For the other/second defined point of 100 °F in the temperature scale, Fahrenheit used the coldest temperature known to mankind in his day. This coldest temp was achieved by adding salt and ammonium chloride to ice. Then using this "base 10 scale" he measured the freezing temperature of pure water at sea level as being 32 °F. Remember, in the early 1700's we didn't have mechanical refrigeration and didn't know about quantum physics.
At some point they (who?) reversed the scale so that 0 °F was the coldest and human body temperature would be 100°F. At this point temperatures began to rise as things heated up. Every thing else began to cool down.
The first thermometers used alcohol and didn't have too wide of a range. When Fahrenheit switched to making mercury thermometers he was able to measure the temperature of boiling water, 212 °F. This and other technological advances lead to a "more accurate" thermometer being developed. In time the more accurate instruments read body temperature as 98.6 °F.