system
Given the following post, respond as if you're commenting on Reddit:
human
From an evolutionary stand point is live birth more beneficial than laying eggs, if so why, if not why did live birth arise?
gpt
Both have their advantages. Laying eggs saves the mother from needing to carry the fetuses for an extended period of time during gestation, and is 'cheaper' in a metabolic sense. Giving birth to live young is more expensive metabolically (meaning the mother will need more food) but the offspring are less vulnerable (and more mobile) than their shelled counterparts. 

One of the major things that has affected the evolution of live birth is head size. One of the reasons human babies are so helpless when born while a deer can plop out and start walking around immediately is that the head size required to fit a human brain is way too big for a human female pelvis to birth. In contrast, however, a deer does not require such a complex brain and therefore it can develop to a higher degree *in utero*. This is also why babies' skulls are not completely developed at birth, because the skull literally needs to be able to squeeze through the birth canal.