Did you have dinosaur for dinner last night?
Did dinosaurs fade into extinction? Or did they evolve into the chicken on your plate, the songbirds in your backyard and the penguins waddling across the Antarctic?The huge question arises because of one small creature.
Archaeopteryx (AR-kee-op-teh-rix), a bluejay-size animal that lived among the dinosaurs, has scientists locked in heated debates about whether dinosaurs evolved into birds 140 years after the first fossil was found.
Archaeopteryx, one of the most important fossil finds, prompts other intriguing questions.
- Did feathers, the most complex derivative of skin, evolve to provide insulation, like fur and hair, and only later prove handy for flying? Or did they evolve for flight first?
- Did bird flight begin with critters running along the ground and leaping into the air to catch prey? Or did it start with them gliding down from the trees?
"Going from being earth-bound animals to flying is one of the great evolutionary mysteries of all time," said anthropologist and science writer Pat Shipman.
Shipman, of Penn State University, explores the debates and leaves you to decide in her new book "Taking Wing: Archaeopteryx and the Evolution of Bird Flight" (Simon & Schuster, $25).
Limestone cutters working in a fossil-rich area in the Bavarian region of Germany in 1860 discovered the tiny impression of a feather that was about 150 million years old. Although no birds were known to have lived near that time, the feather was just like a modern feather.
The next year, stone cutters broke open a slab of limestone and found one of the most remarkable fossils ever found. The delicate and beautiful fossil had the toothy jaw and long, bony tail of a reptile. But it also had wings - and feathers.
All birds, and nothing but birds, have feathers. No birds have teeth.
This clearly was a creature caught in a moment of evolution between birds and something quite different.
The fossils could not have been discovered at a time that would have caused greater pandemonium.
Charles Darwin had just published the theory of evolution in "The Origin of Species" in 1859 and set Victorian society into an uproar. It struck at the Christian belief in divine creation of each type of animal, which in turn justified the social inequities of the Victorian class system as an unchangeable hierarchy of the natural world.
The best argument against evolution was the absence of transitional forms between early animals and later ones.
Then the archaeopteryx feather and specimen - a transitional form - came along in the two years after Darwin published.
Only seven specimens and a feather exist. But they still stir many scientific arguments today.
Ornithologists, who specialize in studying birds, are adamant that archaeopteryx is a bird, not a dinosaur, because it has feathers, which are unique to birds.
Alan Feduccia, an internationally known ornithologist who is the chairman of biology at the University of North Carolina, listed reasons he believes it's a bird - albeit a bird in transition from something else. It has feathers and wings like a modern bird, and has some characteristics that are reptilian, but not dinosaurian.
Archaeopteryx, like reptiles, has simple teeth with no serrations. All theropod dinosaurs, the type of dinosaurs other scientists say ar-chaeopteryx evolved from, have serrated teeth.
"What you find throughout the skeleton is that the features don't match dinosaurs," he said.
Paleontologists argue that some dinosaurs, called theropods, showed bird-like behavior, Shipman notes. Theropods are carnivorous dinosaurs that walk on their hind legs.
Paleontologist Jack Horner, an adviser for "Jurassic Park" and "The Lost World," found dinosaur nests in Montana, and baby dinosaurs which had been fed in the nest long after they hatched. In 1995, an international team discovered about 20 eggs carefully arranged in a nest with a parent Oviraptor seated atop it in a bird-like posture with its legs tucked under it.
Why would a creature that lives on the ground fly? How did flight evolve?
Flying is a tough and complicated process - especially takeoff, which requires enough force to create lift and get the body off the ground.
The gift of flight demanded a big reworking of bird anatomy. Yet stages of evolution aren't set up to reach an ultimate goal. Each change comes about for its own rea-son.
Birds had to develop wings to lift them, keep them in the air and control their flight, Shipman wrote. They developed powerful chest muscles and bones large enough to attach the muscles to.
They had to lose weight, which they did by developing hollow bones filled with special air sacs, and feathers to make them lighter and serve as control flaps for landing.
Archaeopteryx had feathers and hollow bones, but it isn't known if it flew or was just a link in the evolutionary chain toward flight.
But bats and butterflies fly, and they didn't develop all this stuff. Bats, insects and pterodactyls evolved in quite different ways from birds, Shipman pointed out. And they developed anatomical structures for flight and the development of flight in far different ways than birds.
The two hypotheses about how birds began to fly are called "ground up" and "trees down."
Those who argue that birds evolved from dinosaurs tend to favor the ground up hypothesis, while those on the other side favor the trees down hypothesis.
The ground-up argument says birds could have evolved from dinosaurs if the dinosaurs had certain features that would give them the oomph to take off from the ground.
The dinosaurs must have run on two legs and been speedy to get any lift in the first place. It's easier for an animal walking on four legs to evolve to walk on two, freeing its forelimbs to be adapted for wings, than to see a four-legged animal suddenly deriving wings from its front legs. The dinosaur must have developed feathers first for insulation, and they later turned out to be handy for the evolution of flight.
The ground-up hypothesis envisions the ancestor of birds and ar-chaeopteryx as a small, feathered theropod dinosaur that sped across the ground chasing prey. As it leapt to catch insects, it steered better if it stretched out its feathered arms. Eventually it developed feathered forearms that gave it enough lift to get off the ground.
Those who argue for trees-down envision gliding as the first step toward flapping flight. Since gliders live in the trees, the ancestor of birds must have been a tree-dweller and feathers must have originated as a means to develop gliding flight. By this line of thinking, the ancestor of birds couldn't have been a theropod dinosaur because there's no obvious way for it to get up a tree.
Many scientists argue that gliding, which usually involves a membrane stretched from the fore-limbs to the rear legs or rear end, doesn't lead to the evolution of the anatomical requirements for flapping flight, Shipman wrote.
A major strength of the trees down argument is that it's easier to imagine a creature starting to develop flight by stepping off a tree branch and dropping than by conquering takeoff. On the other hand, landing could be deadly. Animals taking off from the ground at least would be learning by skimming low above the ground where they couldn't get hurt much if they flubbed it.
Shipman, who started her research four years ago with an open mind, has come to the conclusion that archaeopteryx is indeed a dinosaur-bird link.
"I do believe that archaeopteryx flew," she said.