I think Neera is correct to say in response to Michael that there are “a whole lot of other premises.” This is why it is important to take Rand as making insightful suggestions, but not as offering a finished product. But if you think any and every version of naturalism in ethics must fail, then it does seem implausible. That is the basic issue here.
Also, speaking of Cicero, I would like to add my favorite statement from him:
Everybody, however, must resolutely hold fast to his own peculiar gifts, in so far as they are peculiar only and not vicious, in order that propriety, which is the object of our inquiry, may the more easily be secured. For we must so act as not to oppose the universal laws of human nature, but, while safeguarding those, to follow the bent of our own particular nature, and even if other careers should be better and nobler, we may still regulate our own pursuits by the standard of our own nature.
– Cicero, De Officiis