When the dust settles on the athletics world championships on Monday morning, one intriguing question will loom largest: whose star will be shining brightest come the 2020 Olympics in Tokyo?
That question has been redundant in recent years, ever since a 21-year-old from the small township of Sherwood Content in northern Jamaica electrified crowds at the Beijing Olympics in 2008.
But the tall guy is taking his final bow in London and so departs the athlete with more magnetism than any other since he took to the grandest stages.
No one has captivated audiences like Usain Bolt. When he walked out before an event, you knew about 78,000 pairs of eyes in an 80,000 seat stadium were focused on him.
He knew it, too, and played up to the crowd, with his gestures, grins and fist pumps with the young helpers before and after his races. Not to forget the now tedious Lightning Bolt move he pulls with great regularity.
In short, athletics, a sport not short on polarising politics and contentious figures, needed him as much as he needed the sport.
So what now? Bolt leaves with eight Olympic and 11 world championship golds (with a last one in the 4x100m relay possibly to come around 8.50am tomorrow, so do yourselves a favour and watch the finale for one of the all-time greats), and seven world records over 100m, 200m and sprint relay.
Who will arrive in Tokyo as the new standard bearer for athletics?
There are some names who stand out. First a question: does it need to be a track athlete? If not, which field athletes are worth a thought? Hammer throwers? No. Discus, shot put, triple jumpers don't resonate, due respect to our own Tom Walsh.
If you work on the basis that running is the purest form of athletic pursuit - after all, you don't need a ball or an implement to hit it with, or a pool or gymnastic mat - it must be a track athlete.
The first individual post-Bolt final was won by a Turk, Ramil Guliyev, who sped to the line for a thrilling, upset victory in the 200m final yesterday. The new age has begun, even before Bolt had left the building.
How about a man who wasn't even in London, Kenya's brilliant David Rudisha, winner of the last two Olympic 800m finals, world champion and world record holder with 1m 40.91s at the 2012 Olympics? A contender, for sure, but he's already 28, remember.
Possibly Christian Coleman, the pocket rocket 21-year-old from Atlanta, who was second in the 100m final and is sure to shine in Tokyo.
No, the athlete closest to taking over the throne is South Africa's Wayde van Niekerk, who also finished second in the 200m final yesterday.
The 25-year-old from Cape Town is a two-time world 400m champion and the Olympic champion - winning that title in Rio from lane eight, which had never been done before at an Olympics or world champs, and in a world record 43.03s.
He's a charismatic figure, and came within a blink of a stunning double in London. Winning the 100m-200m double? Very doable. Both are flat-out sprints. But the 200m-400m pair? That's seriously challenging, requiring two different types of running.
Van Niekerk, a Liverpool fan but don't hold that against him, is inclined to tweet sentiments like 'God is Power'.
But here's a bottom line: can anyone really fill Bolt's shoes?
No one is irreplaceable, but Bolt, in sporting terms, might be the closest to it for the moment. Others may eclipse his achievements. It'll take some doing, but it could happen.
But how about the overall package he brought to the sport: the track brilliance, the colourful persona, the ability to engage with a stadium full of fans. That's a trick you can't teach.