Image: Archaeopteryx
Sometime between the start of the age of dinosaurs and the rise of birds (such as Archaeopteryx, shown in this artist's conception), the number of wing digits was reduced to three on each side. But which digits were lost, and which survived?
By Senior writer
updated 2/10/2011 6:59:00 PM ET 2011-02-10T23:59:00

Birds are believed to be descended from dinosaurs, but some significant changes must have happened as they evolved from their ancestors. A new study involving baby chicks may help clear up a mystery of how one of those changes occurred — how birds got their wing "fingers."

All four-limbed creatures, including dinosaurs, evolved from an ancestor that had five digits at the end of its limbs. These became flippers, wings, hands or paws, and some or all of the digits disappeared altogether in some cases.

Scientists think birds evolved from a group of meat-eating dinosaurs called maniraptors some 150 million years ago, during the Jurassic period. Modern birds have three digits in each of their wings, which means two digits in the forelimbs of these dinosaurs would have had to be lost during evolution.

A question of numbering
But which three digits survived? Paleontologists and developmental biologists have disagreed heartily on this. A proposal made in 1999, called the “frame shift” hypothesis, explained a discrepancy in the evidence, but not everyone accepted it.

The new study, conducted by researchers who transplanted cells from one part of a chick’s body to another, adds to the support for the hypothesis. The results of their study appear in Friday's issue of the journal Science.

If you number the digits so that digit 1 corresponds with our thumbs, digit 2 with our index fingers and so on, the fossil record shows that birds' wings evolved using digits 1, 2 and 3 of the dinosaur’s forelimbs.

However, in a bird embryo, the digits arise from the places on the limb bud associated with digits 2, 3 and 4. This conflict supported those who challenged whether birds were directly descended from dinosaurs.

In 1999, Günter Wagner and Jacques Gauthier of Yale University bridged the two factions by proposing that during development, digit 1 actually arose from the second position (where digit 2 should have arisen), and so on — a frame shift.

"The great thing about the frame shift theory is it makes both things correct," said Ann Burke, an evolutionary morphologist at Wesleyan University in Connecticut, who was not involved in the current study. "Birds are dinosaurs, but developmentally the digits are 2, 3 and 4."

The new evidence
In the new study, Japanese researchers led by Koji Tamura of Tohoku University transplanted certain cells from the feet to the wings and vice versa of developing chicks. These cells are implicated in the growth of digit 4. The researchers found evidence that the last digit of the wing does not correspond to the last digit of the foot. This supports the theory that the wing, unlike the foot, does not have a digit 4, they said.

Then the team mapped out digit development using cell-labeling techniques (enabling them to know where a certain cell ended up once it matured). They found that by 3.5 days of embryonic development, a shift occurs, causing cells in the progenitor region for digit 4 to move forward and grow into digit 3. The same shift occurs for the digits that become 1 and 2.

"I cannot tell you ‘why’ we have five digits and chickens have three digits, although I will be able to correctly tell you ‘how’ they have three digits whereas we have five digits," Tamura told LiveScience in an e-mail.

  1. Science news from
    1. NOAA
      Cosmic rays may spark Earth's lightning

      All lightning on Earth may have its roots in space, new research suggests.

    2. How our brains can track a 100 mph pitch
    3. Moth found to have ultrasonic hearing
    4. Quantum network could secure Internet

The findings accomplish two things, according to Tim Rowe, a professor of paleontology at the University of Texas at Austin who was not involved in the study.

First, "it pertains directly to this seeming conflict between paleontological records and developmental records. It shows in fact there is no inconsistency," said Rowe.

And second, it provides an excellent example of a shift in which one body part is transformed into another during embryonic development, he said.

"We are discovering part of the Holy Grail, which is the evolution of development, how the development of the limb changed during evolution of birds from their theropod ancestor," Rowe said.

This is not the first research to support the frame shift hypothesis, according to Wagner. "I think there is now enough data that shows that the frame shift actually happened," Wagner told LiveScience. "This is going a step further; it shows us the developmental mechanism."

Persistent skepticism
To determine the identity of the last digit in the chicks' "hands," the study used the development of the last digits in a five-digit mouse limb as a guiding model. This leaves Alan Feduccia, an evolutionary biologist at the University of North Carolina at Chapel Hill, skeptical of the results.

Feduccia, who wasn't involved in the study, is an opponent of the predominant scientific view that birds are descended from theropod dinosaurs (which include maniraptors). Rather, he believes birds and theropod dinosaurs share a common, earlier ancestor. This new study gives him no reason to change his mind.

"The experimental results seem to be very equivocal," Feduccia said, "because the bird hand is so highly modified that we don't know that same genetic mechanisms apply to digit identity."

The shift in digits just doesn't make sense, he said. "There is no imaginable selective advantage for such a shift — in other words, why would it occur?"

But he is in the minority.

Wesleyan’s Burke has another issue with the frame shift theory; she believes renumbering the digits of the growing chicks is potentially misleading.

"The dinosaur ancestry of birds is absolutely sound and so well-supported that nobody disagrees with that, so just trying to change the digit numbers in living birds to conform to fossilized ancestry is unnecessary and eclipses important evolutionary change," she said.

