More about the second question: In westerns, normal citizens occasionally participate in the execution of the law. The Andy Griffith Show talks about the 'citizen's arrest'. Was there ever a culture without any kind of police force? Where the community did a kind of self-policing? If so, how effective was it? What would motivate a culture out of one form of policing and into another?
hi! you may be interested in these
overview
examples of early law enforcement
Law enforcement in ancient Egypt, "were there cops in Egypt?" - Ancient Egypt
Did ancient greek cities have any kind of police force? - Ancient Greece
What was law enforcement like in cities of the middle ages? Was there any distinction between law enforcement and soldiery? - Ancient Greece, Ancient Rome, 17th c France
How did the Roman empire deal with police and fire services, if they did exist at all? - Ancient Rome
Were Lictors the Roman Police? - Ancient Rome
What was law enforcement like in Ancient Rome? - Ancient Rome
How were crimes investigated in the past? - Ancient Rome, China, Medieval & Victorian England, Aztecs, 19th c Europe & USA
Before the modern era where did people go to report crimes? Who were their 'police' basically? - China, Greece, Iceland, Japan, Visigoths, Germany
Crime and law enforcement in Middle Ages - Medieval Europe
Law and Order in Medieval cities - Medieval Europe
Who were the local law enforcers in Medieval Germany? - Medieval Germany
What did "law enforcement" look like in the great Mesoamerican cultures at their height? - Mesoamerica
When was the traditional police system created in the UK? - 18th/19th c UK; includes links to a few more posts
Well in the Middle Ages, the Guilds had to take the role of 'police force'(militia) in the cities. They protected the streets with their small militias and every guild had to deliver a small number of men to do so. It was also used to protect the trade in a city and fight of any agressors who tried to attack. The numbers were quite small but effective(oh, well ofcourse it depends on how large the city is). On the countryside, the lord was the policd officer and he had a mercenary force, or used his own farmers(Feodum, farmer gets plot of land in trade for duty during wars) to keep the villages etc. safe. This kept up until the Industrial Revolution. Then the guilds slowly disappeared and the government took over the role of supplying the police force. So it first was in hands by the citizens themselves (the guilds, who were not directly in control of a city, but had an important role) and later became a government duty.
Source: McKay, a.o.(2011) Modern History of Western Society, tenth edition.
this is going to be a bit heavier on sources than on deep commentary but i think it's useful.
well the "posse comitas" or posse in common terms has had a long history in the Anglosphere dating back to the anglo saxon times. Afred the Great's reorganization of local defenses against viking raids lead to the creation of the "shire reeve" or sheriff
I haven't read this paper but the author contributes to a law blog (volukh conspiracy) now hosted at the Washington Post.
http://www.washingtonpost.com/news/volokh-conspiracy/wp/2014/05/15/sheriffs-and-the-posse-comitatus/
http://papers.ssrn.com/sol3/papers.cfm?abstract_id=2427430
from the wapost article
From Anglo-Saxon times until the present, a core power of the Sheriff has been the authority to summon posse comitatus–”the power of the county.” Like jury service, posse service is a mandatory duty of the citizen.
so i would argue this broadly counts: for a long time the local lawman pretty much acted independently deputizing locals when more manpower was needed.
Interestingly backstory radio recently did a program on policing and interviewed professor Alasdair Roberts whose take on the rise of police is that it's not due to a failure of posse comitas and small law enforcement groups to do their job, rather it is due to the need to maintain social order in a time of rising industrialization and political unrest between labor and capital.
http://backstoryradio.org/shows/serve-protect-2/
*A fun sidenote about this is that Andy Griffith literally cannot be the Sheriff of Mayberry because in some episodes he is seen presiding over some trials (violation of separation of powers since sherrifs run the justice system) and really acts as a chief of police for mayberry.