I think that the boundary case is that we at least have a common ancestral single cellular life-form. To my mathematically inclined approach, anything else (such as a the earliest common ancestor being close-to-human, or whether there needs to be a male or female one) is not a logical debate, but rather a biological investigation.

> I think that the boundary case is that we at least have a common ancestral single cellular life-form.

It is possible to imagine several different single cells being our ancestore. If we pretend a lightning strike in the ancestor primordial DNA soup sparked life, then it could have sparked life in millions of cells with a single lightning bolt. There could even have been several different puddles of DNA soup.

In theory it doesn’t, but because the effective population size of humans until recently has been very small, the probability is very high that the last common maternal ancestor lived relatively recently.

An interesting aside is everyone who lived more than 2000 years ago is either your ancestor or has no living descendant. This wasn’t true before 1803 because the Tasmanian aborigines had been genetically isolated since the end of the last ice age [0].

0. https://en.wikipedia.org/wiki/British_colonisation_of_Tasman...

Is this saying that every single human living today has dna from every ethnic group that is still around? I find it hard to believe.

Yes and no. Just by the time you get back 2000 years you have so many ancestors that you are related to everybody alive. There are no isolated human populations so everybody is related to everyone else.

Actually you only have around 1000 effective ancestors from that time as you don't inherit DNA from every ancestor.

But I feel this logic completely ignores "inbreeding" even if it's to the 5th, 10th or 20th degree.

It's like seeing a human as the root of its ancestors tree, and saying that this tree is well balanced all the way to the nodes at depth 100 or so. But surely it's not a well balanced tree, more like a directed graph with many sideways connections. There could be multiple clusters, lineages, more or less narrow, coming from the bottom and reaching the top.

Yes this is true at the genealogy level, but because of the way meiosis works once you get back more than about 12 generation some of your ancestors don’t pass any of their DNA down to you.

I really don't think my two Swedish farmer stock parents had massive African, Native American and Asian heritage. 23andMe didn't mention that either.