Tuesday, 9 September 2014

How Transitional Fish May Have Learned To Walk

Polypterus senegalus
How fish can learn to walk : Nature News & Comment

A fascinating piece of research published a few days ago shed some light on how transitional fish/amphibians may have learned to walk on land as they moved from an aquatic to a terrestrial existence.

The researchers from the University of Ottawa, Canada, led by Emily Standen, took juvenile bichir (Polypterus senegalus) - a freshwater fish from Africa which has a primitive lung as well as gills and so can live on land - and raised them on land for eight months. The control group was raised in water as normal.

The land-raised bichir showed not only a noticeably more sophisticated style of walking but there were marked changes in the skeleton and musculature compared to the control group too.

What this illustrates is a basic principle of biology - a developing organism is 'plastic' in that its environment changes the phenotype so the final organism is not simply an expression of the genotype. This is not at all surprising since we know how things like health, nutrition and exercise can influence the development of human children - something implied in my earlier blog about melanoma, vitamin D and rickets.

It also shows how transitional fish could have become subject to selective pressures on land even before they had left their 'normal' aquatic environment. Those best able to survive on land and which were best able to develop a musculoskeletal system which facilitated survival there would have been differentially selected by the environment, so speeding up the transition to fully terrestrial existence and the evolution of limbs.

It must be so galling being a committed creationist and having to live with such clear evidence not only for evolution but such a clear illustration such as this of how it could have happened.

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1 comment :

  1. Very interesting to read - not only for creationists. BTW: The transition from life in water to life on land was not easily attained. I can recommend some good articles describing this transition process more in detail. First an overview: http://www.learner.org/courses/essential/life/session4/closer2.html (about necessary adaptations to facilitate this transition from water to land).

    Then two articles full of intriguing details: 1) http://evolution.berkeley.edu/evolibrary/article/fishtree_09 (among topics discussed: What came first, gills or lungs?). And 2) http://evolution.berkeley.edu/evolibrary/article/evograms_04 (about the origin of tetrapods).

    By reading those three articles - together with Rosa's own here above - we also learn a lot about the phenomenon called exaptation, i.e. how shifts in the function of a trait during evolution occur (or with Rosa's words: "What this illustrates is a basic principle of biology - a developing organism is 'plastic' in that its environment changes the phenotype so the final organism is not simply an expression of the genotype.").

    Exaptation implies that a trait can evolve because it at first served one particular function, but subsequently it came to serve another. So what happened to the lungs among those fish having both gills and lungs? Well, some of them had their lungs transformed into a swimbladder - a gas-filled organ that helps the fish control its buoyancy. And conversely, both chick and human embryos go through a stage where they have slits and arches in their necks like the gill slits and gill arches of fish. (These structures are not gills and do not develop into gills in chicks and humans, but the fact that they are so similar to gill structures in fish at this point in development supports the idea that chicks and humans share a common ancestor with fish.)

    Read more about exaptation here: http://en.wikipedia.org/wiki/Exaptation .

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