Childbirth is complicated in humans relative to other primates. Unlike the situation in great apes, human neonates are about the same size as the birth canal, making passage difficult. The birth mechanism (the series of rotations that the neonate must undergo to successfully negotiate its mother's birth canal) distinguishes humans not only from great apes, but also from lesser apes and monkeys. Tracing the evolution of human childbirth is difficult, because the pelvic skeleton, which forms the margins of the birth canal, tends to survive poorly in the fossil record. Only 3 female individuals preserve fairly complete birth canals, and they all date to earlier phases of human evolution. Here we present a virtual reconstruction of a female Neandertal pelvis from Tabun, Israel. The size of Tabun's reconstructed birth canal indicates that childbirth was about as difficult in Neandertals as in present-day humans, but the canal's shape indicates that Neandertals had a more primitive birth mechanism. A significant shift in childbirth apparently occurred quite late in human evolution, during the last few hundred thousand years. Such a late shift underscores the uniqueness of human childbirth and the divergent evolutionary trajectories of Neandertals and the lineage leading to present-day humans.
"... during the last few hundred thousand years ..." is a pretty vague time-frame.
With luck* we'll get more paleontological data about this one day. My personal and amateur hypothesis is that changes in female pelvic morphology will correlate closely (but lagging slightly) with neonate cranial volume.
* "luck" = bringing sufficient peace to northeastern Africa for fossil-hunters to range freely over long periods.
"The birth mechanism (the series of rotations that the neonate must undergo to successfully negotiate its mother's birth canal)..."
Holy crap! The little bastards spin as they squeeze out? Look, I don't like the idea of causing the female I might accidentally develop feelings for a huge amount of pain to begin with. It's not like these ginormous heads seem to actually translate into a higher-than-chimp-level intelligence as they were supposed to anyway. Let's just pioneer a cloning chamber kind of deal and cut the ladies some slack. No more spinning big-headed freak babies.
It's hard to understand why the birth mechanism is "more primitive" if it was no harder for Neandertals to give birth than modern humans. Why not just "different"?
Another possibility is that the Neandertals managed to evolve an even more immature level of neural development than moderns, so there was less selective pressure on the birth canal. (This might well explain the strange shape of their faces, it might be associated with a general developmental change that included more immature birth.)
It's hard to understand why the birth mechanism is "more primitive" if it was no harder for Neandertals to give birth than modern humans.
"Primitive" just means that they retained the ancestral birth mechanism.