.

Absent the discovery of a revolutionary fossil bed from that time, researchers will have to get a little more creative to probe these questions. Clarke writes that more modeling could help narrow the range of possibilities by testing hypotheses about the functions of the proto-feathers being discovered. It appears that flight was not the driver behind the origin of feathers, but they provided the raw material that evolution used to sculpt the wings that made terra firma optional.

Science, 2013. DOI: 10.1126/science.1235463 (About DOIs).