This chart - a followup to my previous chart on skink toes - is a collaboration between myself and ~Agahnim for our evolution book. The image relates to an explanation for the reason why theropod dinosaurs appear to have lost their two outermost fingers, but bird embryos form and then lose the inner- and outermost fingers. If birds evolved from dinosaurs, then one would expect that the three "fingers" present in modern birds would have evolved from the three fingers of theropod dinosaurs. However, this is not the case, and this seeming inconsistency has been a common argument of both creationists and BANDits for decades.
I drew the images, and he did the formatting and wrote the following description:
To understand the effects of a frameshift, it’s helpful to imagine an absent-minded postman who accidentally gives several people the mail that was meant for their neighbors. Just as this causes a person’s mailbox to contain mail that normally would have gone in their neighbor’s mailbox, a frameshift has caused the middle two toes of the two-toed earless skink embryo to develop using the genetic code that normally would have been used by the inner two. This is a documented case in living animals where a frameshift has caused the exact same difference that exists between the hands of birds and theropods, so it is clearly possible the same thing could have happened in birds also. And in fact, there is a way to measure whether it happened or not.
If you accidentally receive your neighbor’s mail, you don’t usually need to guess at whether you did. If your neighbor subscribes to a magazine that you don’t, and you see a copy of that magazine in your own mailbox, you can usually figure out what’s happened. What if there were a way to look for the same thing in the hands of bird embryos, and examine whether the genetic code we would normally expect to see used by an embryo’s inner three fingers has ended up in birds’ middle three? Just as receiving your neighbor’s magazine is a sure sign that his mail was delivered to you, perhaps one of the fingers of bird embryos develops using a combination of genes normally reserved for a different finger, which would be a sure sign that a frameshift took place.
This is exactly what’s the case. In all animals that have five fingers as adults, the genes Hox13 and Hox12 are both expressed in the development of the outer four fingers, while the innermost finger (the finger which is the thumb on a human) develops using only Hox13 and not Hox12. If you imagine these five fingers as five neighbors, developing with Hox13 and without Hox12 is the equivalent of a magazine that’s delivered only to the innermost finger and none of the others. On bird embryos, the innermost finger does not develop into anything—only the middle three do—so ordinarily one would expect both Hox13 and Hox12 to be expressed in all three of the embryonic fingers which eventually fuse into their hands. However, what we find instead is that the second-innermost finger of bird embryos develops with Hox13 but without Hox12. To put it another way, the magazine meant for the embryo’s innermost finger has been delivered to its second-innermost finger. Even if it weren’t for the fossil evidence suggesting the ancestors of birds lost their outer two finger, this evidence from Hox genes would be enough to show that the three fingers of birds were originally the inner three, but the genetic postman has delivered their mail to the middle three embryonic fingers instead.
Diagram referenced from: Vargas, Alexander O. and John F. Fallon. “The Digits of the Wings of Birds Are 1, 2, and 3. A Review.” Journal of Experimental Zoology 304B (2005): 206–219.
Great explanation! I had to re-read the part about which Hox genes control what twice, since I wasn't familiar with that part, but for the most part I thought it was well written and made sense.
Interesting that theropod dinosaurs have the innermost three fingers, and birds have the middle three instead. I always thought it was the other way around, and I think this might be because a documentary I once watched on bird evolution explained it that way. I think that was a long time ago, though, so hopefully future documentaries aren't making that mistake!
Another reason for my own confusion, I think, stems from the modification of the human thumb into an opposable finger, which neither birds nor dinosaurs have. So to me, I guess it was easier to think of dinosaurs having the middle three fingers as digits instead. I'm glad this article cleared it up though! Many thanks.
"Even if it weren’t for the fossil evidence suggesting the ancestors of birds lost their outer two finger, this evidence from Hox genes would be enough to show that the three fingers of birds were originally the inner three, but the genetic postman has delivered their mail to the middle three embryonic fingers instead."
Please excuse my ignorance, but I'm having trouble understanding the above quote.
1stly, just to make sure I follow, you're saying that, at some point during the evolution of birds from non-avian theropods, there was a frameshift & theropods went from having Hox13-only thumbs to having Hox13-only index fingers, right?
2ndly, at what point during said evolution did the frameshift occur (I.e. Did it occur when theropods still had functional ring fingers or did it occur afterwards &, as result, theropods re-evolved ring fingers)?
1: That's right. Although since the frameshift caused the embryo's second finger to develop with the anatomy of the thumb, I'm not sure it makes sense to still call it the "index finger" at this point.
2: I don't think anyone knows exactly when the frameshift happened, since it's usually not possible to look at the genetics of fossils. There's one line of evidence suggesting it happened quite early on, though. In Limusaurus, which is a ceratosaur, there are three functional fingers but the two that are reduced are the thumb and the little finger. This suggests that in the common ancestor of ceratosaurs and birds the frameshift might have already happened.
Now that Emily has clarified what this explanation is about, are there any specific parts of it that you think are unclear? I'm asking because I'm the one who wrote this part of the book, so I care about it being as clear as possible.
OK, I'm once again a little confused. After reading your comment, I looked into Limusaurus to remind myself of what I had forgotten. In particular, I found the following, which seem to say that all tetanurans (including birds) have digits 1-3 (as opposed to 2-4). The problem is I didn't completely follow their explanations. Can you help me w/that? I'm also wondering how that fits into the book (Sorry if that's already been explained).
2: Forgot about Limusaurus. That helps clear things up. Many thanks again.
"Now that Emily has clarified what this explanation is about, are there any specific parts of it that you think are unclear? I'm asking because I'm the one who wrote this part of the book, so I care about it being as clear as possible."
1 more question: Will Limusaurus be discussed in the part of the book about frameshifting? Just making sure.
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`anmari has been spreading her infectious positivity throughout our community for over 6 years. Throughout this time Ana has been at the core of all things devious, passionately developing an eclectic gallery, helping organise devmeets, participating in chat events and also recently completed dedicating her time as a Community Volunteer. We are absolutely delighted to bestow the Deviousness Award for May 2013 to `anmari, congratulations! Read More