This is because human have a much larger surface area.
Small organisms have a small surface area and thus simple diffusion meets their transport requirements.
As the size of the organisms increases, its surface area to volume ratio decreases. This means it would take days for molecules to diffuse through a large animal, so humans have a circulatory system with a pump to transport materials quickly around their bodies.
Therefore due to their large size, humans require a specialized transport system.
Does Natural selection or evolution involve with this? it sounds like spiders and other Insects or bugs that have evolved to be able to have limbs in order to survive in a environments that would require extra limbs. probably from genes being passed on and also being mutated positively in order for the newer species to better adapt. the pair of legs could have been increased because of the environment that requires it and also as the mating changes it's basically survival of the fittest.
Stem cells begin to transform into different types of cells in the human body in a process known as cell differentiation.
Cellular differentiation occurs throughout the cell development of a multi-cellular organism. It occurs when the cell changes from a simply zygote into a complex system of tissues and cell types.
Stems cells are cells which have the potential to develop into many different cell types in the body during early life and growth. Stem cells serve as internal repair system in many tissues. When stem cells divide while undergoing cell differentiation, it can either retain being a stem cell or become another type of cell like muscle cell, brain cell, or red blood cell.
