10 August 2011

Wings were handed down from dinosaurs

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New evidence has put an end to the 150-year controversy on how bird wings evolved from the dinosaur hand.
Microraptor

A restoration of the feathered theropod dinosaur, Microraptor flying through the trees. Credit: Flickr user Cryptonaut

Rahonavis,

Reconstruction of Rahonavis, a ground-dwelling feathered dinosaur that some researchers think was well-equipped for flight. Credit: Wikimedia

EDINBURGH: New evidence has put an end to the 150-year controversy on how bird wings evolved from the dinosaur hand.

Bird wing anatomy resembles the digits on the hands of Triassic theropod dinosaurs, but which digit positions gave rise to those seen in modern birds has been unclear. A new study based on genetic maps provides a simple explanation for bird wing evolution, removing the need for complicated evolutionary theories involving digit ‘frame shifts’.

“This work is of historical importance because the difficulty in explaining this evolutionary transition was used as evidence that birds did not evolve from dinosaurs – and was even latched onto by Creationists as evidence against evolution in general,” said lead author Matthew Towers from the University of Sheffield in England of the study published in Nature Communications today.

The ‘frame shift’ debate

Most scientists agree that during the early Jurassic period, birds evolved from small coelurosauria theropods – a diverse group that includes Tyrannosaurus Rex and most known feathered dinosaurs. But the question of how the five-digit ancestral hand evolved into the three-digit modern bird hand has caused much debate.

Research has previously shown that the digits that make up bird wings anatomically resemble digits 1 (thumb), II and III (middle finger) of the hands of Triassic dinosaurs. This is also backed up by the fossil record, which shows a progressive loss of digits IV (ring finger) and V (little finger) in the coelurosauria theropod lineage that gave rise to modern birds.

However previous analysis of the development of modern bird embryos have seemed to suggest that the digits arise in the II, III and IV positions. This disparity was explained through complex theories involving a ‘frame shift’, where the cells responsible for the growth in digits II, III and IV grow forward and into the regions of digits I, II and III.

This conflict was exploited by those who challenged the theory that birds directly evolved from dinosaurs.

Ending the controversy

Now scientists in the UK say they have put an end to the debate by tracking the growth of embryonic tissue. They have identified that digits I, II and III grow to form the wing, which agrees with the fossil record without the need for a frame shift theory.

The researchers grafted embryonic tissue, genetically modified to make a green fluorescent protein, to normal birds. UV light was then applied to the developing chick and the growth progress of each digit was analysed. The results showed that only digits I, II and III grew to form the wing – suggesting that digits IV and V were lost during evolution.

“This work provides a very simple explanation for how the bird wing evolved from the dinosaur hand that disposes of the need for elaborate evolutionary theories such as the frame shift,” said Towers. “Our study provides the first embryological evidence that the bird digits arise in their ancestral positions I, II and III and that a digit frame shift has not occurred.”

‘Fossil record was right all along’

This new analysis technique has provided a key piece of evidence that not only provides a simple solution to a previously complicated issue, but also agrees with the majority of evidence currently available in the fossil record.

“For a long time, the I-II-III dinosaur versus II-III-IV bird hand digit problem has been seen as possible evidence against the origin of birds from dinosaurs. The previous suggestion of a frame shift, which can happen, sounded a bit ad hoc,” said Mike Benton, professor of vertebrate palaeontology at the University of Bristol in the UK, who was not involved with the study.

“This new work applies novel techniques of mapping the developmental fates of embryonic tissues, and presents evidence that current ideas on numbering digits as they develop in chicks are wrong, and that they begin as I-II-III as in ancestral dinosaurs; and finally rejects the key evidence against the origin of birds from dinosaurs.”

Towers added, “I think this study suggests that the theropod fossil record was right all along. We speculate in the paper that the loss of bird wing digits was caused by cell death. In the future we are interested in looking to suppress this cell death and see whether a chicken wing can be returned to its ancestral five-digit condition.”

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