If you include theology, then Akhenaten would be a viable candidate for developing perhaps the 1st known monotheistic religion and concluding that everything in the World came from a single common source.
Here this answer just clarifies that it is known that Lao-tzu or Laozi was born around 600 BC and had written the Tao Te Ching, and as the founder of philosophical Taoism (p.s. sometimes he is also revered as a deity in religious Taoism and traditional Chinese religions.)
In that sense that Confucius is not as earlier as Laozi. There are some ancient documents also recording the event that Confucius politely asks questions to the elder Laozi for some enlightenments.
Traditionally, the earliest philosopher is Thales. But Zoroaster might be a better candidate IMHO. His dates are between the 18th and the 6th centuries BCE depending on the source.
Insofar as I'm aware, the idea of uncompromising duality, namely right vs wrong at the expense of any other option (and as opposed to the Eastern Asian variety which allows for fuzzier alternatives), traces back to him and Zoroastrianism.
That said, methinks I'd take the idea of "earliest known philosopher" with a grain of salt: anonymous thinkers, theologists and law makers were probably thriving in the great river valleys (Mesopotamia, Egypt, Ganges, China) long before the Code of Hammurabi reached us from the 18th century BCE.
Indian philosophy traces it's earliest roots to Kapila, the founder of the Sankhya philosophy which all Indian Philosophy is derived from. Some say that Pythagoras came to India and studied this philosophy during his travels. Kapila is mentioned by Krishna in the Bhagavad Gita. I am unaware of any date given to him, but very very early.
[The Rigveda] is one of the oldest extant texts in any Indo-European language. Philological and linguistic evidence indicate that the Rigveda was composed in the north-western region of the Indian subcontinent, most likely between c. 1500–1200 BCE, though a wider approximation of c. 1700–1100 BCE has also been given.
While it may not be the oldest written source of philosophy, the Rigveda is definitely one of the oldest sources known to day as well as one of the most influential ones.
As such, the anonymous writers of the Rigveda are some of the earliest known and most influential philosophers.
In the Odyssey (supposedly written by Homer around 800 BC) we find among other gems the story about the cyclops Polyphemus:
The philosophical lesson is not to believe nobody to be somebody!
A couple of hundred years later Parmenides of Elea says that you shouldn't believe nothing to be something!
This is essentially the same logical point!
Parmenides is considered to be a philosopher then WHY not Homer?
My vote on the first known philosopher therefore goes to ... HOMER!