Human beings, along with all other vertebrates and 2 phyla of higher invertebrates, have deuterostomia ancestry. Insects, or the Arthropod subphyla Hexapoda and Myriapoda, along with every other organism in the animal kingdom have protostomia ancestry. For now, the difference isn't important. What is important is how the two groups of organisms discussed here evolved to possess the different structures they each needed in order to survive in the environments they lived in to become what they are today.
Insects chose a completely different evolutionary path than the ancestral organisms of human beings, but did so long before they were insects. This is in all likelihood why they evolved so differently compared to us.
During the late Silurian and early Devonian periods (about 400 million years ago), terrestrial environments were largely uninhabited by animals. Low-growing herbaceous plants and the first forests were beginning to flourish, and enough ozone had accumulated in the upper atmosphere to filter ultraviolet radiation from the sun. Animals with adaptations that permitted life on land had a wealth of photosynthetic production available, and unlike in marine habitats, faced little competition from other animals for resources.
However, the problems associated with terrestrial life were substantial. Support and movement outside of a watery environment were difficult on land, as were water, ion (electrolyte) and temperature regulation.
A number of factors contributed to insect dominance of terrestrial habitats during this time period. The exoskeleton of their crustacean predecessors preadapted the insects for life on land. Not only is the exoskeleton supportive, but the evolution of a waxy epicuticle enhanced the exoskeleton's water conserving properties.
This exoskeleton, however, doomed the insect to a limited size potential. It's shell-like design would not support the weight of a significantly larger organism which would need muscles and organ systems far too heavy for such a design to appropriately support.
Insects also have rather inefficient respiratory systems consisting of a highly branched system of chitin-lined tubes called tracheae. Spiracles in the outer exoskeleton prevent water loss during gas exchange through an insect's tracheae. The tracheae then branch off throughout the body and into tracheoles which exchange respiratory gases with every cell in the insect via tracheole fluid. Some insects mimic mechanical "breathing" action and a few even create a one-way diffusion vacuum that draws in more air from outside in through the tracheae.
But this type of respiratory system could never be made efficient enough to allow for a large, metabolically active organism. It simply does not exchange enough gas while at the same time, it takes up a large portion of space inside an insect's body. This makes the organ growth a significantly larger organism would require essentially impossible..
Human evolutionary predecessors are not insects, in fact, even the single celled organisms we eventually evolved from we're not the same ones that gave rise to insects. Our evolutionary path actually made its way from the seas to dry land via body support adaptions that were internal, not external like the insect's exoskeleton. From cartilage to bone, our predecessors adapted structures and formations that were much better suited for a large, heavy, metabolically active line of organisms.
Our respiratory system had evolved to meet the demands of a larger and sturdier organism before our predecessors had even left the oceans as large cartilaginous and then, bony fish, came along. As our predecessors left the oceans, they became more complex, metabolically active and at the same time, efficient. Some remained exothermic (cold-blooded) and consumed high energy and/or high protein diets in order to reach the massive sizes seen in the dinosaurs. Some remained smaller and escaped predation by taking to the skies or by becoming burrowing nocturnal hunters, scavengers, herbivores, or insectivores, warranting the need for either an endothermic or homeothermic (warm-blooded) metabolism. It is thought this is what enabled mammals and birds to fair better than the reptiles during the nuclear winter that supposedly rendered the larger dinosaurs extinct. Once this happened, mammals had the upper hand on land and were presented with the opportunity to successfully evolve into larger organisms without being noticed and consumed by the large predatory reptiles anymore.
So, its descriptive, informational, and long, I know...but now you know why humans are able to survive being the size they are today and why insects could never even come close to matching such proportions.
Source(s): Zoology/Wildlife Science Major
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