You can follow LiveScience writer Wynne Parry on Twitter via @Wynne_Parry.

In addition to Tamura, co-authors of the Science study, "Embryological Evidence Identifies Wing Digits in Birds as Digits 1, 2 and 3," include Naoki Nomura, Ryohei Seki, Sayuri Yonei-Tamura and Hitoshi Yokohama.

© 2012 All rights reserved.

Interactive: Are dinosaurs alive?

Explainer: The world's seven deadliest dinosaurs

  • Copyright 1985 Mark Hallett, "Awakening of Hunger"

    Yeah, it's cliche to say Tyrannosaurus rex was deadly. But the tyrant king was likely true to the billing. Its bone-crushing jaws could splinter prey like toothpicks, after all. And the beast was big, up to 40 feet long, 20 feet tall, and may have topped the scales at nearly 16,000 pounds.

    The king walked on two legs over a vast territory in western North America at the end of the Cretaceous, about 65 million years ago. Scientists wonder whether T. rex was more a lumbering scavenger or a quick and agile predator, but dead or alive, its meals were big, meaty and bloody.

    T. rex wasn't the only deadly dino, however. The globe was filled vicious killers. Click the arrows above to see more.

    — John Roach, contributor

  • Family diner

    Demetrios M. Vital

    Family meals for Majungasaurus crenatissimus were often bloody affairs of the grimmest sort: Kin were the main course. The evidence of its cannibalistic ways comes from telltale tooth marks on Majungasaurus bones that match up perfectly with the size and spacing of teeth in its jaws.

    The 20-foot-long dinosaur stalked the plains of Madagascar about 70 million years ago. At that time, the crime-scene investigators said, pickings were often slim. Their data indicate Majungasaurus fed on dried out "dino jerky" from its plant-eating compatriots as well as members of its own species. Cannibalism was likely just as common among dinosaurs as it is among living animals, but the evidence is rare.

  • Brow beater

    Todd Marshall

    Eocarcharia dinops' brow was swollen into a massive band of bone. The menacing head piece may have been used as a battering ram against rivals and to attract potential mates. Its blade-shape teeth were reserved for disabling live prey and severing their body parts.

    The 110-million-year-old beast and its snout-nosed, gut-and-carcass-scavenging relative Kryptops palaios were discovered in Africa's Sahara Desert. The approximately 7-foot-tall and 25-foot-long duo likely teamed up with a third carnivore and feasted on the long-necked plant-eater Nigersaurus.

  • Vicious rocker


    The toothy Masiakasaurus knopfleri likely speared prey with its forward projecting front teeth and then sliced and tore the captives into chewy chunks with its bladelike rears. This type of tooth arrangement is otherwise unknown in predatory dinosaurs.

    Scientists believe the German Shepherd sized beast feasted on fish, lizards, and other critters on the southern supercontinent Gondwana in the Late Cretaceous period, about 65 to 70 million years ago.

    The first part of this dinosaur's name means "vicious lizard" and the second part is derived from the Dire Straits singer and guitarist Mark Knopfler, whose music inspired the discoverers as they toiled on the African island nation of Madagascar.

  • Biggest carnivore?

    Prof. Rodolfo Coria / Ap

    At about 40 feet long and weighing an estimated 6 tons, Mapusaurus roseae was one of the biggest meat-eating dinosaurs to ever stalk the Earth. Bones of several individuals were discovered in one place, suggesting the giants may have hunted in packs that could have toppled perhaps the largest dinosaur that ever lived — a 100-foot-long plant eater called Argentinosaurus.

    Mapusaurus lived about 100 million years ago and was bigger than well known Tyrannosaurus rex and possibly larger than its older cousin, Gigantosaurus. Its teeth were narrow and blade like, made for slicing its prey. The bones were discovered in the Patagonia region of Argentina.

  • Going green


    Falcarius utahensis is the living image of a vicious meat eater trying to go vegetarian. Scientists aren't sure if the bird-like relative of Velociraptor had fully kicked its taste for flesh, but its meat-cutting teeth had shrunk to leaf-cutting size and its gut had expanded sufficiently to ferment plants.

    Falcarius walked on two legs and stood about 4.5 feet tall. Head to tail, it was about 13 feet long and wielded sharp, curved, four-inch long claws — perhaps to grab a bite when it fell off the wagon? The dinosaur lived during the Early Cretaceous, about 125 million years ago, in what is today Utah.

  • Plant ripper

    No plants were safe from Gryposaurus monumentensis, a big boned, duck-billed dinosaur that could have eaten any vegetation it stumbled across. Its massive skull packed more than 300 teeth for slicing up fibrous greens. Hundreds more replacement teeth rested in its jawbone for the call-up to action.

    The dinosaur lived in the Late Cretaceous, about 75 million years ago on the western side of a giant, shallow ocean that split North America at that time. The plant-munching beast may have reached 30 feet long as an adult and had a 3-foot-long head.


Discussion comments


Most active discussions

  1. votes comments
  2. votes comments
  3. votes comments
  4. votes comments