Biking is a whole lot easier on the body. Just look at the distances and times that both sports have @ elite level and look also at the fact that cycling has no impact on your knees (well, at least not as hard as your whole weight pressing a small point at your knee).

Interesting theory.

The human body is one of the most adaptable objects. If you slowly work towards any goal, you can get there. I imagine that anything done to excess without a slow enough evolution injuries can happen.

Moral of the story, moderation and slow integration makes for a long and injury free life.

I have no ambivalence about this.

3 hours of cycling at my best pace is exhilarating and, while exhausting, harmless, whereas,

3 hours of running leaves me shattered, looking for ice, ibuprofen and 8 hours in the sack.

Similarly, running 21k is exhausting and has given me back injuries, pulled muscles and needs several days recovery, whereas,

I commute 21k morning and night daily by bicycle.

If cycling were more harmful to the body could I commute daily?
I know I couldn't run 21k each way every day.

PoC

Humans are definitely designed to run; We had to hunt animals on foot for centuries. However there is a big difference between running barefoot on the open plains, and running in bulky trainers on asphalt and concrete.

If you run with proper form and work up slowly, you will most likely be able to avoid injury. Long distance runners often put in 100-150 miles a week with no injuries, yet many triathletes get injured when doing 25 mpw. This is due to poor biomechanics and improper form. Overall though, everyone has provided pretty good evidence that cycling is easier on the body. You can bike 2-3 times as long as you can run and it will feel relatively similar.

There are still plenty of distance runners doing over 100/week, especially marathoners in the peak of their base training. You probably won't find many 5k racers training over that, though. Hell, even for 8-10k xc races in college I'd hit 110-120 near the end of the summer. Granted this was mostly base miles with very little speed training, but it's reasonably standard even today.

Just look at what happened to Phidippides! That poor guy DIED from running a marathon. :)

I would guess there wasn't a steadily progressing training plan in place beforehand as well, lol.

As has been said many times, running is much much harder on my body than cycling. Granted, I weigh about 255 (down from 275 in January) right now, so it makes sense that all the bouncing would be hard on my joints.

I would look at the bike fit, because you should be able to bike for a long time without any kind of joint/back problems. I have a ruptured disk in my back, and I've never really had any back problems from biking.

The 1st article it the google search pulls up is very interesting. I'm convinced

I don't know about you guys, but my body is made for sprinting, not endurance (actually, it's probably made more for sitting on the couch and playing video games, but that's beside the point). I think it depends on the person.

Oh, and if you laughed when I said that my body is made for sprinting, then I don't like you.

Our bodies adapted to the environment around us...we were not designed to run fast or far....If that was true we would have stayed on four legs or stayed walking as Bonobos (our closest primate cousin) do...a modified sprint using the arms for movement and balance. We started walking erect because it allowed us to see farther and bear children more easily and use our hands to make tools.
We CAN cover great distances on foot simply by walking...sprinting to escape from something higher up on the food chain. If we were perfect in design...we would not suffer from osteoarthritis or back problems or other running related stuff.
On that note...I ran 100 plus mile weeks when I was in my twenties. (the 70's) and stopped after a few years...I know guys who continued and no longer run, not even to the mail box. Hard to run on knee replacements.
Biking isn't natural either but the impact is less...although the accidents are much more catastrophic.
Running on trails is easier....but the injuries tend to be really nasty when they do happen...
I love 'em both but am far less beat up after a long run than bike...but that's probably because I run slower than when I was a kid.
Just a few random thoughts...