Voy a responder en Español dado que su nombre de usuario y comentarios en otras partes de Reddit están en ese lenguaje.

La lengua que se conoce como Español es en realidad "castellano". Es decir, la lengua del reino de Castilla. Existen otras lenguas en España que no son oficiales y difieren del castellano tanto como el portugués; gallego, catalán, etc. Obviamente también existe el basco, pero esa lengua, propio de una región norteña de España, no pertenece ni a las lenguas romances ni a ningún otro grupo linguistico Europeo que conocemos.

A pesar de que el Estado Español, construido alrededor del reino de Castilla, impuso como lengua oficial el castellano como la lengua universal de sus territorios, los demás culturas preservaron sus tradiciones linguisticas hasta el día de hoy. Por eso no es sorprendente el hecho de que el dialecto que originó el portugués y que evolucionó hacia una lengua propiamente dicha (con normas gramaticales propias, una literatura propia, etc.) lograse sobrevivir, dado que el Estado Portugués no tenía por qué asumir el castellano, especialmente dado que unicamente formó parte de España durante el tardío siglo XVI y la primera mitad del XVII.

Esta particularidad tampoco es propio solo de España. También Francia e Italia tuvieron que normalizar una versión de su lengua por todo su territorio. El historiador Austo-Judio-Inglés, Eric Hobsbawm, comenta en su trilogía sobre el siglo XIX (si mal no me acuerdo sería en La Era del Capital o La Era de Imperio) como políticos del Norte del actual Italia visitaron a Sicilia en el tardío siglo XIX y no se lograban que la población local entendieran a su dialecto dado que las lenguas de Italia habían evolucionado a direcciones diferentes hacía tanto tiempo. La actual lengua Italiana es, realmente, un invento del Estado Italiano y solo fue construido hace relativamente poco.

Si le interesa el tema, surgiero los siguientes textos:

-Eric Hobsbawm: Nations and Nationalism Since 1780: Programme, Myth, Reality (existen también versiones en castellano).

-J.H. Elliot: Imperial Spain: 1469-1716 (también existen versions en castellano)

-Raymond Carr: Spain, 1808-1975 (también está en castellano, y forma parte de la colección del Oxford History of Modern Europe)

Rough Summary in English:

In essence, what I said was that what we call "Spanish" is actually just the language of the Kingdom of Castile, which served as the nucleus for what later became Spain after the incorporation of the Kingdom of Aragón (Eastern Spain).

There were numerous local dialects, such as the ones still used in Galicia and Catalonia, which eventually evolved into their own full blown languages; each with its own grammatical rules and literature. Portuguese started out as a dialect just like the others and due to the fact that the Kingdom was only a part of Spain during the latter half of the 16th and first half of the 17th centuries there was little real pressure to use only Castilian Spanish. In addition, even countries which were part of Spain continued to use their local dialects, even today.

I also stated that this phenomenon isn´t just limited to Spain. Italy, for example, had tons of regional dialects and even full blown languages. Eric Hobsbawm cites a group of Northern Italian politicians who visit Sicily in the late 19th century and who discover that their Italian was incomprehensible to locals. What we call Modern Italian is largely a construct, built out of the need for a unified language around which the Modern Italian State could be built.

So spanish and portuguese are languages that come from vulgar latin and evolved separately. During a time before Spain even existed as a country the language Galician-Portuguese was the language spoken in what nowadays are territory of Galicia spain and north portugal, this language is older than castilian. Galicia became a part of the Castilian Kingdom and the king Alfonso X of Castile even choosed this language over castilian to write and this language was considered a better language for poetry than castilian. Then portugal became an independent kingdom and portuguese evolved separately from Galician as they conquered south portugal from the moors, while Galician remained being spoken in Galicia until this day but lost its relevance and became ostracised from any oficial status as the oficial language of the new country of Spain became the castilian, still despite centuries of being a forbidden language that was not allowed to be used in the court, and the Galician nobility was also punished and taken from Galician to the castilian court, centuries without ever being a written language again, the language survived being spoken by the majority of the population of Galicia, so if Galician language survived in this conditions Portuguese that had its own independent country where portuguese was oficial and spoken by nobility with a written language and literature had no reason to disappear in favour of spanish.

Portugal used to be part of spain until 1139

Not really, Spain as an unified country only appeared much later. The iberian peninsula was home to a number of independent kingdoms with unique languages that grew with the reconquista. See this GIF to understand the spread of the languages trough time: http://lingvo.info/images/lingvopedia/portuguese/map_linguistic_.gif

Portuguese and Galician evolved from Galaico-Português which in turn evolved from Latin. What you call Spanish is only one of the languages spoken inside Spain.