Archaeologist

An archaeologist is a particular type of historian who ventures out into the field to find ancient ruins and study them first-hand. They are interested in the cultures of lost civilizations.

Role and Duties
Archaeologists work to uncover things lost beneath the earth over time, and are eager to make new discoveries about the past. Archaeologists serve a most invaluable role of piecing together the development of countries, people, and cultures.

Archaeologists are not essential to a pirate crew; very few crews are known to have an archaeologist crew mate. However, they are far from useless since their knowledge and experience makes finding ancient treasures easier. For example Robin discovered where the "Lost City of Gold" was in one day, something that took Enel a long time to accomplish.

Void Century Censorship
The World Government does not mind archaeology taking place. However, should any archaeologist (or any person at all) research the Void Century or the Poneglyphs, they risk being sent to jail or killed, and if not captured, a very high bounty may be placed on their head.

Ohara Island boasted the most famous archaeologists in the world, and their Tree of Knowledge held the history of countries from around the world. They were eventually destroyed when the World Government found out they were researching forbidden histories; all save Nico Robin, who had a bounty put on her head as a result of her escape from death, were killed.

Most people in One Piece seem uninterested in the past and are unaware of the Void Century. Archaeologists are not completely scorned by people (unless they research the forbidden histories, then they are considered evil) around the world. However, due to the Ohara incident there appears to be few archaeologists left.

Demons of Ohara
The Demons of Ohara (オハラの悪魔達) were a group of scholars and archaeologists who resided on the island of Ohara in West Blue. The Ohara clan dedicated itself to the study of the past, something the World Government was uneasy about but tolerated so long as no research was conducted on the Void Century and the Poneglyphs that held its secrets. They were called "demons" by the World Government for daring to study the forbidden history, which, by Government regulations, is capable of world destruction.

Exactly how long the Ohara Clan has existed is unknown though the Tree of Knowledge, the great library on Ohara is said to be over 5,000 years old and was apparently planted by the Oharians. The Clan has at the very least been around since the end of the Void Century 800 years ago and were one of the few groups to study it despite this being made a serious criminal offense by the World Government.

When Robin was just 2 years old her mother Olvia and numerous other Oharians set out on a journey to study the Poneglyphs to try and discover more of the Void Century, however during the six year journey the expedition is discovered by the Marines and all but Olvia are killed. Knowing the Marines would link the expedition back to Ohara, Olvia returned to try and warn its citizens but is ultimately too late.

History
After discovering Nico Olvia and numerous other Ohara scholars on their six year journey to research the Poneglyphs, the World Government concluded that it was too risky to allow the Ohara Clan as a whole remain and a Buster Call was initiated. In order to preserve their reputation the World Government spread the lie that the Ohara Clan were attempting to revive the ancient weapons of mass destruction: Pluton, Poseidon and Uranus, earning the Clan the nickname the "Demons of Ohara" and the resentment of the world.

This term has led to a hard life for Nico Robin, the only known survivor of the Ohara Clan, who for that simple reason is hated and feared by nearly all who meet her, the "Demon Child" nickname even stems from her association with the Ohara Clan.

Trivia

 * Though Robin's role on the ship seems a little odd, it was not unheard of for there to be at least one crew member who was knowledgeable on the value of items such as jewels, other treasures or even merchant cargo, along with knowledge on where they originate from.
 * In raids and attacks on others, time along with carry capability of both a ship and its crew would have limited what items the pirates could steal, having someone with knowledge on the most valuable of any given set of items would have proven invaluable to any pirate crew.