Marshall et al's discovery was in January, 1848;
the reason the emigrants became known as Forty-niners is that is when the news really got around the world, but the rush was on within weeks, with emigrants coming from across the Pacific Slope, Hawaii, Mexico, and South America.
And even if it wasn't Marshall in January, given the amount of activity in the Sierra foothills after the end of the US-Mexican War, somebody would have come across gold in one or another of the multiple rivers, creeks, and streams that feed into the Sacramento.
Cripes, it might have been one of the Donner Party survivors.
Best,