Are we not birds? We are DEVO

A recent study of the skeletons of modern birds show that their wings contain a bone that once existed in dinosaur wrists but had disappeared from later evolutionary predecessors.

The idea that evolution is “unidirectional and irreversible” — called Dollo’s law after 19th century Belgian paleontologist Louis Dollo — has been the dominant theory for over a century. It was believed that once an evolutionary change took hold, the previous structure was lost to time. But the findings of a team led by Alexander Vargas from the university of Chile suggests a bone that vanished for millions of years remerged when it again became beneficial.

Four-legged dinosaurs had substantial wrists with as many as 11 bones because of the enormous weight they had to bear. But as carnivorous, two-legged dinosaurs evolved some 230 million years ago, the front limbs took on arm-like characteristics for the job of handling prey. As few as three wrist bones remained in the smaller forelimbs.

But when these meat-eating raptors eventually evolved into birds, the wrist adapted again. A bone called the pisiform, present in long-extinct four-legged ancestors but missing from the raptors, reemerged as an integral part of the bird “wrist.”

For decades, the tiny bone was assumed to be a new structure and named the ulnare. The ulnare “evolved,” it was thought, to give wings extra stability on the upstroke.

But an examination of fossil records together with the evolving embryos of modern birds showed the ulnare to actually be the reappearance of the pisiform.

“While the physical expression of a gene may be suppressed, it doesn’t mean the possibility of generating that structure has disappeared,” said Luis Chiappe, director of the Dinosaur Institute at the Natural History Museum of Los Angeles County, in the February issue of Smithsonian Magazine. “The gene is still there, it’s just dormant.”

That concept was borne out by Harvard evolutionary biologist Arhat Abzhanov in 2011. By altering the DNA in chicken eggs, the Harvard team was able to grow embryos with lizard-like snouts, similar to the chicken’s dinosaur-age ancestors, instead of beaks.

Ethics rules did not allow the eggs to be hatched.

This is not the first time Dollo’s law has been challenged. In 2013, scientists at the University of Michigan determined that the common household dust mite had returned to its freestanding life in the wilds of mattresses, pillows and couch cushions after having previously evolved into a hosted parasites from an earlier, free-range mite.

And a tree frog was discovered to have re-evolved teeth after losing them 200-million years prior.

The re-emergence of primitive traits is not, it should be noted, devolution (or de-evolution) — a widely discredited theory based on the false premise that natural selection always represented “progress” toward more complexity. Still, that’s no reason not indulge in a little musical archeology.