January 8, 2014
Bird Fingers: Understanding Evolution Of Wings In Our Avian Friends
Brett Smith for redOrbit.com - Your Universe Online
While birds’ three fingers appear analogous to our own thumb, index and middle fingers, embryonic evidence has shown that the “pinky” side of a bird’s claw develops first, which some theories have said is evidence of the three bird fingers being an index, middle and ring finger.
The earliest known bird, Archaeopteryx, has fingers that resemble those of the dinosaur Deinonychus, a probable relative. According to fossil analyses, two fingers on the pinky-side of the hand were reduced over the generations in the ancestor of Deinonychus. This evidence supports the identification of these fingers in birds as a thumb, index, and middle finger identification. Also, the genes active in the development of the first bird finger match up with genes in the developing thumb of other animals, and not those of the index finger.
However, some evidence has shown that an embryonic finger begins to develop thumb-side, but quickly disappears before fingers develop on the other side of the hand. This evidence suggests that bird fingers may not be a thumb, index and middle finger, but instead an index, middle and ring finger.
Evolutionary biologists have put forth three theories to resolve this apparent contradiction: birds do not come from dinosaurs; dinosaur ancestors also had three middle fingers; or the thumb, index and middle fingers of the birds somehow migrated to the middle three embryonic positions.
The new study suggests that the thumb, index, and middle fingers are actually index, middle, and ring fingers.
"The appearance -- the so-called phenotype -- of the fingers is determined during embryonic development by the signaling protein Sonic Hedgehog, which emanates from the (pinky) side of the developing limb before any fingers appear,” explained study author Daniel Capek, who conducted research while at the University of Vienna. “This simply means that the concentration of this protein on the little-finger side is highest and decreases toward the future thumb.”
“Therefore each finger precursor (the cells that will develop into the finger) adjusts its gene expression -- and in consequence, its phenotype -- according to the Sonic Hedgehog concentration in its immediate environment,” Capek continued. “We have devised a hypothesis based on molecular and biomechanical mechanisms that is able to explain all the available data.”
According to the study team, a pinky-side reduction in dinosaur evolution proceeded with the little finger being reduced and then lost and the ring finger was partially reduced. However, outer fingers tend to reduce more readily than the more central ones that appear later in development. Therefore, the dinosaurs’ thumb was reduced instead of the ring finger, leaving thumb-side area open for the precursors of the other fingers to grow into. This would lead to a thumb-side Sonic Hedgehog concentration causing the development of an index, middle and ring finger.
"This mechanism explains why the fingers of Archaeopteryx and modern birds have the shapes of the anterior fingers (I, II, III), even though they are actually the central fingers (II, III, IV). At the same time, this hypothesis is consistent with the fossil findings and matches the current developmental genetics results," said study author Brian Metscher, a theoretical biologist at the University of Vienna.
Image Below: This is a photo of the schematic figure of birds' wings. Credit: Brian Metscher