From Fish to Tetrapod

The Basics

In my previous article I described the evolution of fish and how much of the features in which we take for granted today actually evolved within fish. I stopped at the evolution of lobe-finned fishes explaining that these are the ancient ancestors of the human species. This article will now detail how fish went from surviving in an aquatic environment to a terrestrial habitat. The transition occurred in an era called the early devonian period (See fig1).

From the late Silurian to the early Devonian period sea levels had been lowering. This was due to accelerated soil formation and extensive terrestrial vegetation forming stabilised banks which created stabilised banks. The creation of these banks resulted in deeper channels connecting to more shallow waters – a new microhabitat in which fish could live in. This novel environment was most likely the driving force behind the evolution of tetrapods – which are 4 limbed species used for walking. This occurs by a process known as adaptive evolution whereby a series of genetic mutations form a new feature such as limb formation and the ability to breathe in air etc. In this era oxygen level in air was also rising meaning that these shallow environments had quite a high concentration of oxygen. This increase in oxygen levels likely lead to the evolution of much larger organisms.

Timeline

Fig 1: Illustration of the timescale of the evolution of fish to human.

Why land?

Why would fish want to undergo such a stark change in environment? There are many possible reasons including a reduced competition for food (insects and plants), a reduced risk of predation and an increased chance of survival of their young giving them a reproductive advantage. Not only this but walking is less energy expensive than swimming. All of these factors probably contributed to why fish adapted to such an environment and not only survived but thrived. It wasn’t just one species of fish at one specific location that evolved to walk on land but many across the globe with fossils being found in locations such as Poland and North China.

Adaptations needed to go from water to land

By studying fossils it is easy to see anatomical features in lobe-finned fish which pointed towards them being the closest relative to tetrapods – four limbed animals. Exactly how fins evolved into limbs has been under intense scrutiny by both evolutionary biologists and paleontologists for decades. Below are the adaptations that are postulated to have occurred in fish to allow land infiltration.

Fig2: Showing two paleontologists discovering tetrapod tracks in Poland.

Bone Morphology

Fish’s aquatic environment protects them from gravity. However, when they moved from water to land they needed to adapt to cope with the strain of gravity. This required massive skeletal changes allowing them to support their own weight. Lungfish (our lobe-finned ancestor) contained paired appendages such as ancient pectoral fins and more recent pelvic fins. Bone morphology adaptations first began by obtaining a humerus in the upper section of their forelimbs (pectoral fins) and a femur in the upper section of their hindlimbs. The humerus and femur are still found in humans today. The humerus developed attaching to pectoral muscle which would facilitate a push-up action meaning that the fish could support it’s own weight. The femur attached to the hip girdle allowing for a standing motion. At the time these developments where occurring underwater as the fish adapted to it’s new shallow water environment, incidentally much later on they would be fundamental to gaining the ability to walk.

Evolution occurs not only by the gain of new bones but also by the loss others. Many fish skull bones were lost during the evolution of the neck which detached the skull from the shoulders to allow much more flexibility of the neck allowing better visualisation. The evolution of wrists was also an important feature as it allowed for stable contact with the ground as well as support for the body. Ribs also attached to the iluem. Early tetrapods have been found with 6-8 digits on there fore and hind-limbs, this is a controversial fact as all modern day tetrapods have only 5 digits on all of their limbs.

BoneMorphology

Fig 3: Here shows some bone morphology developments in early tetrapods needed to walk on land that are still present in the human body today.

Respiratory alterations

Not only did the bone structure need to change but respiratory organs also needed to adapt to a terrestrial environment. As I mentioned in my previous article water is 800 times more dense than air with 20% less oxygen. There is no salt in water so this no longer needs to be controlled in as stringent of a manner as in fish. There is a giant difference in energy expenditure between ventilation in air in comparison to water, with air needing 25% less energy than water. This needs to be taken into account when making a transition between the two environments. Fish first loss their internal gills, followed by the formation of a three chambered heart with separation of blood by ventricles. This remodelling of vasculature led to the transformation of the swim bladder to the lung which allowed the metabolic rate to slow down dramatically. They also lost their scales to allow respiration through the skin. They changed their excretion products from ammonia which is extremely toxic (diluted easily in water) to urea which is less toxic but its production is energy expensive.

