Were Serbs and Croats one people / tribe in the past? If not, why do they speak the same language?

Currently, Croatian scientists declared that studies show that the Serbian genetics, though very similar, but not identical, to that of Croatians, is older than that of Croatians to the point that Serbs may have fathered the Croatian nation. (I had both a Croatian grandmother and a Dalmatian grandfather. The Venetians came into the Zagora hinterland and helped drive back the Ottomans and then remained, tore down all but one minaret, and helped transition the residents from speaking Old Dalmatian to Veneto. That remained the case even 1 1/4 centuries later when Napolean conquered the Venetian republic but changed when the first Yugoslavian king, a Serb, forbade any Latin language to be spoken. Distant Dalmatian cousins of mine there to this time communicate only in Croatian and consider themselves Croatian citizens. My father in the USA preferred to speak Veneto, otherwise Triestin, I speak English, and my mother spoke Spanish. Languages can change but genetics don't.)

First of all, it's a bit silly to even discuss such topic since we're not historians. Relying on sources such as Wikipedia is worthless. Same applies on the official historical views of such countries (especially the ones that derived from former Yugoslavia) because their primary task is to prove their own "original", "unique", "independent"... origin or background, therefore the identity. Historians who back such theories I'd call regime historians. They existed in every regime, in Balkans as well as anywhere else in the world. So, let's focus on my fellows and countrymen from Balkans or former Yugoslavia region.

I would prefer to avoid answering the question, have Serbs and Croats arrived together to Balkans, are they same of different... Those questions should be answered by independent historians (people, not influenced by political views) and supported by archeological and historical founding (e.g. https://en.wikipedia.org/wiki/Vin%C4%8Da_culture, https://en.wikipedia.org/wiki/Miroslav_Gospel) I'd like to stress, making this comment my intention is not to offend anybody in any way. I am hoping to contribute to general understanding of the topic.

First, I'd like to ignore political maps, present and past. This is also applicable to other countries and nations. So, talking about Serbia, Croatia, Germany, Italy, etc. I won't assume present borders or where they should be. Instead, I'll try to focus on ethnicity, culture (dance, music, national costumes...), language, etc.

I will bravely state that people of now days Croatia, Bosnia, Serbia, Montenegro and mostly North Macedonia are same and speak same language, often with different dialects. Apart of North Macedonia, they understand each other 100% even though they speak in different dialects (of the same language) and often use different words that describe same thing but again those are synonyms of the same language. I have a lot of friends from other Slavic countries, mostly from Slovakia and Poland but from Czechia and Russia as well. Similarity between our languages is so obvious, even though we're living apart for many centuries, one can tell, once upon a time it was one language.

Surely, difference in DNA, language, religion and culture between Serbs and Croats and/or Bosniaks (Bosnian muslims) does exist and it's created through the enforced influence of the occupiers through out the history. Croats have been under the rule of Catholic states such as Venetians, Austro-Hungary, which obviously influenced peoples religion there. Their red/white "checkerboard" exists on Coat of arms of the Czech Republic, Poland uses red/white colours (same as Croats) and white eagle same as Serbs... National costumes of Serbia, Slovakia, Croatia have very similar (often same) colours, pieces of clothing, decorations... So, all that documents great similarities but not being identical. Looking at surnames or origin of some notable people in Croatia, it is obvious that they are not Croatians, for example: Ljudevit Gaj (Ludwig Gay) – obviously a German, Slavoljub Eduard Penkala (born as Eduard Pękała-Duch/Polish), Nikola Tesla - Serbian, Boris Apsen - Russian… This was an example of migrations through the history that would influence the culture, ethnicity, language, etc of a nation.

By the same token, Bosnia, Serbia, Montenegro and Macedonia were under Ottoman Turkish rule which influenced local population with their cuisine, religion, language, etc. So, Croats and Serbs have differences but they are still same people. Like two brothers, educated in different (opposite) schools and married into families with different custom but still very much alike each other in every sense. The territory these people live on, the intersection between Europe and Middle East, being a subject of constant conquest of different foreign powers, is like cursed and destined to suffer all above mentioned foreign influences that have shaped present differences among the local population. We have in our language some foreign words, not many-rather as leftovers which fall into groups of Germanisms, Turkisms… and such influences, understood and taken as negative for our people, are Germanisation, Bulgarization (applied on Macedonia and eastern part of Serbia), Hungarisation...

To my understanding, Germans and Austrians are same people. How? They speak same language with different dialects. That’s normal and often present everywhere. Real name of Austria is Österreich, meaning Eastern kingdom. Kingdom of what? Germany, of course. By the same logic, it is not acceptable to call Slovakia – Upper Hungary, just because it was ruled by Hungary for a long time. Population that always lived there are of Slavic origin, speaking Slovak language. Hungarian language and culture does not belong to Slavic ethnicity. As one german, with humiliating and ironical tone, expressing his own happines, told me:“Slavs are the biggest white race but most divided.“, I believe us Slavs should stop being influenced by foreign powers (economical, cultural, religious, military) and start seeing each other as same. If catholics and protestants in Germany or Britain can live normally together, why we can’t? If we could achieve that (without any brainwash such as communist ideology, forceful atheism, etc.), it would remove a potential for any future wars between us, such as Serbs/Croats, Russians/Ukrainans or Polish.

@Walter. I couldn't agree more with your comment but who am I to tell to others what is their identity and how should they feel? As I said, I am not a historian and the sources are often biased... I can analyze only facts that everyone from that region is aware of but often have been interpreted differently.

