Realistic Star Faring
applied to the Traveller setting.
I forget the exact numbers but IIRC earth has been around c. 5 billion years or so and sentients only about a million years, agriculture only ten thousand years and potentially pre-stellar for only a hundred.
So even with an assumption of one potentially earth like planet/moon per system then if they all formed at different times I think it comes to around one in five thousand systems where the planet has been around long enough for life to have started and long enough for life to reach cavemen.
If the average Traveller sub-sector has say c. 30 systems then a full sector on average has c. 500 systems so that would mean one system had reached an alien version of neanderthals per 10 full sectors.
Sentient aliens with agriculture would be around one per 1000 sectors and pre-stellar aliens one per 100,000 sectors.
The entire Imperium is like 20 sectors.
Those sums are probably very inaccurate but they illustrate the idea that the first species to interstellar is going to find sector after sector of planets that are only partway to being livable and of those that are livable most will only be partway to producing a sentient species.
So if humans were first then I think the logic of it is the first star settlers would expand out exploring in all directions and as the people who do the colonizing will be the sort of people who want to do it then if they produce similar kids the process becomes self perpetuating.
In the Traveller setting most of this early colonizing wouldn’t actually be colonies to live on; it would be building stepping stones for refueling for the next jump but it would be necessary to get tothe next step and the people doing it don’t stop because at the beginning of the process people were selected who were the sort of people who couldn’t stop.
Eventually they’d arrive somewhere that had a real world and pre-sentient alien monkeys. Generally I think taking the top niche in the food chain would prevent any native pre-sentient evolving naturally so that species would be blocked unless quarantined but maybe the humans would uplift them just for the novelty. They’d uplift planets for sure or at least start the process.
So they’d spread everywhere, living in domes a lot of the time while speeding up planetary evolution. In most places their presence stops a native sentient developing but maybe they uplift some themselves gradually adding Wookies and Ewoks to their ship’s crews along the way and just keep spreading because they can’t stop.
So one option – humans are first and spread like a wave in all directions until after 50,000 sectors or so of colonizing mostly rocks and the occasional monkey world they eventually bump into another star faring sentient species.
Which is a bit like the Ancients in the Traveller setting.