It hardly needs to be a difference so extreme. If the atmosphere had half the oxygen that Earth's does, Earth life would struggle badly and most species would simply fail to make a go of it - it'd be like living at 5.5km altitude. The highest human settlement on Earth is at 5km and the people there suffer from chronic health problems, it's a gold mining settlement that wouldn't have anyone living there if they couldn't earn a ton at it. But for the native life that's just Tuesday.