You can outrun most animals

You may think that as a human being you're a poor runner compared to most animals, but it turns out that we are actually some of the best runners in the animal kingdom.  You can outrun most animals -- as long as the distance is long enough.  We suck as sprinters, losing out to every squirrel and rodent -- but, according to this fascinating and persuasive (at least to this layman) article in Nature, "Endurance running and the evolution of Homo" (pdf), "humans perform remarkably well at endurance running," and running "may have been instrumental in the evolution of the human body form."

Sometimes non-triathlete friends make cracks about how my "extreme" training is sick and contrary to nature.  But if this article is to be believed, it's actually entirely "natural" for humans to run very long distances.  The authors, biologists Dennis M. Bramble and Daniel E. Lieberman, cite a variety of arguments, including a bunch of physiological and anthropological gibberish I can't follow but also these facts:

• Humans are the only primate that is capable of endurance running (ER).
• Actually few animals can even do endurance running, except for social carnivores like dogs and "migratory ungulates" like horses. (By the way, as carnivores, you have to wonder how dogs do well in endurance running without a lot of carbs?  More evidence that you don't need a high-carb diet to perform). 
• Speed: "Sustainable ER speeds of humans are surprisingly comparable to specialized mammalian cursors [ie runners] such as dogs and horses....  When galloping, species with high sustainable speeds such as dogs or horses can usually outrun humans....  However, human ER speeds are quite comparable to the preferred galloping speeds that cursors use over longer distances and times."
• Despite our primate ancestry, we somehow evolved to run distances roughly comparable to the longest distances dogs and horses can run. 
• Compared to apes, human legs contain many long, spring-like tendons that function as economical springs for storing and releasing energy as we run (but do not help much in walking). One of the most significant is the Achilles tendon.
• "Another well-developed set of springs important to human running is the longitudinal arch of the foot....  during running, the elastic structures of the plantar arch function as a spring, returning approximately 17% of the energy generated during each stance phase."  (Pity all those people who run in shoes, huh?  They're really losing out!)
• Humans have long legs relative to our body mass compared to many other animals, and it seems to many of our evolutionary ancestors.
• Since our legs are pendulums, the lighter the lower ends of our feet are, the less energy it takes to swing them.  Compared to other primates and to our fossilized ancestors, our feet are a lower proportion of total leg mass (again shoes have got to hurt here), and we resemble dogs and horses in having compact feet and short toes.
• Compared to our fossilized ancestors, humans have large surface areas in their lower (but not upper) joints, which dissipates skeletal impact forces from running.
• Compared to other primates, humans have a "greatly enlarged" gluteus maximus (aka, butt).  "The latter muscle, whose increased size is among the most distinctive of all human features, is strongly recruited in running at all speeds but not in walking."
• We also have overgrown "spinal extensor muscles that contract strongly to stabilize the trunk in running but not walking; and an elongate, narrow waist in combination with a low, wide, decoupled shoulder girdle that have an essential stabilizing function only in running."
• The "elevated shoulders and strong muscular connections to the head and neck" shared by other primates and our own evolutionary ancestors are "functionally advantageous for climbing" and don't hinder walking, but interfere with the body stabilization necessary to run on two feet.
• "The wide shoulders characteristic of Homo act to increase the counterbalancing moments generated by arm-swinging, while also permitting energy-saving reductions in forearm mass." The human forearm is 50% smaller relative to total body mass in humans than chimpanzees, which makes it a lot easier to swing our arms while running.  You'd think the stronger arms would be an evolutionary advantage for our ancestors.
• Heat dissipation: "elaboration and multiplication" of sweat glands, and reduced body hair.
• "The tendency for mouth breathing (but not panting) during strenuous activity."  Apes are typically nasal breathers.

The authors conclude with some interesting speculations on why humans may have evolved as runners, such as persistence hunting -- ie, running your prey into the ground.  (Kind of like what some of my friends do to me when we run together.)  Some native peoples like the Tarahumara of Mexico still do this.

I may still say to myself near the end of a hard run or triathlon, "what the hell are you doing this for?"  But at least I won't have to wonder whether I'm doing something freakish and unnatural.  Born to run, baby!