Rule Five DinoBirds Friday

Once in a while I hear someone make a comment about something being as “extinct as dinosaurs,” which enables me to reply “dinosaurs aren’t extinct at all.  In fact there are more species of dinosaur alive today than there are mammals.  We call them birds.”

Birds have been known for some time to be a branch of the theropod dinosaurs, but the exact mechanisms that brought about the modern form of the bird has been and continues to be widely discussed, but here is an interesting theory that posits that neoteny may have played a part.  Excerpt:

For decades, paleontologists’ only fossil link between birds and dinosaurs was archaeopteryx, a hybrid creature with feathered wings but with the teeth and long bony tail of a dinosaur. These animals appeared to have acquired their birdlike features — feathers, wings and flight — in just 10 million years, a mere flash in evolutionary time. “Archaeopteryx seemed to emerge fully fledged with the characteristics of modern birds,” said Michael Benton, a paleontologist at the University of Bristol in England.

To explain this miraculous metamorphosis, scientists evoked a theory often referred to as “hopeful monsters.” According to this idea, major evolutionary leaps require large-scale genetic changes that are qualitatively different from the routine modifications within a species. Only such substantial alterations on a short timescale, the story went, could account for the sudden transformation from a 300-pound theropod to the sparrow-size prehistoric bird Iberomesornis.

But it has become increasingly clear that the story of how dinosaurs begat birds is much more subtle. Discoveries have shown that bird-specific features like feathers began to emerge long before the evolution of birds, indicating that birds simply adapted a number of pre-existing features to a new use. And recent research suggests that a few simple change—among them the adoption of a more babylike skull shape into adulthood—likely played essential roles in the final push to bird-hood. Not only are birds much smaller than their dinosaur ancestors, they closely resemble dinosaur embryos.

Here’s the key bit:

“The first birds were almost identical to the late embryo from velociraptors,” Abzhanov said. “Modern birds became even more babylike and change even less from their embryonic form.” In short, birds resemble tiny, infantile dinosaurs that can reproduce.

This process, known as paedomorphosis, is an efficient evolutionary route. “Rather than coming up with something new, it takes something you already have and extends it,” said Nipam Patel, a developmental biologist at the University of California, Berkeley.

“We’re seeing more and more that evolution operates much more elegantly than we previously appreciated,” said Bhullar, who will start his own lab at Yale University in the fall. “The umpteen changes that go into the bird skull may all owe to paedomorphosis, to one set of molecular changes in the early embryo.”

Why is this interesting?  Because it mirrors something that happened to humans.  Look at the face and cranial structure of our closest relatives, the chimpanzee, and you’ll see an extended muzzle, a flattened skull, and ears set back on the head.  But look at an infant chimp and you’ll see something much more humanlike:  A flattened face, a more rounded skull.

What Dr. Abzhanov calls “Paedomorphosis” is also known as neoteny, or the retention of juvenile traits into adulthood.  Our species, Homo sapiens, exhibit a tendency to neoteny compared to our recent cousins and ancestors.  And when you examine juvenile dinosaur fossils as well as the developing skulls of young crocodilians, which are the closest living relatives of birds, you see the same sort of retention of juvenile traits.

It’s interesting how biology keeps demonstrating the same tricks over and over again.