Posted: September 9, 2011, 2:00 p.m. PST
Courtesy of Yale University
[Click on the image to enlarge.] In five-digit vertebrates, the thumb comes from the precursor stem cells labeled Pa. While birds have a digit that looks like a thumb, Pa precursor cells die off during development and never produce a digit in adults. As a result, scientists have wondered whether precursor cells in Pb can make a thumb. The answer? Yes.
It was a question that always had two answers. Paleontological evidence said that digits evolved from places 1, 2 and 3 on the hand, or the thumb, index finger and middle finger. But embryonic evidence showed something different: digits evolved from places 2, 3 and 4, or the index finger, middle finger and ring finger. With two results to the same question, researchers were at a lost, until now.
In their study, “Transcriptomic analysis of avian digits reveals conserved and derived digit identities in birds,” published in the scientific journal “Nature,” researchers Zhe Wang, Rebecca L. Young, Huiling Xue and Gunter P. Wagner from the Department of Ecology and Evolutionary Biology of Yale University finally found their answer. Birds have thumbs, but the thumb is in the place where the index finger is. But how?
The idea that digits can move around and change places in development isn’t a new idea, according to Wagner. He, along with his colleague, Jacques Gauthier, first proposed this in 1999.
“Digits can play musical chairs — they get up and sit down at another place,” Wagner explained. “ If you think of it in this metaphor, embryonically, you have five chairs, and chair 1 is the thumb, chair 2 is the index finger, [etc.]. If you think about digits as things that can actually move around, you can think that in the bird lineage that they moved over by one chair, so the thumb sits in chair No. 2, the index fingers moves, [etc.].”
This idea was controversial, according to Wagner, as it made assumptions about what digits are and how they are made in development. But, using current technology, the researchers completed a genomic analysis and found strong evidence that digits can move around (evolutionary speaking).
“We found that thumb [in] the most forward position digit in the wing is, in its gene expression, correspond[ing] very strongly with the big toe of the foot of the chicken,” Wagner said. “This is interesting because they are actually developing in different [places]: the big toe is developing in the first position, while the thumb in the wing is developing in the second position. So there is strong evidence that what we have hypothesized [is] that development program for making a digit can move around different positions. Now, in the wing in the birds, we have digits that used to be on the anterior of the [hand] and have moved over to the middle.”
Why, though? Wagner doesn’t have an answer, but he had heard several theories that he believes might explain why this happens. “It’s clear that birds are derived from a branch of dinosaurs called theropods. They also have tendency of reducing the five fingers to three and, from the fossil records, it’s very fairly obvious that digits that slowly disappear are the ring finger and the pinkie.”
When the theropods evolved and lost their ring finger and pinkie, the remaining digits were functionally very important to them, Wagner explained. “They were used for prey capture, and these same digits are now used for flying. However, the way the hand develops makes the thumb vulnerable to getting lost in evolution,” he said.
Wagner explained that the outer digits can get lost in the development process, as the thumb and pinkie are the last digits to form (versus the other three, which develop quicker). In some animals, such as in horses and cows, no thumbs or pinkies are required to make a hoof.
But , for dinosaurs, when thumbs were so crucial to their way of life, the thumbs simply moved to the next chair. “My favorite theory,” Wagner said. He added that this is all circumstantial evidence and not yet proven. “It was a matter of making sure that three functionally important digits were in the place in the body where they are robustly developed, and they are not vulnerable to environmental or genetic change. The best way [to do this] was to move the three digits over to one place so they are in a less vulnerable position.”