Mar 27 2017

More on Bird Evolution

bird_evoThe fossil evidence for the evolution of birds is currently one of the greatest evolutionary stories we have to tell. It is also a home run for the predictive value of evolutionary theory and is a devastating blow to any denial of common descent.

Darwin published On the Origin of Species in 1859. At that time the fossil record was sparse, although it was enough to establish that the forms of living things was changing over geological time. One of the most dramatic gaps in the fossil record at the time was between birds and other vertebrates. It was believed that birds must have evolved from some type of reptile, but the most recent common ancestor was not known.

This was a great test for the new theory of evolution. If evolution were true then birds must have evolved from something else, and therefore the gap between birds and their closest non-avian relative should eventually be filled in by future fossil finds. If evolution is true, those creatures must have existed.

If, on the other hand, some version of creationism were true so that was created close to its current form, then there would not have to be any creatures filling in the morphological space between birds and their closest relatives. Birds would not have any relatives, they would be an isolated group unto themselves.

Now, of course, creationism does not require anything (like evolution does). Because life was magicked into existence by an omnipotent being, it could take any form desired. In that way creationism (there are many forms of creationism, but using the loose definition that life was created and did not entirely evolve through natural processes) is not falsifiable. God could have made life to look exactly as if it had evolved.

But what we can say is that if evolution were true we would find plausible bird ancestors. If evolution were not true and instead life was created, we would not have to find plausible bird ancestors. In fact it would be quite a coincidence if we did – given the infinite possibilities why would God choose the one that mimics an evolutionary history?

Feathered Dinosaurs

In  1874 the Berlin Specimen of Archaeopteryx was discovered, with a description published in 1884. This one fossil was a stunning victory for evolutionary theory Рit is a beautifully preserved specimen looking like a small theropod dinosaur with clear feather impressions. On close examination it is a wonderfully transitional specimen, pretty much as close as you can get to half bird-half dinosaur. (To be pedantic, birds are now considered a clade of dinosaurs, but I will use the term dinosaur here to refer to non-avian dinosaurs.)

Creationists were not impressed (even though they should have been). They dismissed archaeopteryx as a one-off, a species unto itself that doesn’t prove anything. Duane Gish actually argued that the species was either a dinosaur or it was a bird, but it was not transitional – you could argue it was either because it has features of both.

This was a classic example of filling in a gap in the fossil record and just creating two more gaps. Then something wonderful happened – paleontologists hit upon several fantastic fossil layers from the time that birds evolved producing many well-preserved specimens. Over a few decades the evidence for the evolution of birds from dinosaurs became overwhelming. Scientists not only discovered a plethora of early birds, and half-birds, but they began to discover specimens showing that theropod dinosaurs had feathers.

Over the weekend I went to the Peabody Museum. They have an entire display on feathered dinosaurs, and it is impressive. The evolution of birds went from a complete mystery in the time of Darwin, to one incredible fossil species, to thousands of specimens fleshing out in detail the evolution of birds from dinosaurs. You might say that God, in a fit of whimsy, created Archaeopteryx which just happens to look like a transitional species. But it would take more than whimsy to create an entire series of species clearly documenting the evolution from theropods to birds.

Take a look at the graph above (source here for larger image). There is actually a sequence where more and more bird features appear over time, until you get to fully modern birds.

The Latest Find – Eoconfuciusornis

To give just one example of how robust and detailed this evidence is, let’s take a look at a recent publication: Exceptional preservation of soft tissue in a new specimen of Eoconfuciusornis and its biological implications. This is a specimen from the beginning of the bird branch, which is still primitive but shows clear adaptations for flight. Because there is good preservation of soft tissue scientists can look at some specific features, specifically the skin folds or patagium.

Birds evolved skin flaps in addition to feathers to help them fly.

This fossil helps show how bird wings evolved. The propatagium (the flap of skin that connects the shoulder and wrist) and postpatagium (the flap of skin that extends off the back of the hand and ulna) evolved before the alular patagium (the flap of skin connecting the first digit to the rest of the hand), which is absent in Eoconfuciusornis.

So Eoconfuciusornis has two of the three skin flaps that aid in flight. This is what paleontologists have found over and over again – primitive birds have some, but not all, of the adaptations for flight.

This is exactly why bird evolution is such a great test case. Flight is complex and requires a host of adaptations. However, you can sort-of fly, or fly but be a terrible flyer, and this would still have an evolutionary advantage. Once a species does fly, however, then there is tremendous selective pressure to become better and better at flying. So we would expect to see early specimens that could probably just barely fly, and then over time more and more adaptations come into existence that fine tune anatomy for flight.

That is exactly what we find. There are so many flight adaptations that there are multiple opportunities to see specific evolutionary changes over time. Now that we have a robust fossil record of feathered dinosaurs and early birds  this is exactly what we see.

The fossil evidence for the evolution of birds from theropod dinosaurs is now overwhelming. It is also very important that all of this evidence was discovered after evolutionary theory was proposed. This evidence is therefore a stunning example of the predictive power of evolutionary theory – it predicted that exactly this kind of evidence would be discovered.

The notion that life just happened to look this way is transparently absurd. Still there are those who deny this evidence, mostly through ignorance, but occasionally through active denial. I would like the story of this stunning line of evidence to become better known by the public, for the benefit of those who are not immune to evidence.



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