In this blog post, we will explain the difference in regeneration ability between lizards and dogs from a scientific and evolutionary perspective in an easy and interesting way.
When a lizard is in danger of being eaten by a predator, it cuts off its own tail and runs away with the cut tail as bait. So, does the lizard that lost its tail continue to live without a tail? No, it does not. The tail of a lizard grows back to its original shape even after it is cut off. On the other hand, let’s take a look at a dog. The legs of a dog that has undergone an accident that resulted in the amputation of its legs will not grow back. Once amputated, not only the legs but also other body parts, such as the tail, will only heal the wound at the amputation site, but will not be restored to its original shape. These two cases show that the reactions of living organisms to the loss of a part of their body are different. There are many ways in which organisms respond to changes in their bodies. Let’s take a look at those ways.
First, some organisms respond by regenerating a part of their body that has been lost due to trauma. Let’s take a closer look at the process of the lizard’s tail regeneration mentioned earlier. The lizard cuts off its tail in a dangerous situation. At this point, if the tail is cut off, the nerve cells immediately send a signal to the brain to recognize that the tail has been cut off. When the brain recognizes this, it concentrates a signaling substance called fibroblast growth factor (FGF) near the severed tail. FGF helps the cells in the vicinity to differentiate, that is, to turn into cells or tissues that perform a different role than the one they were performing before the tail was cut off. The cells that undergo this process of reverse differentiation then turn into gemmae. Gemmae have the ability to differentiate into cells similar to stem cells, which then re-differentiate into tail cells, and a new tail is formed through this process. In short, the cells near the tail are affected by FGF and undergo reverse differentiation to become gemmae, which then differentiate into tail cells to form a new tail. However, the lizard’s ability to recover is limited. This is because the new tail is composed only of cartilage and has no bones. Although it looks the same, its composition is not exactly the same as the previous tail. Also, the lizard can only recover its tail, and it has no ability to recover other parts of its body.
Amphibians, such as salamanders, are better at repairing damaged body parts than reptiles, such as lizards. In salamanders, too, FGF is secreted like in lizards to form cartilage and regeneration occurs. However, unlike lizards, salamanders can regenerate even the components of their tails, including bones and muscles. Not only that, but they can even regenerate their legs and even their eyes. The secret to salamanders’ regeneration is hidden in the expression of the ERK gene. The ERK gene is responsible for instructing cells in the damaged area to proliferate. In salamanders, this gene is constantly expressed, which allows them to regenerate.
On the other hand, there are also organisms whose severed body parts do not grow back and remain severed. This is mainly observed in mammals. The aforementioned ERK gene exists in dogs and humans, but does not cause the phenomenon of body regeneration in these organisms. The reason for this is related to the persistence of gene expression. In salamanders, this gene is expressed continuously until the body is fully restored. However, in humans and other mammals, the duration is only about four hours, which is far too short for the body parts to grow back.
Finally, in some cases, a body part that is cut off from one individual transforms into another individual, resulting in two individuals. This is a type of asexual reproduction in which an individual duplicates itself to reproduce, and this reaction can be observed in starfish and planaria. In the case of starfish, this asexual reproduction occurs when the central part called the pyloric stomach is cut off. In the case of Planaria, stem cells account for 15% to 20% of the body. This is 45 times higher than that of humans. Both species have body parts that can differentiate into various body tissues. In addition, the high proportion of body parts that can differentiate allows new individuals to be created beyond the level of regeneration of a part of the body.
Why do different organisms have different regenerative abilities for lost body parts? Evolutionists interpret this from an evolutionary perspective. They argue that the probability of survival varies depending on how each species responds to the cut. As a result, they say that the direction of evolution has probably been towards developing a response that increases the probability of survival. Therefore, they speculate that the duration of expression of the aforementioned ERK gene may have varied depending on the response. In other words, in the case of salamanders, when their body is severed, it is better for them to fully recover than to remain missing a part of their body, even if it takes some time. That is why salamanders have evolved to have a longer ERK expression time. In the case of mammals, it can be said that they have evolved to have a shorter ERK expression time because living with a severed body is more likely to increase their survival rate than spending energy to repair their body. In addition, in the case of organisms that are at a higher stage of evolution, such as mammals, a control mechanism is activated to prevent cells from deviating from their roles. As they evolved, cells were designed to perform various bodily functions accurately as they grew in size and developed the roles of each organ. Since life activities, the sum of the roles of each cell or tissue, can only be carried out smoothly if each cell or tissue performs its role, they evolved to suppress signals that try to move out of their position. As mentioned earlier, the expression of the ERK gene is suppressed. All cells have the same genes, but whether or not a particular gene is expressed depends on where it is located and what role it plays in the cell. This expression control is applied more strictly to organisms with more complex life activities, so it can be interpreted that differences in regeneration ability appear.
When an organism is injured, it can respond in one of three ways. It can regenerate the missing part to restore its original form, it can live with the missing part, or it can maintain its individual or species by having the missing part become another individual. As higher organisms evolve, the principles of life maintenance become more detailed and complex, and this evolution leads to the differences in the reactions mentioned earlier. For regeneration, the cells around the lost part must be able to turn into the lost part. However, the more advanced the organism, the more systematic the system is, making it difficult for already differentiated cells to change in another direction. Therefore, relatively lower organisms such as starfish and planaria can completely regenerate when they are cut and give birth to new individuals. Higher organisms such as lizards and salamanders can regenerate parts of their bodies. Regeneration is almost impossible for mammals, which are higher organisms than lizards and salamanders.
Currently, research is being actively conducted on the principles and mechanisms of regeneration in various organisms. Recently, there is growing expectation that this research will not only expand biological knowledge, but also be applied as a treatment to overcome incurable and intractable diseases. Using the principles of body regeneration in lizards, salamanders, and planarians, it will be possible to find a way to treat people who have difficulty recovering from surgery or who are disabled.
