How far back before we all share common ancestors is a genuine question, surely? Within a small population - as the foundation population of our species must have been - the number of generations further back to ancestors in common isn't far. Having a specific individual that all our species can count as an ancestor doesn't seem outrageous to me although it would only be found by studying our genome; finding the physical remains of that individual rather than the continually reproduced dna that came from them isn't going to happen. Isn't the reason a past species has appeared to be supplanted without intervening forms that evolution within groups isolated by geography and perhaps by territorial behaviour, results in more rapid differentiation than within big populations and when it produced traits that made for competitive success and the ability to colonise new territory, the diaspora of a new form (possibly not interfertile and thus species) followed. The prior intermediary forms occurred in relative isolation and in small overall numbers and most places simply don't suit successful fossilisation.
I haven't been keeping up with the debate here but I'd like to make the point that genes mutate within individuals to be 'dispersed' through a population through time by being passed to descendents. When that new gene has 'fixed' ie effectively carried by the whole population, that whole population has that mutant individual as an ancestor in common. Whilst the same mutation might theoretically occur independently in different individuals, the surest way for a beneficial mutation - for defining traits that predate homo sapiens diaspora - to fix in a gene pool is by inheritance whereas multiple independent mutations, being unlikely random changes within a genome that's quite large looks most unlikely for the spread of a specific gene into a foundation population. We have genes that are universal to our species (noting that sub-populations could lose some and still be homo sapiens) and they arose from individuals who passed them through their descendents through the intervening generations to us. We surely have multiple ancestors in common but one may have been further back than the others.