Friday, July 3, 2015

Fun Friday Facts #113: How Did Dinosaurs Become Birds?

I just saw Jurassic World tonight, and was disappointed to see that none of the dinosaurs were depicted as having had feathers. But that’s Hollywood for you, I guess. Historical accuracy doesn’t sell movie tickets. Abraham Lincoln hunting vampires sells movie tickets.
Also, if scientists did go too far and bring dinosaurs back to life for profit, they’d probably go ahead and tweak the genome to remove the feathers. You’ve got to give the people what they want, and the people want two things: naked, leathery behemoths, and, occasionally, to get eaten.

I heard about the birds-evolved-from-dinosaurs thing when I was a kid, and I’ve always sort of wondered how that happened. Researchers have only recently been able to put together a complete picture of the evolution of birds. For a long time, scientists knew of only one evolutionary link between large, land-bound dinosaurs like the T. rex and the tiny, feathered creatures that seem to line up and take turns going at my birdfeeder (ooh, that reminds me, I’d better fill up my birdfeeder). That link was Archaeopteryx, a birdlike dinosaur that lived about 150 million years ago and weighed about 1.7 lbs (0.8 kg).

Image by Nobu Tamara from Wikipedia.

While researchers previously assumed that Archaeopteryx represented a sudden and mysterious evolutionary jump of just 10 million years, a plethora of fossil evidence uncovered in the last decade has revealed that Archaeopteryx and other early birds were the result of a much more normal, slower progression. Instead of birdlike dinosaurs emerging suddenly about 150 million years ago, dinosaurs began evolving birdlike traits pretty much from the time they first emerged on Earth about 245 million years ago. Modern birds evolved from the maniraptoran theropods, of which the velociraptor is one. As the millennia passed, theropods developed increasingly birdlike traits. The dinosaurs that would eventually become birds also got smaller and smaller over about a 50 million year period. The earliest bird ancestors weighed around 359 lbs (163 kg), but they would eventually shrink down to the size of the smallest modern songbirds.

Birds slowly became birdier, and when the basic components of birdiness – hollow bones, birdlike pelvises, three-fingered hands, wishbones, fully articulated quill feathers, and small size – came together in the Archaeopteryx, conditions were finally ripe for bird evolution to take off. As early birds began learning to fly, they started shrinking faster, and the smaller they became, they more adept they were at flight – large winged animals can glide, but true flight requires the right ratio of weight to wing size. The lineage that would become birds proved itself to be far more adaptable than other dinosaur lines, and shrank 160 times faster than other lines were growing.

The earliest birds also pulled an evolutionary trick that has helped other animals, such as humans, nurture greater intelligence: paedomorphosis. Paedomorphosis occurs when an animal stops developing physically at an earlier stage of its life than its predecessors, in a way proves advantageous from an evolutionary standpoint. Dinosaur embryos had larger, rounder skulls and shorter faces, and as birds evolved, they began to retain that larger, rounder skull shape into adulthood. This allowed adult birds to develop bigger skulls and bigger brains to fill them. It also allowed birds to ditch their ancestral snouts and start developing beaks, which would rapidly become specialized to the needs of each species.

As seen here.
Image by L. Shyamal from Wikimedia Commons.

Their newly small size must have provided dinosaur-birds with an evolutionary advantage, since it allowed them to fill ecological niches left unexplored by their larger cousins. Of course, when a six-mile-wide asteroid slammed into the Earth about 65 million years ago, the small, easily adaptable birds, with their powers of flight and their significantly reduced nutritional needs, were able to ride out the cataclysm, while their enormous cousins dropped dead all around them. And that, friends, is why we have chickens.  

And that, friends, is why we have chickens.
Image by misha3637 from Wikimedia Commons.