The more I read, the more I feel Serbs and Croats are unrelated Slavic tribes which came to Balkans separately and never had shared history or culture. Not one people divided, but rather two unrelated peoples culturally and linguistically merged, as they happened to settle next to each other. Is this true?

Slavic or non-Slavic origin of Croats
Both being Slavic means a rather big degree of relatedness, and most likely similar languages. A much more drastic claim would be to suggest that one of these tribes was of non-Slavic origin, adopting Slavic language and culture, like Bulgars or (to some extent) Varangians. There is indeed a range of such theories, tracing the origins of Croatians to Goths, Iranians or Avars - see Origin hypothesis of Croats. There is also a question of whether Croats themselves constitute a h*m*geneous group or rather a combination of peoples of different origin.

Language
In either case, the South-Eastern Europe was settled by Slavs in the 6-th and 7-th centuries, and the first Serbian states emerge in the 8-10th centuries - see Serbs. The term Croat emerges in the 9-th century and is widely attested in the 10-th century - see Croats. On the other hand, the Serbo-Croat language, as a language distinct from Old Church Slavonic, spoken by a much broader group of Slavic peoples, is attested only from the 10-th century. In other words, it is fair to say that by the time the distinct language emerged, the two people had already been linguistically and culturally close.

Religion
The first contacts between the Pope and Serbs and Croats can be traced to the 7-th century, and the Christianization started in earnest soon thereafter. The Great Schism between the Roman and the Eastern Christianity took place in 1054. To quote Wikipedia:

The identity of ethnic Serbs was historically largely based on Orthodox Christianity and on the Serbian Church in particular. The conversion of the South Slavs from paganism to Christianity took place before the Great Schism. During the time of the Great Schism, Serbian rulers including Mihailo Vojislavljević and Stefan Nemanja were Catholics, with the former being a vassal of the Papal States. In 1217, the Serbian ruler Stefan Nemanja II was crowned by Pope Honorius III of the Catholic Church. However in 1219, Nemanja II was crowned once again by the newly independent Serbian Orthodox Church. This shift solidified the Christian Orthodox religion in Serbia.

Genetics
Finally, as far as the genetic evidence goes, both people seem to be of Slavic origin, although Croats represent a mixture (see Genetic studies on Serbs, Genetic studies on Croats):

An analysis of molecular variance based on Y-chromosomal STRs showed that Slavs can be divided into two groups: one encompassing West Slavs, East Slavs, Slovenes, and western Croats, and the other – all remaining Southern Slavs. Croats from northern Croatia (Zagreb region) fell into the second group. This distinction could be explained by a genetic contribution of pre-Slavic Balkan populations to the genetic heritage of some South Slavs belonging to the group.

Note however that such genetic studies are not incontestable either, since what is taken as a signature of Slavic or non-Slavic origin is somewhat subjective.

The Serbs and Croatians, were and are part of the larger South Slavic ethno-linguistic and ethno-racial group. What primarily distinguished Serbs from Croatians-(or Croatians from Serbs), was religion, namely, a sectarian difference.

Both Serbs and Croatians have been Christians since the Middle Ages, though the Serbs have been and are still, Eastern Orthodox-(or of the Byzantine rite), whereas Croatians have been and are still, Roman Catholic-(or of the Roman rite). Serbia and Croatia are South Slavic Balkan lands, though they have different historical identities.

The Serbs lived under Greco-Byzantine Christian rule for centuries (and, for a short while, the Serbs founded their own independent state during the Late Middle Ages, though remained steadfastly loyal to Eastern Orthodox Christianity). However, the Croatians had peripheral contact with the Greco-Byzantine Empire-(comparatively speaking) and early on in their history, became increasingly associated with-(as well as conquered by), The Roman Catholic Venetians. The Serbs were conquered by the Ottoman Turkish Muslim Empire for much of the Modern era, though the Croatians were conquered by-(or were a quasi-tributary state under), Venice for centuries-(and unlike the Serbs, the Croatians, with the help of the Venice, defeated the Ottoman Muslim imperial onslaught). The Serbian Christians have had a more Eastern cultural and religious orientation, whereas the Croatian Christians have had a more Western cultural and religious orientation. Yet, both Serbs and Croatians, are fellow Christians

So, as you can see, the sectarian identities of both Serbian and Croatian Christians was very much formed under the historical circumstances surrounding each country.

According to the entry concerning the Southern Slavs it is quite probable that the common language source would be due to common ancestry:

The Byzantines broadly grouped the numerous Slav tribes into two groups: the Sclaveni and Antes.[8] They are both first encountered in the lower Danube region. Some, such as Bulgarian scholar Vasil Zlatarski, suggest that the first group settled the western Balkans, becoming one of the forerunners of the linguistic group that became the Bosnians, Serbs and Croats,[9] whilst offshoots of the Antes settled the eastern regions (roughly speaking), becoming one of the ancestors of the Bulgarians.[8][10]

So at this time (the 6th century),they were considered to be a single group by the Byzantines.

At some later point, they are referred to, again by the Byzantines, as two separate peoples:

Constantine VII in De Administrando Imperio recounts in 30th chapter "ancient Croatia, also called "white", is still unbaptized to this day, as are also its neighboring Serbs... plundered by the Franks and Turks and Pechenegs..."

(emphasis mine) from entry on White Croatia

So there is a basic line of thought which would place both the Croats and Serbs as a common people with a common linguistic root, considered by the Romans as a single group the Sclaveni, but which hundreds of years later were reffered to as two distinct groups by Constantine VII( reigning from 913 to 959).