We all know that Spanish, French, Italian and Portuguese are romance languages, this means that they were all derived from Latin. For this reason, they all can be quite similar, but how did Spanish emerged? Well, we have to take in perspective that its social and demographic circumstances affected it significantly. This is what happened with Latin, it had distinct varieties in different cities of the Roman Empire and after its dismemberment, it separated them and Classical Latin decreased in its use. This originated the languages mentioned above.
As I mentioned, demography brings a lot of repercussion in how a language evolves. Spanish originated in the Iberian Peninsula, this through the influences of the dialects that lived there like Castillan, Astur-Leonese, Navarro-Aragonese and even Basqan. But how could dialects affected so much Latin? First of all, the nucleus that originated Spanish was the Vulgar Latin, the consequence of this is that the language was more vulnerable in a social aspect. Therefore, it was gradually influenced without a Norm or a Dictionary that could've stopped its evolution. After many centuries, in XV the Spanish that we know now surged.
In XVI, America met this language through its colonization of the Spaniards. This originated a different influenced Spanish, for it took through linguistic interference from the indigenous languages. Although, the difference never got to be that significant to be considered as another language. At this present moment, Spanish, next to Mandarin, is the second most spoken in the world, with 437 million of native speakers and 590 million of people that uses it as a secondary language. For instance, if you want to learn another system of communication apart from English and don't know which to use, pick this one; because even though it is next to Mandarin, it can be thought as the first if you consider the demographic distribution.
Adios!