The Bible claimed Adam was the first person but what does science say?
Scientists sometimes speak of the mitochondrial Eve or the Y chromosome Adam, but these were not literally the first humans, but rather the only humans whose maternal or paternal lineage still live on in every man and woman today.
According to Richard Dawkins, there was no first person because evolution happens so gradually that no single individual was sufficiently different enough from their parents to be considered a member of a new species (i.e. Homo sapiens).
But if there was no first member of a species, how can scientists measure how long a species has existed (a standard measure of evolutionary success)?
The simplest definition of a species is a group of organisms that are reproductively compatible. How reproductively compatible? At least more compatible than modern humans and Neanderthals were since these are often considered different species: The male offspring of such unions were infertile if they had a Neanderthal father.
So a species has clear boundaries when used on contemporaneous populations. We can often draw geographic lines telling us where one species begins and ends in space, but how do we draw such lines in time? When does a species begin and when does a species end? If everyone is the same species as their parents (in the sense that they’re technically reproductively compatible if close enough in age), then drawing a line between parent species and daughter species seems arbitrary and unscientific.
I propose that the first member of a species is he or she who could not produce fertile offspring with any fertile member of the parent species. So if modern humans evolved from Homo heidelbergensis, the first modern human is the first fertile person who could not produce fertile offspring of both sexes with at least some (likely the most archaic) fertile H. heidelbergensis. Similarly, the first heidelbergensis is the first fertile one who can produce fertile offspring of both sexes with at least some fertile erectus etc.
Pumpkin Person 
Let’s say you track offspring fertility across 10,000 generations. Define k as the generation of interest, and k+N as the first generation after k that is not cross fertile. So N is how many generations after k at which cross fertility is lost.
It’s possible that for k=100, N=2000. And for k=101, N=2500. And for k=200, N=9000. And for k=8000, N=500. So cross fertility degrades over time due to drift. But this doesn’t make the division point between species any clearer. Even if there may be periods of time of rapid change and for those periods N is smaller than usual. Just as blue transitions to green in a rainbow, where you mark the division between rainbow colors is in the end arbitrary.
More generally where biologists mark species divisions is instrumental, defined towards a particular purpose you have in mind. So it often varies by subfield in biology. There’s no way around this, many things are this way in nature.
As for mammals, the rule of thumb is species lose compatibility after about 2 million years (from memory, can’t recall exact number) of divergence. But of course this varies widely, and gene flow can continue across barriers (eg polar bears and brown bears) for a long time, which can maintain cross species fertility for longer than it might have been otherwise.
Sorry. Realized your point here was the first person is measured as against contemporary living human population. I guess that does work ok.
Though I suspect how far back you could go would still vary somewhat depending on which current human population you choose. For example Khoe San might differ (either direction, to be clear) from, say, Native Americans.
Well i wrote the article going from derived species to ancestral species, but let’s try it the other way:
The first sexually reproducing organism was the first member of the first species (defined by reproductive compatability)
Now i would say we could define the first descendant of that organism who it wouldn’t be reproductively compatable with (had they lived at the same time) as the first member of the second species.