How and Why did Fish Start Walking? A Short Evolutionary History of Limbs.

So, let’s say that you are a fish.  You have gills which work amazingly well for taking oxygen out of the water and helping you not die, and you have fins which help you swim around and avoid predators, also helping you not die.  Your body is streamlined and quite good at cruising through the water and you have paired fins at your head end and tail end which help you change direction quite easily.  These are amazing adaptations which came about through millions of years of evolution, and they are still very helpful to those critters that didn't head off down the path to living on land. 

If we travel back in time to the end of the Devonian period (around 385 million years ago) we would find that there were two types of fish.  One group, the ray finned fish, was very popular and had thin, beautiful fins which were very helpful for swimming in the open water.  They are still the most common type of fish.  The second, much smaller group, were called lobe finned fish, and they spent most of their time hanging out on the floor of lakes, rivers, and streams rather than cruising open bodies of water.  Despite being fish, these lobe finned critters had adapted to a life of bottom feeding and were not nearly as good at hunting or evading predators in the open water.  Their bodies were less streamlined and more flattened because flat is good if you are a bottom feeder.  The lower your profile the less likely you are to be eaten, and being eaten is never good.  

Eusthenopteron (right), a critter that lived about 385 million years ago, is a great example of this flat, lobe-finned critter.  It had a low profile and tended to spend all its time hanging out in shallow seas.  Its fins were still somewhat adapted to swimming, but it is currently thought that it had the ability to "crawl" along the underwater rocks.  The bones of its fins are the blueprint for the bones of your arms and legs and are thought to be the start of the evolution of all tetrapods, or 4 limbed animals.  You can see the pattern in the picture at the top of this post with one bone (pink) attaching at the shoulder of the fish, 2 bones (blue and yellow) attaching to the pink one, and lots of little bones below those two which would be similar to the fingers of later tetrapods. 

Tiktaalik (left), discovered by a team from the University of Chicago led by Neil Shubin, is a 375 million year old critter which is even more amphibian-like.  Shubin sometimes jokingly refers to it as a "fishapod" because it still has characteristics of fish but is well on its way to having 4 limbs like a tetrapod.  It is a great example of a transitional fossil.  It is more flattened than Eusthenopteron and spent most of its time in the shallow rivers during the late Devonian period.  The bones of its limbs have evolved to become more arm- and leg-like, but the main difference between it and its other, more fishy relatives, is that it had the ability to rotate the lower bones in the fin like a wrist.  And, based on the fact that it has a sturdy shoulder structure, this critter definitely used these limbs to support its weight.  Another aspect of this fishapod is that it could move its head from side to side and up and down, something that fish cannot do but is a characteristic of tetrapods.

Following on the heels of Tiktaalik comes another of the transitional critters that helped pave the way for tetrapods to take over land.  Acanthostega (right) lived about 365 million years ago and was, most would agree, more salamander-like than fish.  It had definite limbs - no fins for this one - but due to the structure of the bones it was unable to put any major weight onto those limbs.  It was still, like Tiktaalik, mostly aquatic, but the size and strength of the shoulder bones and the fact that its pelvic (hip) bones were attached to the spine instead of free floating suggest that it relied entirely on these limbs for movement.  Baically, it "walked" along the bottoms of the rivers and streams, only occasionally venturing out of the water. 

Around 360 million years ago, Ichthyostega (left), came around.  At a little over 4 feet in length, this is one of the largest of the transitional critters we are going to discuss.  It was one of the first of the transitional tetrapods to be discovered and for quite a while it was the only transitional tetrapod we had.  It has a fishy tail but everything else about it is amphibian.  It had lost its gills, and its tail fin is significantly smaller than would be necessary for swimming.  The bones in the front and rear limbs show the "one bone, two bones, little bones, fingers" pattern that is characteristic of all tetrapods, and its shoulder, pelvis, and backbone are so robust that scientists agree that this thing definitely spent quite a bit of time on land.  It most likely only used its front two limbs to walk around.  It is still not considered an amphibian, but it is the most amphibian-like of all the transitional critters we have seen so far.

Eventually, around the same time Ichthyostega was cruising around, the amphibians - the first true tetrapods - evolved.  

Based on the evidence, it is obvious that all land based animals with 4 limbs started out in this manner.  We, along with dogs, cats, squirrels, whales (yes, whales), dinosaurs, birds and other animals with 4 limbs owe our existence to a group of critters that, for some reason, left the water and invaded land.  Why would this happen?  Wasn't the water full of things to eat and places to live and other fishy things to mate with?  Yes, it was - that isn't the problem.  The real problem is that, from time to time, the "things to eat" category included fish that would rather not be eaten.  Predators were everywhere and some of them, like the armored Dunkleosteus (right), were massive and not terribly picky about what they ate.  

Imagine that you are a small lobe-finned fish living about 380 million years ago.  Even though your large fins are constantly mocked by the other, more svelt and popular ray-finned fish in the area, you are perfectly content living near the edge of the water, crawling around on the rocks, searching for food.  You come across a tiny shrimp and are about to dig in to what will most likely be the best meal of your young life when out of the corner of your eye you see a shadow.  This shadow might be nothing, a piece of floating wood or one of those ray-finned meanies, but it might be one of those massive predators that the Devonian is so chock full of.  You decide not to chance it and run away, but since you are so small compared to the predator you won't get far before your latest meal becomes your last meal and you are killed and eaten in some horrible way.  What does a small lobe-finned fish do?  The only thing that you can do - scamper into shallower water to avoid this predator.  

You scoot to your left to begin the hopefully life-saving journey away from the predator, but NO!  It has seen you!  Crawl little lobe-fin, crawl!  So you put on a little burst of speed and pull out your trump card.  Your stronger fins - the ones the other fish make fun of  because they are so large and weird - allow you to leave the water to avoid being lunch.  You scamper up out of the shallow water and onto land just as the predator makes its move, and due to the fact that you are no longer in the water the predator makes a quick change of direction and eats one of the ray-finned fish that were unable to get out of the way.  You perch on the rock for a second to gloat and make sure the predator is gone and then dive back in, ready to resume your search for a little shrimpy snack.

This predator avoidance strategy is one of the more likely reasons for tetrapod evolution.  If you are not in the water you are not going to be eaten by anything in there.  As it turns out, until amphibians evolved and truly started hanging out on land, most land animals were more like millipedes and other insects and therefore were not really something the first tetrapods needed to worry about.  At this time, land was the safest place to be if you could get there.

This is just part of the story of how our ancestors got up and changed the world.  I'll do a post on lung evolution in the future to help round out the major transformations needed for life on land.