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.