Many popular science articles cite high sequence conservation of mitochondrial DNA and the Y chromosome among living people as evidence that all humans are descended from a single male and single female human ancestor.
Is sequence evidence necessary to make this assertion, or does it follow tautologically from the definition of speciation? What would a speciation pedigree look like for a species that does not pass through a single-individual bottleneck?
The assertion is not tautological because the definition of speciation does not involve single individuals.1
From Wikipedia:
Speciation is the evolutionary process by which populations evolve to become distinct species. [emphasis added]
Speciation is a process of populations, and it is not necessary for only a single pair of sexually reproducing organisms to be the sole ancestors of a species.
There did exist one female ancestor, dubbed mitochondrial Eve, who is the most recent matrilineal ancestor of all living humans. Because mitochondria are passed from mother to offspring, and do not recombine, that individual's mitochondrial DNA is the ancestor of all human mitochondria.
Similarly, there existed one male ancestor, Y-chromosomal Adam, who is the most recent patrilineal ancestor of all living humans. The Y-chromosome is passed through the father, and most of it does not recombine, so that individual's Y-chromosome is the ancestor of all current human Y-chromosomes.
Note that mitochondrial Eve and Y-chromosome Adam do not have to and almost certainly did not live at the same time.2
The age estimate for mitochondrial Eve is about 150,000 years ago.
The age estimate for Y-chromosomal Adam is about 200,000-300,000 years ago.
These time frames are around or after the origin of anatomically modern humans, so it is correct to call these ancestors humans. (This is didn't necessarily have to be true, it's possible these individuals could have predated the separation of humans from other species.)
Finally, is sequence evidence necessary to make these assertions? For knowing the existence of mitochondrial Eve or Y-chromosomal Adam, no; their existence is a logical consequence of constructing a pedigree (see coalescent theory). However, for dating these ancestors and being able to claim they were human, some examination of the DNA is necessary, and DNA sequencing provides the most precise estimates.
1 We'll ignore asexual organisms, where the definition of species becomes especially difficult.
2 I will add these additional cautions from the mitochondrial Eve page, that equally apply to Y-chromosomal Adam.
Caution: This does not make Mitochondrial Eve the first woman, or the first human, or the first member of a new species. Further Caution: This does not mean that other women alive when Eve was do not have descendants today; they simply do not have living descendants who are descended only through female links. Yet Further Caution: If a person were to be discovered whose mtDNA showed a pattern of mutations of greater time depth, then the status of Mitochondrial Eve would be reassigned to the most recent female ancestor shared by both that person and the person we now call Mitochondrial Eve.
