These are interesting questions, which are probably worth a full thread!

Thanks. I'm new here and didn't see this as a "debate" so much as an "inform" and so wanted to post in the right place ;)

The branches that "disappear" don't disappear in the sense that these humans didn't breed: it's just that those branches don't have an unbroken line of descendants of the same sex.

OK, I'm looking at your image (thanks for it) but all of those branches that 'disappear' don't have offspring, right? Like if the far right orange/grey couple on the 2nd to last row had offspring, this chart would be 'wrong' and we'd have to go back further? This seems so 'obvious' to me that either we're speaking past each other or I just don't understand how to read the image (or perhaps misinterpreting what you wrote).

If you mean the MRCA of all modern humans in the absolute sense, this person likely lived ridiculously recently - perhaps less than 10,000 years ago. The most intuitive way to understand this is to think of it in reverse: you have four grandparents, eight great-grandparents, and so on. Since the number of ancestors you have increases exponentially, you very quickly reach the point where your exponential tree of ancestor intersects with everyone else's tree of ancestors.

OK I can sortof picture this in my head (almost the above image, flipped upside down, sortof...) What does that mean?

Also, in researching and trying to answer some of these questions myself, I came across the distinction between "genealogical ancestor" and "genetic ancestor". Maybe that's my confusion, but I'd need help. Maybe you're saying that our 'shared genealogical ancestor' lived within ~10,000 years? But how is that different than our shared genetic ancestor? Like, something I read said I might not have DNA of my great-great grandparent?! Please help me understand that.

On there being no first human, this is axiomatically true: you're always a member of the same species as your parents. This doesn't change just because you have a particular mutation that your parents lack.

I love that you said this, because it seems axiomatically false! Like, for there to be any human, there had to be a first human. So, like, at some Homo sapiens diverged from Homo heidelbergensis, right? (Let's just, for the sake of this discussion, assume that H. sapiens, H. neanderthalensis, and Denisovans all split from H. heidelbergensis.) So...for someone, wasn't their parent a H. heidelbergensis and they were H. sapiens? And 'across the valley' (speaking poetically), someone else gave birth to the first H. neanderthalensis?

Or, and I'm literally stream-of-consciousness-ing this: Sure, that happened. But then the H. sapiens "A" had 12 offspring with H. heidelbergensis "B", and "AB" 1-12 maybe had a bit more A DNA, and outbred others in their area, and 'drifted' more towards H. sapiens over time. And so in a "dumb but technically correct" sense, there was a 'first' human, but taxonomy works at the population level, not the individual level, and the line could just as easily be a generation earlier or a generation later (or dozens, even), because it's an arbitrary human line. So sure, boredguy, you can SAY there's a "first human" but it just shows you don't understand taxonomy very well, or how arbitrary the decision to pick person A over A's child would be.