The best way to find an "average human" for an NPC Extra is to take a look at some of the statblocks in the official books. If you look at the typical "soldier" in the Allies section of the core rulebook, you'll see it has d4 in Smarts, d6 in all other attributes, d4 in Stealth, and d6 in the other skills (including combat skills). The "experienced soldier" has two attributes at d8 and the rest at d6, combat skills at d8, and a couple of combat Edges.
If you have SWD you can also look at the Fires of Ascalon one-sheet adventure in the back, where you'll see the "Village Fighting Men" have Smarts d4, other attributes d6, Fighting d4 and Notice d6.
There are some even better examples in the Fantasy Companion, but to cover some of the more "average" types:
- Bandit: d6 in all attributes and relevant skills.
- Citizen: d6 in all attributes, d4 in combat skills, d6 in other relevant skills.
- Mercenary: Vigor d8, all other attributes and relevant skills at d6.
- Courtier: Smarts d8, other attributes d6, primary skills d8, Fighting d4.
- Town/City Watch: d6 in all attributes, combat skills d8, other skills d6.
- Militia: d6 in all attributes, combat skills d6, other skills d4/d6.
So we can see that citizens (covering "everything from farmers to crafters") and courtiers would typically have Fighting d4, while most bandits, mercenaries and village militia would have Fighting d6, and the Town/City Watch (described as competent guardsmen) have Fighting d8. The Horror Companion also has a "typical cultist" who has Smarts d4, but all other attributes and skills d6, as well as a "Police Patrolmen" who has Spirit d8, other attributes d6, combat skills d8, other skills d6-d8.
So I would argue that most humans have d6 in all attributes, sometimes with a single d4 or d8. Most would also have d6 in skills related to their profession, or d8 if they're particularly well trained, but only d4 (or unskilled) in other skills.
Regarding the ratings for different die steps, it's worth taking a look at the section on Professional Edges, which "reflect many years of practicing a particular trade". A character with a Professional Edge is described as being "far more competent in his chosen field than most others - far more than just having a d12 in that occupation's skills". Most Professional Edges require d8+ in the appropriate traits, and many of them grant a +2 bonus to certain skills.
Thus the Professional Edges help to differentiate between a skilled professional (d8+2) and someone with exceptional talent but without professional training (d12). Both get the same result on average (if you ignore the Wild Die), but the professional is far less likely to make silly mistakes (12.5% chance of failure at TN 4, instead of 25% chance for d12). For a Wild Card, d8+2 is far superior to d12 - not only because the +2 is added to both the trait die and wild dice, but also because it means the character will only fail at standard difficulty tasks if they roll snake eyes.