Almost all long-distance runners have heard it at some point: "Humans are not made to run that long." These words are usually uttered by concerned family members or even physicians.
I often respond that humans are not made to do many things, like climbing Everest, diving 1000 feet, swimming in Artic waters, or trekking across the Gobi desert. Our large brains help us figure out how to do things that our body is "not meant to do".
But it turns out that at least for running long distances, we may not be going against nature. Long distance running may be an evolutionary adaption that allowed us to run our prey to death.
According to Harvard scientists, Daniel Lieberman and Campbell Rolian, humans may have evolved to run for extended lengths of time, most likely for obtaining food, and was the catalyst that forced Homo erectus to evolve from its apelike ancestors. To learn more, read this (thanks Lorinda!).
Long gone are the days when running was considered to be detrimental to our health. I even remember reading that running a marathon shortens your life by a year! Nonsense. A long time ago, if you were not able to run for a long time, you may have starved. So there, I have given you ammunition to respond to the humans-are-not-made-for-running comment.