Tuesday, August 25, 2009

Elbow Joint from an Evolutionary Perspective

It is such a pity that I have been ignoring this blog, but that was for good reasons.

(By the way, you can skip this part and go to "Elbow joint from an evolutionary perspective" if you are not interested in what I've done since my last post :)

First, I passed my preliminary exam, and was told by my dissertation committee that I could graduate in a year or so, if I am done with the experiments I proposed. (Which is not gonna happen and we all know that, but oh well, wishful thinking is what keeps a grad student rolling).

Second, I've been to Turkey, visited my husband's village where his dad was born. This was my first time and I am utterly in love with the place. You can check out the two videos I shot and edited here and here, and the pictures my husband took are here.

Finally, I have been really busy with the volunteer project I am involved in: The Hard-workers for Evolution. Relatively good things are happening in Turkey, the press is covering more evolution related stories, there has been TV discussions (even though dominated by creationists, it is a start because now we finally see the Turkish academics standing up for evolutionary theory). As a part of this popularity -thanks to Darwin 200- a philosophy magazine asked our group to contribute articles on evolution for their Fall issue, which will have evolution as special focus. I am not yet sure whether the article I have been writing with a friend will be published (I'll send it and it is up to the editor) but I have been reading lots of great books and have learned a lot while writing it (mostly on the history of evolutionary theory(ies), evolutionary psychology (EP), feminist views on EP, Dan Dennett's articles, memetics etc...) I hope I'll end up writing on this stuff here soon.

After this long apologetic intro, the real thing: While writing my grant proposal for the preliminary exam, I read a lot on evolution of the limbs and skeletal elements. All that reading (I am talking tens of papers here) turned out to be a paragraph in the grant. So I decided to share that favorite paragraph of mine (with slight changes) here:


Elbow Joint from an Evolutionary Perspective

Paleontological information we currently have about paired appendages reveal the main steps of limb evolution (figure on the left, adapted from Shubin et al., 2009). First, a taxon called Sarcopterygii (lobe-finned fish) arose 418 million years ago (MYA) and these fish had the main three bones amniotes have today in their fins: humerus, radius and ulna. Consequently, they possessed the structure that would evolve into the elbow joint in the amniotes. Evolution of wrist joint began 400 million years ago with the pectoral fins in primitive fish. The first example of an organism with functional wrist joints is Tiktaalik roseae (Shubin et al., 2006). The paleontological analysis of T. roseae revealed that it was one of the tetrapod ancestors who used its forelimbs to crawl out of water. As a result, the arising of organisms with a wrist dates back to 375 MYA. However, these animals still did not have an autopod ("hand") with digits ("fingers"), which is the significant evolutionary novelty in tetrapods that makes them different from their fish-like ancestors. Acanthostega gunnari is the earliest fossil record that has digits (365 MYA). In all these taxa mentioned so far the humerus, radius and ulna or their homologs (i.e. an elbow or its homolog) are present. What differentiates between these taxa is the presence or absence of wrist and digit joints. In consequence, there is around 55 million years time difference between the evolution of elbow joint and joints of the autopod. Interestingly, the gain-of-function and loss-of-function studies on tetrapod limb and joint development reveal different phenotypes for the elbow joint and joints of autopod (phalangeal -finger- joints). For example, GDF-5 knock-out and Indian hedgehog knock-out mice have fused phalanges but the elbow joint does not show any abnormalities (Storm et al., 1999; Koyoma et al., 2007). Both of these genes are expressed at elbow and phalangeal joints, but knocking them out affects only phalanges. These differences imply that different types of joints in the limbs might be developing through different mechanisms, which most likely are a result of cumulative adaptive evolution. Also, most research on joint development has focused on phalangeal joints rather than larger joints of the limb. Considering these evolutionary differences, it will be important to understand how these larger joints develop.


References:

Koyama, E., Ochiai, T., Rountree, R. B., Kingsley, D. M., Enomoto-Iwamoto, M., Iwamoto, M., et al. (2007). Synovial joint formation during mouse limb skeletogenesis: roles of Indian hedgehog signaling. Annals of the New York Academy of Sciences, 1116, 100-12.

Shubin, N. H., & Alberch, P. (1986). A morphogenetic approach to the origin and basic organization of the tetrapod limb. Evol. Biol., 20, 319-387.

Shubin, N. H., Daeschler, E. B., & Jenkins, F. a. (2006). The pectoral fin of Tiktaalik roseae and the origin of the tetrapod limb. Nature, 440(7085), 764-71.

Shubin, N., Tabin, C., & Carroll, S. (2009). Deep homology and the origins of evolutionary novelty. Nature, 457(7231), 818-23.

Storm, E. E., & Kingsley, D. M. (1999). GDF5 Coordinates Bone and Joint Formation during Digit Development. Developmental Biology, 27, 11-27.

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