Resemblance between us and fish

Fish have backbones and skulls just like us. All creatures including us are descended from ancient fish. The tangle of nerves inside humans is also the basic wiring found in fish. The skeleton of every animal on earth had the same patterns of bones in a limb.

The in between of swimming animals and walking animals

Early four legged animals with fins at the end of their body are called Stegas. The transition from animals with fins and animals with limbs has been a very complicated question but in that process there would have had to been a point where there was both. Ichthyostega is an animal with both limbs and fins. It’s forelimbs could push the top half of its body off the ground but the back end has paddle like hind limbs which are useful in water for swimming with but on land act as stabilizers to stop from falling over.

Environment of swimming and walking animals

These valleys that these unknown animals with fins and limbs used to be a giant flood plain filled with rivers and swamps and streams. Inside these plains filled with bodies of water was diverse life.

Embryos of people and fish

In the early life of animals they start as a single cell and divide repeatedly until a body emerges. One of the most important embryos were fish. They were one of the most important because in their embryos was the basic body plant to become our own. If you see an early fish and human embryo side by side they look almost identical. One of the similar features of fish and people are gill arches. Which in fish they turn into components of the gill apparatus. The structure in humans become portions of our lower jaw and middle ear. Many of the muscles, nerves and bones you use to talk and listen correspond to gill structure in fish.

Adaption of gonads

Fish have gonads next to our heart. Testicles in humans have to drop to a lower place because they can’t stand the head unlike cold blooded animals which can. In the early stages of life humans gonads were up over by the heart just like fish then dropped mirroring evolution. When the gonads dropped to a lower level there then creates a weak spot in all males where their guts can fall through. This leaves the male vulnerable to specific types of hernias. The body wall in a male is now weaker because of that reconfiguration.

Limb development using chickens

Take an egg of a chick and cut a hole in the top of it. Then toss the top part of the egg and you now have an exposed chick embryo. You can learn what parts of the embryo they need to form by removing little bits or moving around tissue and seeing what it does. One guy names John Saunders experimented on chick embryos. Saunders did an experiment where he took a group of cells from one side of the bud and placed it on the other side of the bud. About a week later that same chick embryo had grown a second set of digits almost identical to each other. That tiny patch of cells somehow was telling the digits where to form.

Sonic Hedgehog

Individual genes can do complex things including guiding the formation of entire body parts. One gene called “Hedgehog” in a fruit fly sends out an organizing signal. It told different cells to do things in a particular order depending on how close they were to the source of the signal. They found that the fly Hedgehog gene plays a role in chickens too. They found that the gene was active in exactly the same patch of cells identified by Saunders. They wanted to know if this gene called “Sonic Hedgehog” is the key signal for making the array of digits in the hand. In an experiment they found out that Sonic Hedgehog was the source of the signal responsible for generating the pattern of the digits. This gene also shapes the paws of mice and other animals and even our own hands.

Extra Digit

People with an extra digit (extra finger or toe) often have mutations that alter the effect of their Sonic Hedgehog gene. If we turn down the signal of Sonic Hedgehog which forms our hand into fingers fewer fingers would be made. If we were to increase the effect of it though we would get extra fingers.