Evolution in A Centaur’s Life

Evolution in A Centaur’s Life

six limbs

As I have said before, Darwin’s Theory of Evolution is a major underpinning of modern biology and A Centaur’s Life is unique in the fact that it makes evolution a major plot point. Other anime might mention evolution as causing something, Newtypes in Gundam, or the human-like Invid in Robotech, but it is in passing and no real explanation is given or expanded on. In A Centaur’s Life evolution is used to explain how and why humans as we know them don’t exist, and the earth is populated entirely by monster girls with six limbs. The evolution of life on Earth in the real world and in A Centaur’s Life is the same until the evolution of fish.

Fish Evolution- Paired fins

evolution in a centaur's life

The first land animals evolved from fish, and legs slowly evolved from fins over a long period of time, so A Centaur’s Life is correct in starting its evolutionary change with fish. It tries to make the case that each pair of fins of a fish, (pectoral, and 2 pairs of ventral fins) each evolved into a pair of limbs for a total of six limbs. As seen in the above diagram from the manga, pectoral fins evolved first and then a mutation (gene duplication) caused the pectoral fins to duplicate twice, leading to a 6-finned fish. The manga is correct in showing a 6-finned fish, as some fish do actually have 6 fins in three sets of 2 (pectoral, pelvic, anal) as seen below.

fish fins

There are two main problems with this explanation. The first one is that if gene duplication, a mutation where an entire gene sequence is duplicated and added to an organism’s DNA, does happen, it would not lead to the duplication of an entire fin. (Think of it like adding an extra B book to a set of encyclopedias or having two copies of the same program on your hard drive.)  There are many genes involved in the development of fins/limbs and copying one gene would not cause an entire set of fins to just emerge in a new location on a fish. That being said, gene duplication is common in certain groups of fish, and it does drive the evolution of new species, so in that sense A Centaur’s Life is correct that gene duplication can drive evolution, it just stretches things a bit.

The second problem I have is the way it describes the evolution of fins, because at the current time there are 2 competing theories on how fish fins evolved, the gill arch theory, and the dermal fold theory, but each of them has one thing in common. This common thread is that the scientific evidence points to the pectoral and pelvic fins evolving at the same time. I haven’t found any information on the anal fins, which appear to have evolved after the pectoral and pelvic fins. Now I know some of you are probably thinking, ok, so you have a problem with how A Centaur’s Life described fish evolution, but they did get the main thing right and that is that is fish have 3 sets of paired fins, for a total of 6. Well, there is a little more to the evolution of fish and eventually life on land, so let’s continue.

Fin Evolution- Ray finned fish vs. Lobe finned fish

Many of you have probably noticed that the previous diagram is of a ray finned fish, and amphibians actually evolved from lobe finned fish. Ray finned fish are characterized by fins that are made of bony spines covered by skin. ray finned fish

Lobe finned fish, on the other hand, have large muscular fins, and then tend to have fewer fins than ray finned fish. There are only two groups of lobe finned fish: the coelacanth, which still have small dorsal and anal fins and are highly maneuverable, and the lungfish (see below), which have fewer fins and are less maneuverable. Now you might be thinking that this is a negative, but these fish focus on hiding from predators by living in/on the river bottom. The large tail is a positive when trying to force your way through the mud at the bottom of a body water. A larger tail would help in this movement. The large muscular fins also help in its ability to travel on land between bodies of water, as it has the ability to breathe air for a short period of time, hence the name lungfish.

lobe finned fish

You’ll notice that the dorsal and anal fins are gone, replaced by a much larger tail. While the back set of fins might look like the anal fins, they are not. The pelvic fins have slowly moved backward over many generations as seen here. These changes would be driven by the previously stated environmental factors.


Thus, the move to 4 fins/limbs happened even before amphibians evolved, with the evolution of the lobe finned fish, and specifically the lungfish, as it is an ancestral lungfish that evolved into the first amphibians.

limb evolution

This means that A Centaur’s Life explanation of evolution is wrong right from the start and busted. Don’t worry though, I’m not going to let it end here because there a few other points I would like to make. A Centaur’s Life says that having 6 limbs is easier than 4 limbs because each limb would have to carry less weight; but would having 6 limbs instead of 4 limbs really be easier on the organism? Well, I have an answer for that one as well.

