This question originally appeared on Quora: Why did we evolve to have such weak vulnerable bodies when you would think evolution would have given us better body armor and protection for organs? Answer by Adriana Heguy, Genomics researcher.
First of all, “why” questions regarding the evolution of any trait, behavior, etc. of any organism are difficult questions to answer, in most cases impossible, as we do not know what conditions were present when the trait evolved, or was selected against. We also do not know the genetic background of the organism at the time the trait was favored. One can generally answer this type of questions with a generic but not terribly useful observation that of something evolved, or did not evolve, it’s because it was not adaptive, or at the bare minimum, it was not neutral or a spandrel.
But the main problem with this type of questions is that many people mistake evolution for design. Evolution does not design organisms. Evolution does not have a big table where it draws organisms like Ninja Turtles or Marvel comics characters. Evolution molds organisms based on structures and features that are already there. In the case of humans, we are primates and there isn’t a single primate that is armored. There are a few armored mammals belonging to the Order Edentata, such as armadillos. And their armor is protective only to a certain degree, as anybody who has seen a dog catch an armadillo can attest (I had the misfortune of seeing a Doberman dispatch an armadillo in seconds, some decades back, in my home country).
Given that we are primates and that we evolved from either arboreal or savannah type of primates (this point is not exactly clear yet, we need more fossils), it really does not make sense to evolve thick skin. It made sense to evolve to have sweat glands and to be nimble and capable of climbing trees, walking or running long distances. We took an evolutionary path that favored hand dexterity, a big brain to support hand-eye coordination and throwing, and to support sophisticated communication to thrive in complex, cooperative social environments. We may have soft bellies, but we are not a weak animal. We are an agile, intelligent social mammal. We are the most deadly predator of all mammals, and we are also opportunistic and flexible, and adapted to life from the tropics to the Poles. From an evolutionary point of view, we are being very successful—much more than any armored mammal I can think of.
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