Air causes dryness issues. It has often been said that we are 98% water but we have had millennia to adapt this characteristic. Fish were used to living in aquatic environments that now they need to develop methods to with-hold moisture to prevent dehydration. The formation of the choana catered for this. This duct connects the external nostril to the mouth, and has another opening that allows connections to the eyes. This is a primative version of the lacrimal duct found in humans today which allows the passage of secretions to moisturise the eyes and nasal cavities.

Sensory adaptations

Sensory organs also adapted to life on land as the lateral sensory system of fish is adapted to aquatic environments allowing for the detection of pressure fluctuations in water. This sensory system is still used in amphibians but has been negated by higher vertebrates. Eye lenses also needed to adapt to the different refractive index between water and air. Not only this but fish eyes are used to being surrounded by a wet environment, tetrapods therefore needed to evolve eyelids as well as tear ducts to keep the eye in a moistened state.

Examples of Transitionary Fish->Tetrapod Species

It is important to know that not all of these adaptations came in one species but that they developed over millions of years with each novel fish containing features that were slightly better for life on land than the last. Below is a diagram which illutrates and dates some of these transitionary species.

1. Tiktaalik

A Tiktaalik fossil was found in the Canadian Arctic and dated back to 375MYA. It is commonly known as a ‘fishapod’ as it is still a fish but has many tetrapod characteristics. At first glance many of it’s features are extremely fish like with scales and fins, but on closer inspection Tiktaalik has a flattened skull with eyes on the top of it’s head. It actually appears more like a crocodile than most fish. It has a functioning neck, as stated in the bone morphology section this is an adaptation toward terrestrial living. It has ribs similar to early tetrapods. Although we know that Tiktaalik was capable of swimming and propping itself into a push up position it is doubtful that it spent much time on land. It is more likely that this species spent most of it’s time in shallow waters as it’s front fins have too limited of motion to walk in the way amphibians do.

Fig 4: This image highlights the crocodile like appearance of Tiktaalik.

2. Acanthostega

This tetrapod species was probably one of the first vertebrates to climb up onto land about 365 million years ago but is still thought to have spent most of it’s time in aquatic environments. This presumption is made due to it’s underdeveloped ribs which would be unable to withstand a land environment, as well as it’s extensive tail fin. This creature did not contain a wrist, ankle, or knee that could support the species weight on land. It’s spine was based on the notochord which would be unable to cope with it’s weight on land. It also contained a lateral sensory system indicative of an aquatic surrounding.

It’s paddle like pectoral and pelvic fins contained 8 digits in both fore and hind limbs. Tiktaalik’s was connected to an ulna and radius . Hindlimbs contained a femur connected to a tibia and fibula. It only contained a small patch of scales on it’s stomach but the rest of it’s body was completely scale free.

3. Ichthyostega

This is a direct decendant of Tiktaalik, and a critical link to the creation of humans. This early tetrapod arrived in the late Devonian era, it had many similar features to acanthostega. It had a tail fin and undifferentiated spine. It also had odd overlapping ribs, which would have protected the lungs and allowed for an increased inhalation when outside aquatic environments but would have drastically inhibited side to side movements. It had primative fore and hind limbs, that acted more like paddles than walking devices. Unlike acanthostega however ichthyostega had the ability to hold itself up aswell as having elbows with slight forward and back movements. The also contained knees which were relatively flexible but had no ankles. For more watch this video:

http://animal.discovery.com/tv-shows/other/videos/animal-armageddon-ichthyostega.htm

Tune in for my next article on how early tetrapods further evolved resulting in the emergence of amphibian species.

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