Can we actually live on another planet, what about gravity?

I was talking with some people at work yesterday, and we were looking at a series of scale images that show the difference in size between the planets, the sun, and other stars.

One interesting thing I noticed was just how much smaller Mars is than Earth. Then, while looking at the statistics for Mars, I noticed that the gravity there is only about a 1/3rd as strong as the gravity on Earth.

So I was thinking...say in the future we actually learn how to terraform another planet. We go live there. Good for us, right?

Well, what about the gravity? Our bodies have evolved with the gravity of our Earth as a constant. In other words, it's been a constant force on our cells since the earliest days of evolution from amino acids to where we are now.

How would our bodies, or any organism really, react to an elongated state of lesser gravity? Would it cause a shift in evolution? What about someone who was born on Mars and then moved back to Earth? Would they die because their heart can't bear the strain of pumping under the increased gravity?

Seems to me that the issue of gravity might be the biggest obstacle to overcome when considering actually colonising other planets once we manage to create the wherewithal to do so.

What do you think?