For years, man kind has been pushing themselves, and nature to the limit. I sat back recently, and looked at how far we have come. Back in the day of the spartan, (I could be misinformed on this) 5'9 was tall for a man, and 50 years ago, the men you trained for their roles in 300 would have been passable as world class body builders. Records are constantly being broken, and limits are forever being pushed. It wasn't even that long ago, that the 4 minute mile was an unattainable goal, but when the previous world record was broken, and a runner broke that 4 minute mark, the previous record holder bounced back, and reclaimed the title. In your opinion, how is it that men (and lets not forget women) are constantly achieving the unachievable? Is there even a limit to human potential, and we just have not reached it yet?
It should be remembered that in the more highly developed and widely practiced sports the "unachievable" is a tenth of a percentage point away. Yes, there was a new marathon world record set recently but the time wasn't shocking (2:04:26 instead of 2:04:55 and Geb largely credited environmental conditions - as well as increased training mileage - for the speed) so few outside the running world noticed. Nothing like Beamon's long jump in 1968 ...
The sports in which enormous advances are still made - high altitude climbing might be used as an example - are those where environmental conditions play a big role, and those which are so complex that figuring out how to do something takes years, or those where the psychological component has been the limiting factor, and those with relatively few participants. Track and field is not an example: anyone can participate, the history of training and preparation is well-documented, the consensus psychology is stable i.e. everyone knows X can be done, it's simply a matter of timing fitness/conditions/brain/etc.
Have limits been reached? No. But I think there's a good argument that, bio-mechanically, the human body is very close to those limits in certain endeavors.