Bird embryos uncover homology and evolution of the dinosaur ankle : Nature Communications : Nature Publishing GroupScience is like a detective story. You accumulate evidence, examine it and let it lead you to the truth. But in that body of evidence there may be a few little pieces that seem to be pointing to a different truth or to falsify the truth that everything else seems to be pointing to. The question is, how strong is this little bit of contrary evidence? Can the overall conclusion live with it or is it fatal so that some other truth has to be found to accommodate it and all the other evidence?
Often, the problem can be solved by a closer reassessment of this evidence. Things look very different under a microscope.
Take the evolution of birds. Lots of independent strands of evidence point inexorably to the conclusion that the immediate ancestors of birds were theropod dinosaurs, i.e., the group that include Tyrannosaurus rex which walked on two legs and had reduced forelimbs. Lots of intermediate fossils have been found showing feathered dinosaurs and early birds with teeth, and almost all the evidence showed homologous structures in the skeletons of birds that were present in these dinosaurs; inactive genes have been found in birds which, when activated, make bird embryos begin to develop a dinosaur jaw or a dinosaur foot.
But just like a good detective story, where the audience needs the plot explained to them in case they hadn't been following it, "There's just one thing I still don't understand, Lieutenant!"Why did the ankle of birds not look like it had evolved from a dinosaur ankle? If it didn't, birds didn't evolve out of dinosaurs. There is no way evolving birds could have got their ankles from something other than their ancestors and if the ancestor whose modified ankle they inherited wasn't a dinosaur, their ancestor wasn't a dinosaur.
Now, this little vestigial piece of doubt has been removed by closer examination, and, as you would expect of a theory which corresponds with the truth, the last little piece of evidence is not a problem after all and fits perfectly into the picture. The ankle of birds has evolved from the ankle of the bipedal theropod dinosaurs.
The reassessment and closer examination was carried out by a team from the Biology Department of the University of Chile, Santiago, Chile. Their findings were published in Nature Communications last May in an open access paper. My thanks to Facebook friend Bil Wight for sending me a link to it.
The problem centred around the precise details of how the cartilages and bones in bird ankles develops in the growing embryo to produce the 'ascending process' (ASC). The problem was that in theropod dinosaurs this appears to arise from a developing bone called the astragalus and in birds it arises from the calcaneum. The team used several different techniques to follow this embryological development in six different orders of birds, including the tinamou, a member of the ancient Paleognathae order, and believe they have shown that the ASC actually arises from it's own center and not from either bone, so these differences are more apparent than real so do not pose a problem for the accepted view of bird evolution.
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See original paper for details. Copyright © 2015, Rights Managed by Nature Publishing Group. |
The anklebone (astragalus) of dinosaurs presents a characteristic upward projection, the ‘ascending process’ (ASC). The ASC is present in modern birds, but develops a separate ossification centre, and projects from the calcaneum in most species. These differences have been argued to make it non-comparable to dinosaurs. We studied ASC development in six different orders of birds using traditional techniques and spin–disc microscopy for whole-mount immunofluorescence. Unexpectedly, we found the ASC derives from the embryonic intermedium, an ancient element of the tetrapod ankle. In some birds it comes in contact with the astragalus, and, in others, with the calcaneum. The fact that the intermedium fails to fuse early with the tibiale and develops an ossification centre is unlike any other amniotes, yet resembles basal, amphibian-grade tetrapods. The ASC originated in early dinosaurs along changes to upright posture and locomotion, revealing an intriguing combination of functional innovation and reversion in its evolution.*
Bird embryos uncover homology and evolution of the dinosaur ankle;
Luis Ossa-Fuentes, Jorge Mpodozis & Alexander O Vargas;
Nature Communications 6, Article number: 8902 doi:10.1038/ncomms9902
*Copyright © 2015, Rights Managed by Nature Publishing Group. Reprinted under Creative Commons CC-BY license
In fact, it seems the apparent similarity between the way bird ankles develop and the way tetrapod amphibian ankles develop is because, as part of their adaptation to bipedalism, the theropod dinosaurs simultaneously evolved some functional innovations and partially reverted to an amphibian mode of embryological development in respect of this feature. Birds have simply continued on this trajectory. So, what looked like a fatal piece of evidence is nothing of the sort and fits neatly into a consistent picture of bird evolution from the theropod dinosaurs.
No doubt, creationists can explain this inexorable process of more and more evidence mounting up for this evolutionary origin of birds, objections to it being dismantled and shown to support it instead, and never a single piece of evidence ever being found which supports a creationist view.
Or maybe not...
'via Blog this'
It's fascinating how our ongoing investigation of the evidence is enabling us to fill in more and more details of how life developed. When it comes to persuading creationists, though, I think we long ago reached a point of diminishing returns. The evidence for evolution is already more overwhelming than the evidence for pretty much anything else in the world. If creationists were amenable to persuasion by evidence, they wouldn't be creationists.
ReplyDeleteThe exception, of course, is those (mostly young) who reject evolution because they simply haven't seen the evidence yet. That's what public education is supposed to address, but all too often doesn't.