Scripture makes it clear that Adam and Eve were just humans. They were not of any of the labels that have been placed upon the other lineages, ethnicities, and nationalities of people living today or of any of the lineages that are now extinct such as the Neanderthals, Cro-Magons, and Denisovans.
All men are descended from Adam; every lineage of men whether existing today or no longer in existence was formed from one blood. (Acts 17:26) Just as all plants and animals were created to reproduce after their own kind as has been observed since the beginning of time, so man can only begat man. (Gen. 1:12, 20-31)
I am afraid you may have received your education from a source whose information is either inaccurate or out of date. Homo erectus was never considered to be the first man to walk upright. In fact, there is now extensive anthropological and paleontological evidence that homo erectus coexisted with man which alone disqualifies it as being a missing link.
The origins of homo erectus is not clear since their remains appear throughout much of the world and their remains are not necessarily based on a single so-called sub-species of man and therefore, it is difficult, even for evolutionists to identify who or what homo erectus may have been, though many of the homo erectus remains discovered may be human; similar to Neanderthal or even the same. Every so-called missing link between ape and man to date has either been an ape no longer living, the remains of a man, or based on scant remains that could belong to any creature.