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.  

You are Awesome. A Quick Evolutionary History of Your Accomplishements.

You are a winner.  I'm not just saying that to garner your affection, although I am not against that, but rather because it is the truth.  You are a winner because, to begin with your most recent amazing achievement, as a sperm you beat out an estimated 300,000,000 other sperm for the coveted title of being you.  This race was long and arduous and, while other sperm made fateful errors and turned the wrong direction at the fallopian tubes or were ensnared by the vaginal mucous before even reaching the uterus, you persevered and ended up fertilizing that egg.  So you got that going for you, which is nice.

Additionally, even before you won the sperm race, you were descended from winners.  Your direct ancestors were all strong enough and fast enough to be able to survive until it was time to mate, and when it was business time they were attractive enough to actually get a mate.  Had even one of your ancestors been a bit of a dud with less than adequate skill in the survival department you wouldn't be here now.  You are the product of over 3.8 billion years of evolution.

But this planet of ours, where that evolutionary process happened and continues to happen, seemed, at times, to be downright antagonistic to the squishy things that were attempting to flourish on it.  From volcanos belching lava and tiny bacteria spewing forth nasty toxic gasses to meteors impacting the surface and making life significantly more interesting than it already had been, there have been times when it was incredibly difficult to be a living thing.

Life, and therefore your ancestors, almost come to an abrupt end on five different occasions in the past.  It was during these Mass Extinction events that your ancestors survival skills were really put to the test.  The first of these occurred about 450 million years ago and wiped out 60-70% of all species.  The second, around 370 million years ago, resulted in the deaths of about 70% of all species.  The third, the Permian extinction event, is the subject of the rest of this post and will be discussed in detail in a bit.  The fourth occurred around 200 million years ago and wiped out 70-75% of all species including many of the competitors of the dinosaurs, helping them rule the land for another 140 million years or so.  The fifth happened about 65 million years ago and is the one most people know about.  It was responsible for not only the deaths of all of the non-avian (not birdlike) dinosaurs but also the extinctions of around 75% of all species.

The third event, the Permian extinction, occurred around 250 million years ago and is known as the Great Dying due to the fact that almost 95% of all species went extinct. Your great-great-great (x140 million or so) grandparents, the reptiles, had evolved only about 70 million years prior and were attempting to make a go of it on this crazy death planet.  The reptiles almost immediately split into two main types, the synapsids and the diapsids.  These were sort of like sibling groups in that while they were similar in a lot of ways, there were also some key differences, the most obvious of which, assuming you have access to these things, is the number of skull holes (or, should you wish to impress people at a tea party, temporal fenestra) each had.  As in all sibling rivalries there was some competition, and there was a winner and a loser.  The winner, at first, were the synapsid reptiles which grew to fairly decent sizes and were essentially the kings and queens of the Permian period.  Some, like Dimetrodon, could reach 15 feet in length with a massive sail-like crest on its back that allowed it to regulate its body temperature.  These huge reptiles are often mistaken for dinosaurs because of the large size and scary teeth, but they predated dinosaurs by around 70 million years.

This sibling rivalry lasted, with the synapsids in the ascendency, until the Permian Extinction event.  The synapsids, and indeed almost everything else, were basically wiped out during this time, and this opened up the door to their rivals, the diapsids.  The diapsids had bided their time while their synapsid relatives dominated, staying small and inconspicuous.  After the Great Dying the diapsid reptiles began their dominance, leaving the synapsids to the fate of small stature and inconspicuity, assuming that is even a word.  Spell check doesn’t have a problem with it so I am leaving it in.  

The diapsid reptiles eventually went on to evolve into such hit animals as the pterosaurs, crocodiles, dinosaurs and, eventually, birds.  The synapsids hung back and began the process of evolving to survive.  They stayed small and adapted to the cooler, darker night conditions.  They developed hair to insulate themselves and assist in temperature regulation.  They developed more refined jaw bones and modified ear bones to aid in hearing.  They began to produce milk from glands on their stomach to feed their young, and when the time was right, roughly 65 million years ago after the Cretaceous mass extinction event, evolved into all of the groups of mammals that we have today.  Dogs, cats, platypuses, squirrels, elephants, whales, primates and eventually you - all of them descended from that one group of reptiles that refused to die when almost everything else did.  

And that is why you are a winner.  Your ancestors survived everything that this cranky old Earth and solar system could throw at them and came out on top.  You are #1.  

Or, to put it like one of my college biology professors:

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