Is there one true form of Islam? Who is the human leader of Islam on the earth?
Islam is a unified religious code based on well-defined and intelligible sources, the Qur'an and the Sunnah. It is there in its original form as communicated to the world by the Prophet Muhammad (sws). The Almighty made sure that the basic teachings of Islam are not put in the hands of some individuals facing the danger of being lost. These were communicated and disseminated in the entire generation of the Companions of the Prophet (sws) who in turn communicated it to the following generations and so on. The Holy Qur'an embodies the doctrines and the Sunnah contains certain practices which Muslims need to perform in their daily life. Thus these practices are kept alive in the Muslim community is never exposed to loss.
All the Muslims generations over the history are acquainted with these two essential parts of the religion and have never let go of it.
Your question seems to stem from the fact that Muslims are divided in different schools of thought and sects. It is an astonishing fact that the entire Muslims sects have a consensus on the basic components of the religion. The differences only occur in the minor details or explanations of the sources and in practical implications of some of the directives. Still the number of differences in these small details is much smaller than the agreed upon ones.
Muslims all over the world believe in and practice these directives in their individual and collective life. Some of the Muslim societies have adopted the Shari`ah as the supreme law of their countries.
There is no such system of archbishopric personalities in Islam. The final authority rests with the person of the Holy Prophet (sws). He communicated the religion to the world and taught it to all and sundry. He explained that the certain parts of the religion are preserved in the Qur'an and the Sunnah. None after him holds the divine status.