One explanation for the gigantic scale of pre-historic life
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Imagine that you are designing a large box shaped structure with four legs at each corner. The structure's legs have a maximum stress level that they can withstand before they break. If you are a good designer you will make the optimum use of your materials and design the legs so they are as thin as possible. This is the optimum form, and the maximum size, for that particular design.
Now imagine that the proportions of this optimum form of design are maintained but it is now placed somewhere with a gravity only one half of the earth (0.5g). Since the large box shape must remain in proportion to the legs it results in a reduction in the stress within the legs. This is due to the lower gravity reducing the weight of the structure.
In order to use the maximum leg stress available, and keep the proportions the same, the whole structure can be increased in size until the maximum leg stress has been reached. This is the maximum size for that particular design in a reduced gravity. For a gravity of 0.5g the structure could be twice as big.
Now apply this knowledge of structures to animals. Large animals are structures which have evolved the optimum form to achieve their maximum size. They are structures which are limited in size by gravity and will vary in size as gravity varies.
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A mathematical examination of the maximum size of life
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