Portrait making and photography are two different things. Portrait making belonged to the painting genre while photography was more of a capturing the moment kind of thing. However, when you evaluate the early periods you find that there are just as many pictures of landscapes as there were of portraits. This helps to stabilize the fact that photography did not evolve from portrait making.
Yes, portraits were the major type of photo that was taken in that era. However, the presence of an equal amount of landscape photos offsets this claim. Hence, we can safely say that photography did not evolve from the portrait making.
You really cannot say that photography evolved from portrait making. The reason that a lot of people believe this is that during the real early days of photography there were a lot of portraits that were taken. When you think of pictures from back during the 19th century, the first thing that comes to mind are portraits. The thing is that there were just as many pictures taken during that same time of natural landscapes and pretty much anything else from that time period.
A good example is from the time period of the American Civil War. While there were a lot of portraits of officers and soldiers, there were also a lot of photos taken of the battlefields and the wounded and dead from fighting. It would just be a hard argument to try and convince someone that photography evolved from portrait making.
While portraits were very prevelent during the early days of the camera and photography, landscape photo started around that same time. You could make a statement that photography evolved from portraits but then I believe that there would be a stronger argument to the opposite.