Vertebrates- Six Limbs vs. Four Limbs

One point I like to make to my students is that nature is inherently lazy. Evolution favors changes that increase the organism’s ability to survive and reproduce in the simplest and easiest manner. The complex systems we have today are a result of countless small changes over millions of years. There are a number of ways I could explain why having vertebrates with six limbs wouldn’t be more advantageous over vertebrates with 4 limbs, but I’m going to use the one that makes the most sense in the widest number of situations. The reason for this is that, if you remember your biology classes, evolution selects for the traits that are the best fit for the environment each animal lives in, so what is beneficial in one environment might not be beneficial in another environment. Thus, I’m going with the one reason that would be true across a wide range of environments and that is food intake.

For example, in the human body your arms make up 10.6% of your body weight. This means that if a 200-pound individual had an extra set of arms (six limbs), those arms would weight around 21 pounds. This means the individual now weighs 221 pounds. Now let’s compare the calories needed to maintain that additional weight.

Four limbs

Age 30

Height 6’0”

Weight 200 pounds

Exercise 3-5 times a week

Calories 2,953

six limbs

Age 30

Height 6’0”

Weight 221 pounds

Exercise 3-5 times a week

Calories 3,101

Just based on this rough estimate (because calorie calculators don’t have a setting for number of arms, which would certainly change the math), our individual with six limbs needs at least an additional 150 calories a week just to survive. Do keep in mind the number is probably higher, when you consider all of the extra skin, muscle, bone, and nerves needed for an extra set of limbs. Even if you doubled the number, an extra 300 calories doesn’t seem like all that much in this day and age. However, you have to consider all of the other animals in the anime with 6 limbs that do not have our ability to grow food, plus consider the fact that humanity didn’t always have the ability to grow food during the course of our evolution. Taking this into consideration, an animal with six limbs would be at a disadvantage against its 4 limbed counterparts just based on the extra food needed to survive.

six limbs

This does not even take into account the increased complexity of the skeleton needed to support six limbs.


I can’t even begin to speculate on how the skeletal system would change to support an extra set of limbs, or how that would change the location of the internal organs like the kidneys, which sit closer to the back than other organs. Then there is the brain to consider, which would also need to adapt. The frontal lobe, which controls movement, would need to be larger to account for the extra muscles controlling six limbs. Additionally, the cerebellum, which is involved in coordination, would also be enlarged.


So our monster girls with six limbs would have larger heads to account for the increased brain size, which does not appear to be the case.

six lims

The only way an individual with six limbs would have a chance of evolving is if having the extra limbs provided a distinct advantage to surviving and reproducing over its 4-limbed counterpart. I think that nature has probably provided the answer for us since there are no known 6 limbed vertebrates alive today or found in the fossil record. So I don’t see a situation like A Centuar’s Life happening anytime soon.

6 Limbed people in the real world

Wait a minute, I thought you just said that vertebrates/humans couldn’t have six limbs. Yes, I did say that, but there is always the exception to the rule in biology and here is the exception. There are very rare but recorded cases of couples having children with six limbs. To explain this, I first have to talk about twins for a moment.

Single child- one egg cell fuses with one sperm cell

Fraternal twins- two different egg cells each fuse with a different sperm cell

Identical twins- one egg cell fuses with one sperm cell, the resulting embryo then spits in half to form 2 individuals

Conjoined/Siamese twins- one egg cell fuses with one sperm cell, the resulting embryo does not completely split into two individuals

In extremely rare cases the split is not equal and isn’t complete, leading to a parasitic twin. This twin is connected to the larger sibling and never fully develops. The results can vary widely depending on how and when the twins began to separate and when the smaller twin ceased developing. In two recorded cases this resulted in the individual being born with extra arms and legs. In one of those cases the girl was considered an incarnation of Shiva, a Hindu god with six limbs.

six limbs

In my research on parasitic twins, I ran across a genetic condition called Dipygus where the body essentially becomes confused (for a lack of a better term) during embryonic development. This results in the body growing duplicate parts and in one case this involved a woman growing an additional pelvis and set of legs. In both cases some of the extra limbs are less functional and they can impair the function of the stronger limbs. The extra limbs can be removed with surgery to return normal function to the remaining limbs.


While A Centaur’s Life’s use of evolution to explain how all of the characters with six limbs came to be is an interesting bit of world building, it just doesn’t make sense scientifically.


That being said, it really doesn’t really take away from the slice of life monster girl antics, if that is something you enjoy. I am far from done with A Centaur’s Life as I will be looking into the biology of each of the different species, so stay tuned for more. If you have any comments or questions please leave them in the comments below.






Kardong, Kenneth V. Vertebrates: Comparative Anatomy, Function, Evolution. McGraw-Hill Education, 2018.

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