All the big news was Usain Bolt winning his 9th gold medal in the sprints (I can't embed that video), and the Americans being disqualified for an ever-so-slightly premature handoff. But there is other interesting news here. Runners of West African descent have dominated the sprints for decades. But the Japanese team took the silver. If you watch the video, it was close to even-up among the top six teams at the final baton exchange. The Japanese anchor runner, Asuka Cambridge, surged equally with Bolt at the beginning, until the Jamaican hit top speed and pulled away from the pack. The American team and the Canadian team, composed entirely of blacks of West African descent, did gain on him at the end, but he ultimately held on.
It is important to note that this is a team event. Cambridge is half-Jamaican, so if we were inclined to be dismissive we could regard him as a one-off exception based on that. The 20-year-old phenom Yoshihide Kiryu I heard about briefly 2 years ago, and one could possibly write that off as a single exception as well, the lone outlier in a fairly populous and highly-competitive country. But Japan has two more, Ryota Yamagata and Shota Iizuka, who ran the first two legs. They look to my eyes to be very slightly behind everyone else at the halfway point, but not by much. The Japanese may depend on outliers (as if all 100m specialists weren't outliers to begin with) for medal hopes, but they are not outclassed entirely. The other teams in the finals were Great Britain - which has one runner of mixed Iranian and Moroccan descent among the West African others - Trinidad & Tobago, and China.
The Chinese team, which finish 5th-6th is also rather obviously not of West African descent, but they also have an enormous pool of 1.3B people to draw on, and a wink-wink forced breeding program of athletes. (Yao Ming was not accidental. Though he seems to have become a very decent individual after it all.) Still, they're worth noticing. The Asian teams also favor a type baton pass, developed originally by the Japanese, that creates a marginal advantage.
One genetic advantage of West Africans is a greater concentration of fast-twitch muscle fibers. Many Jamaicans have a strong version of the ACTN3 gene, intensifying this. Kenyans and neighbors who live at high altitudes above the Great Rift Valley have more slow-twitch fibers, which is advantageous in distance running. But those distance runners have some other genetic advantages as well, including narrow ankles and calves, without loss of thigh strength, and this highlights an important point. Genetic "advantage" is real, but can be an elusive thing. Ashkenazi Jews have enormous numeric and verbal advantages, but their visual-spatial abilities are a tick below average. In some settings they will stand out, in others, they are just regular guys. Since the basic muscle-fiber difference was discovered in the 1970's there has been some refinement in the understanding, plus a lot of crap from fitness "experts."
There is more than one way to skin a cat. Improved oxygen-processing or lung capacity are also genetic advantages that other groups can parlay into distance running. It wasn't that long ago that there were Caucasian sprinters, especially Russians, who were medal hopefuls. Advantages can be slight, and there can be other ways of navigating to the top. I don't know what it is the Japanese have got going on. There may some specifics, or it may be a subtler advantage that they have a significant population with no major athletic negatives to draw from.