I also believe someone posted development notes for the game that explained originally humans came from Earth but then due to RPG fashion they went generic, in the original concept only 1 human colony ship made it to the Frontier the 2nd ship never made it, they where thinking of a ship along the lines of the Warden if I remember right... either way I say do what feels right for your vision of your Frontier Universe.

For me I have decided Old Terra exists, but as to where it fits in timeline wise to SF is very questionable. I have decided the SF Humans are descendants of a colony that survived all the stuff that happened in the old GW timelines. This allows me to draw on GW ideas for background to the SF Humans if I need it, but place them far enough away from GW Space to have only a vague knowledge of what went wrong on Terra. It also allows for other domestic creatures from Terra to exist logically, but that is me. Humans in SF can be equally from a different place per passages already quoted by others.

SF timelines like GW or MA timelines can hurt the head a bit. Use what works for you & junk or move what does not work... there is a wealth of ideas, approaches and game visions in the forums that effect timelines. 

I like ideas in Zeb but they often don't work with what I want so I adjust as needed. As I am not handing out timelines to players I look at them as guidelines for me more than them. 

Pick a start date in SF time for your adventures... that is the real start date, then decide what has happened in SF prior (you can be vague) and what has not happened... for instance if you use Zeb, when are you starting, is it before or after the Blue Plague for instance, that becomes your focal point then to order or reorder and move forward from. Basically pick an event you know you want to either be recent history, or be something Players live through adventure wise and that is what you then build the timeline around, you will diverge from "canon" but you would anyway if you think about it as no two refs are going to be doing the same things, we all tweak ideas presented in the books, modules, sci-fi and various game systems to our needs. 

Remember the first rule: Have Fun! 

Population could be about 10k^5 for just the catastrophe surviving humans on the dozen planets they've scattered across... that would allow for about 8 space-aged maglev-cities of roughly one million (in just human populations alone... not counting the other half dozen races heavily populating most multi-species settled worlds along side the humans of the most populated cosmopolitan settlements) on each planet... and still be conservative (and too evenly distributed) enough to drop in another half dozen megaport-cities across the whole sector at ten times that in population each.

Many discussions on this have been held, but -- recap; it is evidenced that a viable genetic population can start with only a couple dozen, but no sane attempt would be intentionally conducted on less than 500 (about the size of a healthy village or oversized hamlet) and preferably around 10k... the later being the equivalent of dropping a planned city, onto a remote location, with no guarantee of return visits, all in one go.

My numbers go from three colony ships of around 30k each, plus one yazirian planetary exodus of around 10million a bit late to the negotiations (many more yazi launch into the deadly fires of the space wars than any other... and similarly to the vrusk, they have their own reserved planets in a sizable exclusionary zone... so some balance is maintained...) to a total reckoned Sector Consensus very roughly between 80^12 and 8^18 after all cataloged sentients are guesstimated, P.F./Fy 1.
That is; by the time the federal establishment is in full force 800 years after the grandly celebrated historical First Contact (Dralisite-Vrusk) and the initial plans for mutual trade and colonization (before humans, or even yazi, were discovered) far too many individuals (organic, clone, unspecified, synthetic, and mechanical) for public records to actually account for have been included into the various communities in a variety of capacities and recognition... and the exact numbers are largely dependent on the prejudices of the local census bureau about the constitute declaration of sentient populations, and their importance to the jurisdictional colonial and civic charter.

And all race in colonization ships to meet near a perfectly good planet and no one notices a nice planet & set up colony ASAP? They just all go back to their ships and twiddle the fingers? 

I have begun to think maybe everyone already was going to colonize & had actually headed out to do that... got to where ever they where going in the Frontier, set up at least 1 colony and then had the "um we are not alone" discovery? Then they chatted & agreed to meet.  

One more thought "19-8PF The area beyond the Greater Morass claims hundreds of exploration vessels. Though it is believed that most Frontier races came from beyond the Greater Moras, all information about their origins have been lost. Exploration of the area ceases and it is named the Vast Expanse."... note the word "believed" and only about 300 or less years has passed between contact? No one is 100% sure where their HWs are? I know this has been debated, but what if they all ended up in the Frontier by accident? Colony ships that went off course? What are the odds 3 races would pop out in the same area of space by accident... just saying that how did the information get lost for ever? I think this is intriguing...  

A technology issue, jump engine issues like you suggest or, sabotage of computers, or MAAO's scenario where The Morden's AI simply abducts people, animals, plants & supplies from Old Terra/GW and jumps away... to where no one knows but away from alien invasion fleet. All that makes some sort of logical sense for Humans trying to do the  "recalculating" of what happened.

A Natural Phenomenon that caused a colony fleet to get lost.

An ancient piece of defense technology? Say a race found a nice spot (probe said it was okay) but when they showed up with their fleet the ancient super science gizmo gets activated and propelled them to the Frontier... oops on triggering automated non lethal defense system.

Anyhow if everyone was Lost In Space that makes more sense to me. If everyone ended up in an unknown Galaxy to boot, there would probably be even less chance of getting back to HWs.

The Frontier Sector is obviously only ONE sector in the galaxy, the one where the four spacefaring races met. They must have come from their own worlds, and there's no good reason why those worlds shouldn't be there, off the edges of the map. And even worse, there's no good reason why those worlds shouldn't be expanding into their own sectors.

So unless you can convince your players to stay in the Frontier as a matter of convenience, you're really obliged to start detailing those other sectors. You need to account for them if you start the Second Sathar War. They're going to be economically and militarily powerful. Either that, or you need to pick Frontier systems that are really the races' home worlds (e.g., Theseus for humans).

The matter of home worlds has always been a thorn in the game's side.