Well, it really depends on what you mean by historian.

If you are referring to oral historians - such as tribal elders and storytellers who would pass on a tribes history through song, myth, and legend - then historians first appeared alongside the first stirrings of human civilisation eons ago. Storytellers like Homer and Hesiod, who told of the history and myth of the Greek civilisation, are among these oral historians and date back far into antiquity.

But if you are referring to 'true' historians, scholars who research a wide variety of sources from which they formulate and author a historical enquiry, usually in the written form, then we can pinpoint the exact period in-which they first appeared; this period being the Classical period of ancient Greece (500 to 400 BCE). It was during this period that the 'first' historian, the "Father of History" to quote the great Cicero, lived - this being a man by the name of Herodotus of Halicarnassus.

Herodotus (484 - 425 BCE) was an Ionian Greek, he was born in Halicarnassus, a city-state which sat on the coast of the Aegean Sea in the westernmost region of Asia Minor (modern-day Turkey). From what we know of him Herodotus was a traveller who, over the course of decades, spoke with people from across the known world to formulate a source base from which he authored his Enquiry. Authored circa 430 BCE, Herodotus' Histories - as we refer to it today - is split into nine books and primarily examines the history of the Greco-Persian Wars (498 - 448 BCE), while also telling stories of the wider world outside of the Greek world. Herodotus' Histories is the widely regarded as the foundation of western history.

Herodotus and his Histories would forever change the nature of the telling of history within western society - as historical storytelling shifted from the primordial method of oral history being told in the form of song, myth, and legend to the 'traditional' method or written history. Historians after Herodotus, such as his contemporary Thucydides, would follow his example of researching a wide array of sources from which they would formulate their enquiry. Though many historians - including Thucydies - would come to criticise Herodotus and his methods of the collection and inclusion of sources, to the effect of giving him the moniker of 'the Father of Lies' to compliment his 'Father of History' moniker; Herodotus is, for all intents and purposes, the first historian in history.

I'll leave you with Herodotus' motiviations for writing his Enquiry in his own words:

"Here are presented the results of the Enquiry carried out by Herodotus of Halicarnassus. The purpose is to prevent the traces of human events from being erased by time, and to present the fame of the important and remarkable achievements produced by both Greeks and Barbarians; among the matters covered is, in particular, a history of the hostitilities between the Greeks and the Persians."

Hopefully this answer is helpful. Feel free to ask me any further questions.