It would have had one sort of similar, but we would have still needed to adjust it to live properly, and we would undoubtedly need suits or some sort of shielding to live on mars, even during that time. The issue with mars is that there was never a global magnetic field. This is a problem because our friendly neighborhood Sun likes to constantly give off charged particles. This is whats called solar wind. On Earth this isn't that big of a deal, because we have a magnetic field which redirects these particles (some of which are absorbed by the atmosphere, or what you may know as the northern lights.) But even on Earth, when Sun hiccups (solar flares) sometimes with a burst of these particles and we have to shutdown our satellites to save their electronics. A solar flare would have the potential to kill us if we were on Mars, and solar wind would be extremely detrimental to life, imagine trying to raise a baby inside the Chernobyl reactor, it won't work. This is also why Mars looks like it does today. Solar wind literally stripped away the atmosphere, making it extremely thin and primarily composed of CO2 because CO2 is heavier than oxygen and water. The oceans boiled off a few billion years ago. Similar to this is Venus, which is more like Earth than Mars is. Solar wind stripped its atmosphere of light elements, but since Venus is larger than Mars, it retained most of its heavier elements, which caused a thermal runaway effect. Too much CO2 in the atmosphere couldn't go through the CO2 cycle, and so the oceans boiled off. Now its the hottest planet in the solar system. rains sulfuric acid, and we can't see the surface from space, as opposed to Mars which is extremely cold and boring.