Evolution just keeps surprising us.  Scientists just recently believed they have proven that ancient fish carried the sequences or ”switches” – the instructions if you will that govern the timing and development of certain gene activity – that would tell their later descendants how to grow limbs or digits.

This mind-blowing discovery was engineered by scientists at the Department of Organismal Biology and Anatomy at the University of Chicago. They successfully transplanted the genetic switch from a fish into a mouse embryo.  This DNA material from fish actually activated the limb generating regions in mice, suggesting that species that are in evolutionary terms 400 million years apart still retain some connection.  In other words, the trigger for generating limbs still exists in fish.

If I’ve piqued your curiosity, let’s figure out how this worked.  It seems that the idea came to the scientists with the famous 2004 discovery in the Canadian arctic of a what’s called a transitional fossil – a hybrid if you will of a fish with fins just starting to develop a skeleton to crawl out of the ooze and develop into a tetrapod (four legged creature).  The fossil was called Tiktaalik.

It was the similarities of Tiktaalik as a bridge between fish and limbed, land-dwelling creatures that led scientists to want to find out how the switch developed. Tiktaalik’s fins contained the skeletal structure so similar to those of early land dwellers, so just how similar were they?  So they started a study to determine the similarity of the shared physical and genetic traits of each animal.

It was previously thought the switch for limb development was only present in evolved land dwellers.  But now, armed with knowledge there WAS some genetic material shared by a creature that straddled the leap from swimming to walking the earth, scientists posited that even the predecessor to the hybrid might have contained something in its DNA to stimulate the switch.  They were right.

The scientists isolated a switch region called CsB; this is an area known to stimulate limb development in rats, mice, frogs and…wait for it… humans.  It is also shared with two fish species - the skate and the zebrafish.  This entire grouping is important because their last group of shared ancestors existed before the four-legged “tetrapods” took their first steps.

The scientists dug deeper and found that sequences were shared between the fish and the tetrapod species.  They took CsB switches from the fish and used them to stimulate limb growth in the tetrapods of the group – in this case, mice.  What’s even more amazing is they took CsB from mice and used it to generate the beginnings of skeletal limbs in the fish! However, when it was tried with other species of fish, the experiment didn’t work; just the ones with our shared CsB switches.

So that’s how reaching way back on the evolutionary tree, we might find the wee beginnings of our arms and legs in the genetic code of the Tiktaalik that crawled from the ooze.  Who said evolution didn’t have a leg to stand on?

^{+++++++++++++++}

^Sources:

^{http://www.astrobio.net/pressrelease/4087/ancient-fish-gave-%20us-a-hand}

^{http://www.eurekalert.org/pub_releases/2011-07/uocm-gsf070611.php}