system
Given the following post, respond as if you're commenting on Reddit:
human
When did we know that birds are descended from dinosaurs?

I remember back in school, some 25 years ago, that I was taught that *maybe* birds and dinosaurs were related but that there was no real evidence.

Did paleontologists (or whoever knows these things) not know back then, or was my teacher's information just outdated?
gpt
Almost as soon as the first *Archaeopteryx* was formally described by Richard Owen, other scientists noticed the many similarities between this "first bird" and certain dinosaurs. Within 5 years—so, 1868—you had Thomas Huxley loudly banging the "birds and dinosaurs are related" drum, as well as Cope and a few others. When the 1870 "Berlin *Archaeopteryx*" revealed its dinosaur-like teeth, the comparisons to *Compsognathus* were hard to miss. Owen himself never liked the idea, but a lot of contemporary biologists did. It was a fairly popular, but not *universal*, hypothesis within biology.

But in the 1920s, a Dane by the name of Gerhard Heilmann argued—quite persuasively at the time—that birds could *not* have evolved from dinosaurs. Dinosaurs, you see, had at some point in their evolutionary history lost their collarbones (clavicles). Birds *have* clavicles (fused together into a single "wishbone") and it was a popular idea in the '20s that once a trait was lost it could not be re-evolved. So birds must have branched off the reptile line before dinosaurs did. Heilmann put birds as descendants of the "thecodonts", a (now-obsolete) group containing the crocodile-like ancestors of dinosaurs, pterosaurs, and modern alligators and crocodiles. Heilmann's position was attractive enough that the "birds are dinosaurs" idea was effectively put to rest, and for the next several decades the textbook answer was "birds are some kind of thecodonts, but not *dinosaur*\-thecodonts."

It was *Deinonychus*, discovered in 1964 and described formally in 1969, that started pulling scientists back into the "birds are dinosaurs" way of thinking. Ostrom spent several more years after *Deinonychus* comparing *Deinonychus* and *Archaeopteryx*, arguing that their skeletal similiarities couldn't be the result of simple "convergent evolution". (Alan Grant in *Jurassic Park* mentions this specific argument of Ostrom's: "And look at the half-moon shaped bones in the wrist. No wonder these guys learned how to fly.")

This was still just one hypothesis, however; there were still a lot of "birds are thecodonts" advocates, and there was a competing hypothesis that, based on skull features, birds were *crocodylomorphs*. All three arguments were swirling around paleontological circles.

In 1986, Gauthier published his cladistic comparison of birds and dinosaurs, and showed that—by sheer number of shared anatomical traits—birds pretty much *had* to be some kind of theropod dinosaur. Birds weren't "thecodonts", they weren't crocodylomorphs, they were *definitely* dinosaurs. If your teacher was telling you 25 years ago that there was "no real evidence" that birds were dinosaurs, they were giving you out-of-date information.

Of course, there was also no Internet 25 years ago, so although your teacher was at least a decade behind, not having access to the most current information was a little more understandable than it would be today.

The final clincher for any holdouts (absent a handful of ~~morons~~ "contrarians" like Alan Feduccia) was the explosion of feathered bird fossils and feathered dinosaur fossils that started coming out of China in the '90s. Some dinosaurs were even originally described *as* birds, so covered with feathers were they, and only after publication recognized as bird-like dinosaurs instead of dinosaur-like birds. This would have been right around 20 years ago, just after your school experience. By the end of the '90s pretty much every paleontologist was back on board Cope and Huxley's 130-year-old "birds are dinosaurs" train, and hopefully your teacher was, too.