How the Evolution of Fish Relates to Human

The Basics

From the plains of Africa to the rain forests of Panama, the planet has undergone global infiltration by the human species. We have adapted to survive in the hot temperatures of the equator as well as the sub-zero temperatures of the poles. This dynamic ability of the human race is the reason why there are currently over 7 billion people on the planet, and increasing by ~200,000 each day. But why is the human population soaring at such a rate? There are species much larger, stronger and faster than us but have only a fraction of the human population today. The success of the human species is not due to one distinct feature, but an accumulation of features working in synchrony to produce a species ready to not only predate but to defend itself against predation. This accumulation of only the best features over millions of years is known as evolution. The survival of the fittest to quote Mr. Darwin himself!

Fish are incredibly diverse. There are roughly around 26,000 types of fish which have adapted to survive in every nook and cranny of the ocean, from shallow water to deep sea and from fresh water to salt water. Survival in water is no easy feet, water is 800 times more dense than air with 20% less oxygen. Trout is an example often used to illustrate the sheer awesomeness of fish. The trout can remain motionless in water. This is possible by varying the amount of water in their ‘swim bladder’ which allows them to regulate their buoyancy. Fish also have they most efficient respiratory organs of any species both in and out of water – watch out Micheal Phelps! Below is a diagram which is a summary of this article, I will begin with fish ancestry, continue onto the evolution of jawless fish and finish with more recent fish types. I will refer back to this diagram throughout the article.

Fidh Evolution

 

Fig 1: A summary diagram taking you through each stage of fish evolution.

Fish Ancestry

Evolutionary biologists often use the example that humans came from monkeys. I would like to take you further back in evolutionary time and show you that many of the features in which we obtain today actually originated in fish around 400 million years ago long before the arrival of monkeys! It’s weird to think that fish are one of our most important ancient ancestors. The first fish came from a currently unknown species of protochordate (Oooooh our first big science-y word)! Below is an illustration of a chordate with some of the characteristics pointed out. As you can see they have some basic similarity to humans but lack alot of our complex organs. This is our starting point.

chordatefinished

Fig 2: Chordate characteristics

Early Fish

The first fishes were jawless creatures known as agnathan fishes. They used a method of filter feeding or sucking to obtain food and could be described as passive eaters. These fish did not contain a stomach. When they ‘suck’ in their food it travels along their pharynx where mucous is secreted to allow the food be absorbed by the intestines. Early fish also have developing eyes, touch and smell organs but no paired appendages (two fins). Although a lot of jawless fishes are now extinct there some extant examples including hagfish and lamphrey (See fig 2).


JawlessFish

Fig 3: Hagfish (left) and Lamphrey (right)

The Rise of Jawed Fish

The agnatha then evolved to have jaws and were known as gnathosomes. It is postulated by investigating the embryology of sharks that jaws evolved from gill slits with the upper gill slit forming the upper jaw and the lower gill slit forming the lower jaw. There are many types of jawed fish:

  • Carboniferous fish which are now extinct
  • Cartilaginous fish which include sharks and rays
  • Canthodians which have heavy spines on all fins
  • Bony fish which are the dominant type of jawed fish (19000 species).

1. Cartilaginous fish

I will begin with a brief description of cartilaginous fish. There are 850 types of cartilaginous fish known today. They have extremely well developed sensory organs another feature shared with humans. Their strong jaws allow them to predate on much larger species, therefore making the switch from passive feeders to active predators. They are also equipped with swimming musculature making them streamlined, flexible and fast. These types of fish have a skeleton composed of calcified cartilage, an adaptation to aid quick maneuvering during predation. Shark’s cartilaginous skeletons support their paired fins on both their pectoral and pelvis. It has been said that sharks are ferocious predators and are a threat to humans, but only a fraction of sharks actually prey on humans with the highest frequency of shark attacks located off the coast of Australia. The exact techniques sharks utilise while hunting are now known. Sharks can sense their prey from up to 1km away through their olfactory organs (Their olfactory organs are so sensitive they can detect chemicals in their environment at a level of 1 part per 10 billion). Olfactory organs are not their only method of long distance sensing, they also detect low frequency vibrations through special receptors – mechanoreceptors. At closer range, sharks switch to vision as their primary source of detection, but as their prey becomes extremely close they become difficult to see due to shark’s lateral vision. To compensate for the lack of visual at this point sharks use the bioelectic field around the fish (all animals have bioelectric fields) which are also detected by special receptors – electroreceptors.

CartilFish

Fig 5: Shark sensory organs.

Sharks have evolved to have incredibly sharp teeth. They have 2 rows of teeth, the first row is functional but the second row develops more slowly in order to replace the first row when they become worn. They contain a similar upper cavity to humans, they have an eosophagus that leads down to a J-shaped stomach. They also have a liver, a pancreas and a straight intestinal tract. Male sharks contain an anal fin which is used for copulation. After fertilisation the female produces goo-ey yolky eggs which contain tendrils that will cling onto the first thing they make contact with.

2. Bony Fish

The second type of gnathosome are the bony fish. These are the biggest group of gnathosome fishes, containing over 96% of all fish. They are characterised by their skeleton composed of bone which replaces cartilage during development. There are two types of gnathosome: ray finned and lobe finned. I will discuss lobe finned fishes in more depth as they are suspected to be the ancestor of the first four legged animals, amphibians. They have paired lobe fins that move in an alternate fashion – it is quite similar to a horse trotting. Lobe finned fish have other interesting features such as a hinge for it’s jaw so that it can predate on much larger fish. It also has a notochord that acts as a backbone, a tri-partitie brain (fore-brain, mid-brain, hind-brain), kidneys, a heart with increased vasculature and with a set of much more developed sensory organs.

BonyFish

Fig 6: A comparison of the coelacanth fin to the ichthyostega an early genus of amphibian in the upper Devonian period. Great illustration of fish evolving to tetrapod.

Lobe-finned fish are very rare, two types include lungfish and coelacanth. Above is a diagram of a coelacanth with lobed fins which will eventually (over millions of years) evolve into limbs which will be used on land. There are many hypothesis as to why fish left the water for a land environment, the leading hypothesis is that fish which lived in shallow water were subject to drought conditions. Throughout these droughts the fish which favored dry environments propagated. Another hypothesis includes some fish were simply driven out by bigger fish. They then looked to land for sources of energy through plants and insects.

In conclusion, about 450 million years ago the adaptive evolution of fish shaped the human race as we know it today. Through the development of an anterior/posterior body plan and notochord in chordates, the development of sensory organs in agnatha, stomach in cartiginous fish, kidney, nervous system, circulatory system, bone, hind limbs and fore limbs and many more in gnathosomes.

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