The marathon is dominated by African runners. David Epstein in a relatively recent interview mentions about a specific tribe in Kenya called the Kalenjin, "There are 17 American men in history who have run under 2:10 in the marathon...there were 32 Kalenjin who did it in October of 2011". The times and number of African runners reaching those times times rarely achieved by their racing counterparts is impressive. Below is a graph showing the top 50 times recorded by Association of International Marathons and Distance Races (AIMS) over the past few years.
The Boston Marathon is perhaps the most sought after race for marathon distance runners. At least in the US, qualifying for the Boston Marathon can be the pinnacle achievement for an avid runner's career. As one would expect, this race draws runners from all over the world who seek the prestige and purse of winning the Boston Marathon. Over time the winners of this race have changed, as arguably, the physiology (and arguably culture) of runners has become more of a factor since access to the race has become easier over time (for more on Kenyan physiology and culture as running determinants, see this Radiolab podcast). Like in all marathon races, the times are getting lower and African runners have shown a clear dominance over the last several years. In the graph below you can see the descent into Boston Marathon winning times that 30-40 years ago were unimaginable.
Here are the same times and years broken out by continent instead of country. Notice the break in dominance of winning this marathon from Europe/North America to Africa in the mid 1980s. Prior to this time, the race enjoyed a larger amount of variety in countries/continents winning the race.
The grayish line intersecting these points is basically a confidence interval (95% confidence interval). One could interpret any point within this grey area as a time that would not be a statistical outlier or a time that could be expected to win the Boston Marathon. The interesting thing about this graph is how the gray area is now widening in the past few years. This is partially because of the fastest marathon ever run is included in this graph (This was done by Geoffrey Mutai in 2011, which did not count as a world record formally because of the change in relief of the Boston Marathon). Notwithstanding this time, we also see times more recently that have historically been run by North Americans, Australians, Asians, and Europeans. Though it is clear that Africa demonstrates clear dominance in this marathon and others, the times that African participants have been running are not insurmountable from a historic perspective.
This "widening" of race time expectations I believe provides opportunities to continents and countries who have run races at this speed in the past. The question now becomes how many runners in these continents/countries can currently run at these paces. There are some. Both Ryan Hall and Dathan Ritzenhein are US runners who have run marathons in 2:08, which would make them both very competitive with the recent winners of the Boston Marathon.
Stripping out the African countries we can see the times of other continents over the past several years. In fact all of these times fit into the range of "expectation" (95% confidence interval) of the most recent races.
Running this fast a race must take into account multiple other factors such as weather, injury, etc. However, based on the data of previous races, the times produced by these runners in the graph above would have been very competitive if not won previous recent years' marathons. There may not be as many challengers in other continents, but those challenging African runners stand a chance. More recently if a non-African runner had run the Boston Marathon in what would possibly have been their best race, they would have had a great chance at winning.