Karaism is the original Judaism which has existed throughout history under various names incl. Righteous, Sadducees, Boethusians, Ananites and Karaites, all of whom obeyed the Torah with no additions.
Karaism has been around since God gave his laws to the Jewish people. At first those who followed YHWH's laws were merely called "Righteous" and it was only in the 9th century CE that they came to be called Karaites. The question of why God's followers are today called Karaites is really a question of the origin of the other sects. At first there was no reason to label the righteous as a separate sect because there was only the one sect which consisted of the whole Jewish people. Throughout history a variety of sects appeared and it was only to distinguish the righteous from these other groups which caused them in different periods to take on such names as Sadducees, Boethusians, Ananites, and Karaites.
Biblical Period- The Righteous
In the Biblical Period people are described as falling into two categories: the sinners and the righteous. Very often the people were led into sin by false prophets who claimed to be relaying the message of God. In some periods the majority of Israel followed the false prophets and those who remained loyal to YHWH were but a small few (e.g. Elijah at Mt. Carmel). God sent his prophets "from morning till evening" calling on the people to repent but all too often it was only by punishing the nation with a great calamity that YHWH could get them to listen. Much of Biblical history is a repeating of the familiar cycle of sin, punishment, repentance and rescue.
Second Temple period- The Sadducees and the Boethusians
The first reference in the history of Israel to more than one sect takes place some 200 years after the close of the Biblical period, in the first century BCE. Various sources tell us of two opposing sects, the Sadducees (Zadokites) and the Pharisees. The Sadducees followed the Torah as it was written while the Pharisees believed in a second "Oral" Torah which they added to the real one. The Second Temple period saw the rise of several more sects among them another group which only followed the written Torah called the Boethusians and a sect which added several books to the Bible called the Essenes (a.k.a. the "Dead Sea Sect").
Like the Karaites who were to follow them, the Sadduccees and the Boethusians continued the tradition originated by Moses (Dt 4,2) of keeping the Torah's commandments with no addition. We often hear in ancient literature that the Sadducees denied the doctrines of the immortality of the soul and reward and punishment in the hereafter. Whether this is accurate or not is of little consequence since they arrived at these beliefs based on an honest interpretation of the Bible (even if most Karaites disagree with them on these doctrines). The Pharisees on the other hand believed that the interpretation of a particular teacher was divine and elevated these teachings to the level of the Torah itself. After time this doctrine got carried away and they claimed that these teachings originated from God himself in the form of a second "Oral" Torah. They even went so far as to claim that when two teachers taught diametrically opposed interpretations of the Bible that both interpretations were from God! The third major sect, the Essenes, had a Bible which consisted of more than our 24 Books and as a result had practices which do not originate in our Bible such as a solar calendar.
How long these three sects continued to co-exist is unknown. It is often thought that the Essenes and Saducees ceased to exist with the destruction of the Temple in 70 CE. However this seems unlikely as writings of the Essenes appear as late as the 10th century which seems to indicate that they survived well after the destruction of the temple. References to the Sadducees and the Boethusians continue to appear in post-70 CE literature and they also seemed to have survived for some time.
Middle Ages- The Ananites and the Karaites
In the early middle ages the Pharisees continued to thrive. They began to call themselves Rabbis and only used the name Pharisees when remembering historical events from the Second Temple period. In the 7th century the Islamic Empire swept the Middle-east. The Muslims had no interest in imposing Islamic religious practice on the Jews and gave them a degree of autonomy under a system known as the Exilarchate. The Exilarchate had been founded hundreds of years before under Sassanian rule but until now only had influence in Babylonia and Persia. Overnight the Rabbanites turned from a localized Babylonian phenomenon into a political power which stretched throughout much of the Middle-east. From the 3rd-5th centuries the Babylonian Rabbanites had developed a body of religious law known as the Babylonian Talmud which they now imposed on every Jew in the Empire.
Resistance to the Rabbinites was fierce, especially in the eastern provinces of the Empire which had never even heard of the Talmud. The historians tell us of Jewish leaders whose resistance against the Talmud put them in direct conflict with the Islamic government, which had empowered the Rabbis and given them full authority over other Jews. One resistance leader who refused to accept the Talmud was named Abu Isa al-Isfahani and it is said that he led an army of Jews against the Muslim government. Other attempts to cast off the Talmud were also undertaken but all failed and the Rabbanites and their Talmud seemed unstoppable.
Then in the 8th century a last glimmer of hope appeared in the form of a shrewd leader named Anan ben David. Anan organized various non-Talmudic groups and lobbied the Caliphate to establish a second Exilarchate for those who refused to live according to the Talmud's man-made laws. The Muslims granted Anan and his followers the religious freedom to practice Judaism in the way of their anscestors. Anan himself was not a Karaite; although Anan rejected the Talmud he used similar irrational methods of interpreting Scripture as the Rabbis, such as intentionally taking words out of context. Anan's followers became known as Ananites and this group continued to exist down until the 10th century. On the other hand, those Jews who continued to practice the Tanach-based religion of their anscestors became known as Bnei Mikra ("Followers of Scripture") which was also abbreviated as Karaim ("Scripturalists"), in English "Karaites". This name derived from the old Hebrew word for the Hebrew Bible: Mikra, Kara. The name Karaim, meaning "Scripturalists", distinguished these Jews from the camp of the Rabbis who called themselves Rabaniyin ("Followers of the Rabbis") or Talmudiyin ("Followers of the Talmud").