How were the ghettos and the Yellow Star the first acts of dehumanization by the Nazis?

The forced concentration of Jews into ghettos and the requirement that Jews in Nazi Germany and occupied areas wear Yellow Stars were acts of dehumanization, but were not the first acts of dehumanization in Nazi Germany. The Nuremberg Laws of 1935 had already stripped Jews of their German citizenship, essentially making them subjects of the state, and had decreed that Jews could not marry or have sexual relationships with non-Jews. These laws were informed by the Nazis' belief in eugenics and in the superiority of Aryans over other types of people, including Jews, Slavs, Gypsies, and people of African descent.

The Nazis first developed the idea that Jews should wear stars to identify them after Kristallnacht, a pogrom in which synagogues, Jewish businesses, and Jewish homes were destroyed in 1938. In addition, about 100 Jewish people were killed during Kristallnacht. When Germany invaded Poland in 1939, Jews in Poland were forced to wear Yellow Stars, and the Nazis enforced this rule in all the countries they conquered. The one exception was in Denmark, where King Christian X bravely said he would also wear the badge if it were instituted. Danish Jews never wore the Yellow Star. The badge had different styles in different countries (see the link below), but the intent was the same—to identify and isolate Jews. 

The star was intended to be dehumanizing, as it clearly made a separation between Jews and other people. The star identified Jews as isolated and different from their countrymen and also had the effect of making it easier for Nazis to begin to round Jews up in a plan of forced ghettoization. The Warsaw Ghetto, for example, was established in 1940. From there, about 250,000 Jews were sent to the Treblinka concentration camp. Over 300,000 Jews eventually died in the Warsaw Ghetto from starvation, disease, and deportation to concentration camps.