Tuesday, January 14, 2014

Tiktaalik gets four-wheel drive, but doesn't get evolution out of a rut

Artist's depiction of a pair of Tiktaalik in water.
Credit: University of Chicago, Neil Shubin.  Image source.

In the evolutionary history of life on earth, the transition of vertebrates from water to land was a key step, so much that it has become a symbol of evolutionary belief (such as the legged Darwin fish).  The exact nature of that transition, however, is still being investigated by paleontologists.

Lobe-finned fish, with their large muscular fins, were once considered to be the key transitional forms between fish and amphibians.  Though they were thought extinct, living lobe-finned fish, called coelacanths, were found still living in the Indian Ocean in 1938.  Further study of these creatures showed that their muscular limbs were not used to "walk" on the sea floor, as scientists had previously hypothesized, but to maintain a constant, steady, rhythmic paddling motion in the water.

Scientists then searched elsewhere in the fossil record for links between fish and tetrapods (walking animals).  Creatures like Eusthenopteron and Panderichthys were placed just on the fish side, while Acanthostega and Ichthyostega closed the gap from the tetrapod side.  Then, in 2004, a fossil was found on Ellesmere Island in Canada that seemed to be the perfect link.  Two years later, Tiktaalik was introduced to the public as the ultimate proof that land animals evolved from out of the sea.  It had the ribs, lungs, and neck of a tetrapod, but the gills, scales, and fins of a fish, with seemingly transitional limb bones and joints.

This week, Tiktaalik is once again making science news headlines.  Recent expeditions to the site of the first Tiktaalik fossil have turned up hind limb bones, which were not included in the original description of the extinct creature.  The newly published description reveals that Tiktaalik had a relatively large pelvis, indicating strong muscle attachment.  The authors of the study claim that this enhances the view that Tiktaalik was part of the transition from water to land, and also that hind limbs were much more developed in the first amphibians than previously thought.

As the authors note, the Tiktaalik is still technically not a tetrapod.  Although the pelvis is quite a bit larger than those of other fish, it only consists of a single bone (tetrapods have three pelvic bones: the ilium, ischium, and pubis), it does not connect to the vertebrae, and there is no bony connection between the two halves of the pelvis.  These features (or lack thereof) indicate that the hind limbs of Tiktaalik could probably not support weight out of the water.  The authors suggest that the ribs near the pelvis eventually lengthened and contacted the pelvis, forming the first attached pelvic girdle that could be used to walk on land.  However, there is currently no fossil support for this explanation.

There is no doubt that Tiktaalik was a strange creature.  Though a fish, it was able to turn its head from side to side, owing to the absence of the bony plate that most fish have over their gills.  Unlike the coelacanth, Tiktaalik's eyes face upward on its head, and it appears to have been able to breath air through a set of holes behind its eyes, implying that it spent much of its time on the bed of shallow ponds or streams.  It is therefore reasonable to propose that the muscular fins were, in fact, used to "walk" along the bed, or even to traverse short distances across dry ground to other bodies of water.  If I may add my own speculation, it may be that the bony gill plate was unnecessary in Tiktaalik because it was able to anchor itself in moving water, which would flow across the gills without muscular assistance.

Air-breathing and fin-walking abilities do not necessarily imply evolutionary relationships with fish and amphibians, however, as they are present in a number of modern-day fish.  The climbing gourami, or climbing perch, is a fish found in southern Asia and Africa that breaths air while using its gills to crawl across the land in search of new ponds.  Some catfish and the infamous northern snakehead fish are capable of breathing air while scurrying across the land.  Mudskippers are possibly the most well-known fish capable of terrestrial locomotion, and yet they, along with all other examples mentioned here, are in a completely separate class of fish from the one that is thought to have produced tetrapods.

If one assumes that tetrapods must have evolved from some creature, then Tiktaalik is, admittedly, the most likely candidate for the key transitional form.  However, the mere existence of Tiktaalik does not suggest an evolutionary relationship between fish and tetrapods.  As with the climbing gourami, the catfish, the snakehead, and the mudskipper, Tiktaalik fits just fine in the creationist view as a unique creature designed with a specific set of characteristics that enable it to live in a particular environment.  Placing it on a spectrum of varying creatures does not prove that the spectrum is a linear progression.

2 comments:

  1. Great post answering my latest questions on evolutionary biology theory. Thanks Brandon!

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    1. Thanks Matt, I'm glad you find it helpful!

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