A Mitochondrial eve is defined as “the most recent woman from whom all living humans descend in an unbroken line purely through their mothers”. The most recent woman.
That’s not the first human, since she clearly has human ancestors.
There is no first human because evolution ocurrs as a transition. Our ancestors slowly evolved and evolved, gradually acquiring human characteristics until species emerged that became gradually very similar to us. At what point do you say humanity starts and our not-exactly human ancestors end? Even today, no one is the same exact human as me, because my genes mutate so I’m not a perfect combination of my parents genes, just like no one else is, and all humanity except subsaharan Africans have Neanderthal DNA, yet we’re all equally human aren’t we? There is no clear cut point of what constitutes a human and what constitutes a close-but-not-quite
Mitochondrial Eve is the most recent female ancestor from whom all humans descend from their mothers. But not the first human at all