Humans

Since they outnumber most other races, usually are amongst the most technologically advanced and organized of the races, human beings remain the dominate race in the world. Most islands are inhabited by humans, even in the Grand Line, where most of the stronger races such as the Fishmen and Giants are found.

Appearance
The same as in real life, however they come in several sizes ranging from "normal" to much taller (such as in the case of Jerry).

Differences between humans varies greatly; islands not allied with the World Government form a number of tribes including (but not exclusive to) the Dwarves, Longarm Tribe and Snakeneck Tribe. Other tribes are expected to exist as Ape-like humans including Masira, Shoujou and the Coffee Monkeys have also appeared; Masira himself claimed openly to be human and not ape although was not insulted when called a "monkey". As of Thriller Bark, humans with demonic features also appear in the storyline.

Biology
Mostly the same as in real life.

The strange tribes of the Grand Line are also human, however they have significant appearance alterations from "normal" humans. Their advantages over normal humans have yet to be explained. It has yet been explained how large sized humans occur in humans even amongst families that produce normal sized humans. Where Sandersonia and Marigold were normal human-sized in the past, as well as that their older sister was not actually taller than her siblings, but currently somewhere above normal human sizes. Amongst the Kuja in particular, there is a notable significance that every child born is female and no males are ever produced. The alterations of other tribes biology like this have yet to be explored.

Humans can however breed with Merfolk to produce a human-merfolk child. However further interbreeding with humans removed all apparent merfolk features. After just two generations, the only thing that remains is a "human" with an incredible swimming ability. Since Giants are a race of extremely large humans, which can breed with Fishmen to produce a Wotan, one can presume Fishmen also breed with normal humans, although fishmen/human half-breeds have yet to appear in the storyline.

Inter-species Relationships
The weakness of the humans in the One Piece world often makes other Races such as the Fishmen come to think lowly of them. Often this reaches to the point where they inflict heavy racism onto Humans themselves, although not all Fishmen typically think this. Often this is met by humans with returned racism towards the Fishmen, sometimes leading to bloodshed. Part of the problem is that Humans tend to think of Fishmen and the Mermen as nothing more then fish, dismissing them as "men" thus alienating any chances of them being viewed as equals with humans.

Amongst the humans themselves, there is an apparent problem with the non-World Government allied tribes. Their freakish appearances (such as having long arms or legs) are often a lure for kidnapping crews at Sabaody Archipelago, who sell them as slaves because their selling rate is much higher than a regular human. These tribes and races caught up in slavery are often sold for a higher price then "normal" humans. In sharp contrast to the Mermen and Fishmen, Giants are respectfully regarded as "men" by humans.

Overall Strength
Most of the inhabitants of the One Piece world are normal Humans and are one of the weakest races on the planet. While the average human is as weak as in Real Life, with enough training they are able to train themselves to be able to do incredible stunts such as using Super-Human Strength and incredible abilities. Normal Humans are more likely to consider the usage of Devil Fruits than any other creature in the world, and are the leaders in Devil Fruit research.

Some humans are fortunate to be more than twice the size of a normal human being. Naturally, with no training, these tall humans are amongst the strongest men and women in the world and their size makes other humans reconsider attacking them. Even then, compared with true Giants, their size is nothing, as to Giants these tall humans are nothing but overgrown ants. An example of this would be when the Kairiki Destroyers of the Franky family (enormous by human standards) were totally outmatched in combat by Oimo and Kāshī, who are actual giants.

While the normal human is physically much weaker than races like giants and fishmen, their wide range of attacks, skills and abilities make up for them to be more versatile fighters. And even with the physical strength of other races taken into account, there are a handful of humans that surpass even their great advantages against them, although it it is not always in physical strength but usually in more tactical thinking.

Trivia

 * While all races are "man", humans in particular seem to be more inclined to decide what the meaning of "man" is. Thus it would seem that within the One Piece world, to be called "human" and "man" may in some cases not mean the same thing. In particular the World Nobles may to use discriminative termology to describe their slaves as "barely human" and will go so far as to burn the hoof of the dragon onto thier slaves flesh - from this day forward the slaves are forever known as "less than human".
 * Tony Tony Chopper classifies himself as a "man" because of his Devil Fruit abilities of the Hito Hito no Mi, although he is a reindeer by species.
 * The existance of strange tribes humans can be alikened to the documentation of early explorers and researchers. They often returned from places like Africa, Asia and the Americas with accounts of tribes who have manipulated their appearances in strange or even what would be considered to such men as grotesque ways.