Why is Buddha needed? In order to attain Nirvana one must be able to  understand that his or her self is an illusion , error or is like a dream. This understanding doesn't come naturally. Naturally one finds that there is a self. Naturally one finds that he or she is behind feelings ,thoughts ,perceptions. Most humans and animals (I guess ) would more likely observe and conclude that I am angry, I am coming , I am going , I am doing this or that , I am suffering etc.. He fails to see that he is made up of aggregates. He fails to see that all aggregates dissolve. He fails to see that his self will dissolve too because it is made up of aggregates.He clings to the thought that that was me. Clearly we see that in order to attain Nirvana a sophisticated Teacher is required because Nirvana requires a deep understanding of self. I wonder whether a teacher is always required for Nirvana or nature provides a clue to non-existence of self.

So my questions are :

Is it possible for humans or animals to attain automatic Nirvana? (Here automatic means without any help from external teacher) 

If yes , then what is the need of Buddha ? 

EDIT : For more clarification on the meaning and nature of the word Automatic in the above question I add the following:
I say automatic Nirvana in the above question because all aggregates must dissolve automatically and complete dissolution of Self is Nirvana.No amount of Karma or no kind of karma can keep the Self alive forever.Self by nature is liable to pass away as an error , illusion or dream.

Here I quote,from the book Buddha by Dr. Hermann Oldenberg ,page 216, about the things which no one can stop from happening:


  There are five things which no Samana, and no Brahman, and no god,
  neither Mara, nor Brahma, nor any being in the universe, can bring
  about. What five things are these ? That what is subject to old age,
  should not grow old, that what is subject to sickness, should not be
  sick, that what is subject to death, should not die, that what is
  subject to decay, should not decay, that what is liable to pass away,
  should not pass away—this can no Samana bring about, nor any Brahman,
  nor any god, neither Mara, nor Brahma, nor any being in the Universe."


If no one can stop the dissolution of Self from happening then we see that eventually everyone will attain Nirvana with or without a great teacher like Buddha.
In Buddhism, rebirth will occur endlessly without the attainment of Nibbana (which does not happen naturally or by some sort of evolution). There is no automatic nibbana. Natural spiritual evolution is a Hindu idea (see Chapter 26 of Autobiography of a Yogi by Paramahansa Yogananda) and not a Buddhist one. 

In Buddhism, all beings are not naturally progressing towards Nibbana. It is possible to be reborn as a Brahma, then later as a pig as in Dhammapada 338-343:


  On one occasion, while the Buddha was on an alms-round at Rajagaha, he
  saw a young dirty sow and smiled. When asked by the Venerable Ananda,
  the Buddha replied, "Ananda, this young sow was a hen during the time
  of Kakusandha Buddha. As she was then staying near a refectory in a
  monastery she used to hear the recitation of the sacred text and the
  discourses on the Dhamma. When she died she was reborn as a princess.
  On one occasion, while going to the latrine, the princess noticed the
  maggots and she became mindful of the loathsomeness of the body, etc.
  When she died she was reborn in the Brahma realm as a puthujjana
  brahma but later due to some evil kamma, she was reborn as a sow.
  Ananda! Look, on account of good and evil kamma there is no end of the
  round of existences."


The Self disappears and arises again endlessly through rebirths - which is explained through dependent origination. As stated in Milindapanha 3.5.5, rebirth is like light of a candle passed from one candle to another. It can be passed on forever. Also see this answer.

It is indeed possible to become enlightened (i.e. attain Nibbana) without ever encountering the teachings of a Sammasambuddha.

Such beings are called Pacceka Buddha or Pratyeka Buddha (see here and here), which literally means "private Buddha" or "lone Buddha". You can find some details in this question. A Pacceka Buddha would not have the ability to teach others and start a Buddha Śāsana.

While it is possible for someone to become free from suffering this way, it is immensely harder than if someone learns it from a Sammasambuddha (or Samyaksambuddha).

My analogy here is that Albert Einstein is a "Buddha" of physics who discovered the Theory of General Relativity and taught it to others through his groundbreaking published paper and his lectures to trained physicists. While it is possible for others to discover this on their own, it is immensely difficult. It is easier to learn it from Einstein and from those who have learned it from him.