The five fingers of our feathered friends

Date:

Share post:

In general, land vertebrates have five fingers or toes per hand or foot. Many animal groups, however, have modified this recipe in the course of evolution. For example, camels have only two and horses only one fully developed toe. Similarly, birds’ wings have only three fingers.

The five fingers of our feathered friends
This is a photo of the schematic figure of birds’ wings
[Credit: Brian Metscher]

However, the early trace of a fourth finger on the posterior (little finger) side of the hand can be found in embryonic birds. So the question arises as to which fingers the bird wing actually hass: thumb, index, and middle fingers (I, II, III) or the index, middle, and ring fingers (II, III, IV)? Theoretical biologists at the University of Vienna have examined and clarified this problem in a current article in the Journal of Experimental Zoology.

In most tetrapods (land vertebrates) the fourth (ring) finger is the first to develop in the embryo. And in birds, the finger on the outside of the hand (posterior, the pinky side) appears first, which suggests that this is the ring finger. However, it can be shown that on the thumb side (anterior) an embryonic finger begins to develop, but quickly disappears – this would have to be digit I. These data argue for an identification of the fully-formed fingers as the index, middle, and ring fingers (II, III, IV).

However, the three fingers of the earliest known bird – Archaeopteryx – resemble those of the dinosaur Deinonychus, with whom Archaeopteryx was probably closely related (as was the famous velociraptor). Successive fossils show the reduction of two fingers on the posterior side of the hand in the ancestor of Deinonychus, and thus support the thumb, index, and middle finger identification (I, II, III) of the bird fingers. Also, the genes active in first bird finger correspond with those of the developing thumb in other animals, and not those of the index finger.

To resolve this contradiction, three approaches had predominated: 1) Birds do not come from dinosaurs after all; 2) the dinosaur ancestors of birds had also the three middle fingers (II, III, IV); or 3) the three anterior fingers (I, II, III) of the birds were somehow moved to the middle three embryonic positions. In fact, none of these theories can explain all the existing data.

For the birds: 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 signalling protein Sonic Hedgehog, which emantes from the posterior side of the developing limb before any fingers appear. 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. We have devised a hypothesis based on molecular and biomechanical mechanisms that is able to explain all the available data,” said lead author Daniel Capek, who conducted research in the group of Dr. Brian Metscher and Prof. Gerd Müller in the Department for Theoretical Biology at the University of Vienna for this project and is currently a PhD student at IST Austria.

According to this hypothesis, a posterior reduction in dinosaur evolution actually proceeded with the little finger being reduced and then lost, and the ring finger was partially reduced. However, it is generally much easier to reduce the outer fingers than the more central ones, as they appear later in development. Thus the first finger is reduced (“thumbs down”) instead of the fourth finger, leaving an anterior area open for the early precursors of the other fingers to grow into. This would lead those developing fingers to encounter an anterior-like Sonic Hedgehog concentration and then to develop accordingly – more like I, II, and III.

“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,” says Brian Metscher of the Department of Theoretical Biology at the University of Vienna.

Source: University of Vienna [January 07, 2014]

ADVERTISEMENT

spot_img

Related articles

Human impact has created a ‘plastic planet’, research shows

Planet Earth's oceans and lands will be buried by increasing layers of plastic waste by the mid-century due...

Earth’s interior cycles contribute to long-term sea-level and climate change

Ancient rises in sea levels and global warming are partially attributable to cyclical activity below Earth's surface, researchers...

Scientists pioneer method to predict environmental collapse

Scientists at the University of Southampton are pioneering a technique to predict when an ecosystem is likely to...

Large mammals were the architects in prehistoric ecosystems

Researchers from Denmark demonstrate in a study that the large grazers and browsers of the past created a...

Stretched-out solid exoplanets

Astronomers could soon be able to find rocky planets stretched out by the gravity of the stars they...

Continental drift created biologically diverse coral reefs

An international research team has studied the geographical pattern of the evolution of corals and reef fish. Their...

Geologists produce new timeline of Earth’s Paleozoic climate changes

The temperature of a planet is linked with the diversity of life that it can support. MIT geologists...

NASA mission provides closest ever look at dwarf planet Ceres

A NASA mission led by UCLA professor Christopher Russell has released new images of the dwarf planet Ceres,...