Evolution of Fish to Reptiles

Scientists believe amphibians came from fish ancestors. Why do they believe this? Well, their DNA shows them to be very closely related, we have fossil transitional forms and fish and amphibians also share many traits.

Lungs

The first step to survival on land is breathing oxygen. Fish didn't decide to start breathing oxygen. Every organ comes from a prior organ that was also useful.

Lungs evolved from a fish's swim bladder. It helps fish manage buoyancy in the water. It's an oxygen sack. "Physosmous" fish gulp to gain buoyancy and burp to lose it. The bigger these bladders got, the more blood vessels and the longer they could survive in low oxygenated water giving them an advantage in reaching food sources previously unavailable to them.

These fish swam in thicker water, with further food sources just out of reach on land.  Survival in the low oxygenated water also meant they had less predators. Gradually with each generation the fish with the most oxygen absorping swim bladder/lung were more likely to survive and produce offspring.

Many fish use shallow bays to lay eggs. These bays have less predators and give the young a better chance of living longer.

Today there are fish with lungs called Lungfish.

https://youtu.be/vGj3XJjPIuU

Stronger

Early fish were then able to climb up submerged branches or embankments. Over a million years of doing this a fish one day came all the way out for a brief period. It moved poorly and tired quickly but the reward of a new food source paid off. Over a million years the fish with the strongest fins could get more food and had greater chance of survival and producing offspring.

https://youtu.be/tRDwzQ3gz8Q

Thin fish fins give less propulsion, wider fins more effective and efficient. Wider feet better for walking on soft surfaces, like snowshoes.

Walking

Some of today's fish can travel on land.

https://youtu.be/5wTQLcKNsJE
https://youtu.be/9ZdavNHGUho

https://youtu.be/N-Ie-K-361Q

https://youtu.be/qWyLtYrTLYo

https://youtu.be/apFZ1eMFOFk

https://youtu.be/PN9Rc5DrOzw

https://youtu.be/mKxRe0hAQmg

Tiktaalik

https://youtu.be/o5Z4mPQBjqA - tiktaalik

https://youtu.be/J7rN3_tSb7g

https://youtu.be/E1h4kgt2520

Mudpuppy

https://youtu.be/h13u3sZ6i-U

Muskippers


The fish spent more and more of their time out the water like modern day mudskipper who only spend 20% of their time in the water swimming.

https://youtu.be/CAQuoH_fOWM

https://youtu.be/meKZQfwW6K0

Incanthostega 

Ichthyostega is another extinct animal we only know from fossils. They had stronger legs than Tiktaalik and Mudskippers but still would've moved awkwardly on land.

https://youtu.be/GynpF2Zj128

Salamanders
Salamanders can be aquatic, semi aquatic and predominantly terrestrial. They have lungs and most can spend a lot of time out of water. Some keep gills for a few months and then use lungs for the rest of their lives. Some never lose gills and primarily use them and barely use their lungs.

https://youtu.be/nysfZS9ggqQ

https://youtu.be/MjAiWnzU2aE

Lizards

Salamanders and geckos and very often confused. 

all Geckos
Anole
skink
collared lizard
Iguanas 
Monitor lizard 
Komodo dragon

https://youtu.be/28FzV5OHqMU

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