Descent with modification is a component in the theory of evolution. It is the idea that every species shared a common ancestor at some point in time, and the more recent the ancestor is, the closer related the two species will be. Our guiding question for this lab is, “Does mammalian brain structure support or refute the theory of descent with modification.” From the data I have collected, I believe that descent with modification is strongly supported by the structure of mammalian brains. To provide evidence, I observed the general aesthetics of the brain, and took measurements of key components. The data I used consisted of measurements of the overall brain size, cerebellum length, brain stem length, and the number of hemispheres that divide the brain. I collected data from ten different animal species;Humans,raccoons,gorillas, polar bears, elephants, domestic dog, guinea pig, sloth, harbor seal, and the indian flying fox(bat). All mammal brains have shared features, for example, the cerebellum and brain stem, which is something that signifies a common ancestor. However, there are significant variations in size, and in general, the shape of the brain and its different structures.

Cerebellum length:

Human
4 cm
Gorilla
2 cm
Domestic Dog
2 cm
Elephant
5 cm
Guinea Pig
1 cm
Raccoon
2 cm
Indian Flying Fox
1 cm
Polar Bear
4 cm
Sloth
2 cm
Harbor seal
3 cm
Although there is definitly a correlation between the overall size of the brain and the length of the cerebellum, humans definitely have a disproportionatly-sized cerebellum. But in general, there was no true differentiation among species, most stayed within the expected proportions.

Brain Hemispheres:

Human
2
Gorilla
2
Domestic dog
2
Elephant
4
Sloth
2
Raccoon
2
Guinea pig
2
Polar bear
2
Indian flying fox
2
Harbor seal
2

The only outlier is the elephant, but the additional 2 hemispheres is most likely due to the elephant’s enormous brain. I also found that the hemispheres for the guinea pig looked vastly different in comparison to the human brain. It was much more elongated and linear, while the human brain is much larger and rounder.

Brain stem length

Human
3 cm
Elephant
2 cm
Gorilla
1 cm
Polar bear
4 cm
Sloth
2 cm
Raccoon
2 cm
India Flying fox
1 cm
Harbor seal
3 cm
Guinea Pig
1 cm
Domestic dog
3 cm
The elephant has a surprisingly small brain stem, and the dog on the other hand, has a large one in comparison to its size. This further supports my reasoning, because although the structures appear to be similar, the predominance of the brain stem is larger in certain species that require more quick and reflexive actions in order to thrive.

Brain size

Human
10 cm
Elephant
20 cm
Gorilla
6 cm
Polar Bear
13 cm
Raccoon
6 cm
Guinea Pig
4 cm
Sloth
5 cm
Harbor seal
6 cm
Indian Flying Fox
4 cm
Domestic dog
9 cm

The elephant, of course, has the largest brain, but if it were a contest of proportions relative the rest of the body, humans have the “largest” brain. From the data collected, there are no outliers that refute the theory of descent with modifications, as all the mammals I have chosen, have the same brain structures with slight variations in proportions and aesthetics.