One interesting question is why larger species are multicellular. Why can't there be human sized single cell organism. One plausible answer is that the surface area to volume ratio is significantly larger in multicellular organisms. Surface is where all the nutrient absorption occurs. The surface is where the cell contacts with the external medium. On the otherhand, the interior of the cell is where the nutrient consumption occurs. In some sense the nutrient need is proportional to the total . So to sustain a larger cell with higher volume, more surface is needed. That's were being multicellular comes in. Take 1 ball and divide it into two balls. The area increases. Do that again for the two balls and so forth. At the end, you have the same volume but increased surface area. If balls are difficult to visualize, think about painting blocks. Suppose you have a 3x3x3 block. Paint the surface. Separate the blocks into 27 separate blocks. Now, some surfaces are not painted. This thought experiment suggests that the surface area in the separated blocks. So, by being multicellular, organisms can increase the surface to volume ratio.
Observe. Ask why. Even if it doesn't get answered immediately, it changes how you think about the world. There are lots of stupid questions... but some of the seemingly stupid question may be the most important. No risk, no gain.
ref:
I think this is something I heard in one of my lectures but written with my own spin. No solid ref.
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