S2E2: Evolution of Quadrupeds

Hello everyone! Welcome back to the Entangled Bank podcast! This week’s topic explores the evolution of quadrupeds. Nikki, Sarah, and Jamie will be discussing how quadrupeds began in the water, transitioned to land, and then returned back to the water. So turn up that volume and get ready to learn about the development of limbs!

As mentioned above, our story begins in the water. Paired fins (a fin on each side of the body) actually began as a single, pelvic pair before becoming the double paired fins we’re familiar with today. This development is a result of Hox gene expression. According to  Edwards (1989), there are two theories why paired fins evolved: structural use and functional use. After much research and observation, Edwards found that functionality (movement, feeding, defense, etc.) is the most likely reason. Fins are not the only type of aquatic limb; there are non-paired limbs as well, such as octopus and sea stars.

Some animals felt it was time to move out of the water. In fact, many fossils have been found in Greenland, including ichthyostega and acanthostega. Interestingly, the closest living relative to tetrapods is the lungfish. We know this because of the structural and skeletal adaptations that helped species move on land. There were, however, some challenges to moving on land. Limitations included having the animal evolve from being warm blooded or cold blooded. In addition, there are trade-offs to living terrestrially. There are more resources, but breathing with lungs costs more energy. The most interesting thing that evolved in animals that moved onto land was flight. Birds have unique adaptations that help them fly. Fun fact: birds are living dinosaurs!

So, moving onto land required a lot of energy. Why move back into the water? We’ve discussed the evolution of limbs, but some species have evolved to lose their limbs. This is typically seen in reptiles…slithery lil snakes. Studies have shown that certain genes stunt limb growth, thus why snakes don’t have any. There are a few theories on why snakes lost limbs, one being for burrowing, another for swimming. Some snakes - pythons and boas - are basal, meaning they still have a pelvic girdle. The reduction/loss of limbs was so those specific animals could adapt to life back in the water. Development of limbs was due to Hox gene expression, but loss of limbs interestingly is not due to Hox genes but rather selective pressures. Regardless of reason, lack of limbs make for more efficient swimming.

           

And there ya have it, listeners. Quadrupeds have had an interesting journey in and out of the water, and now you have the information to tell your friends about it. So get

up and use those limbs to spread the word of evolution!