Buddhism: Morality & Biology I have relapsed out of Buddhism two times in my past. Currently, I am between reconciling and rejecting it yet again. I decided to make this post to ask questions that I feel will help me either reconcile or reject Buddhism. I have categorized my questions into two areas: morality and biology. Before I write my questions below, please note the following information: I was/have been part of the Theravada Mahasi Sayadaw tradition under Yuttadhammo Bhikkhu's teachings and have studied both Buddhism and science in the past as a vigorously dedicated layman in both fields.
Section 1: Morality

How is it possible that those who achieve Nibbana are morally infallible?
How is it that certain mind-states are objectively unskillful (evil) or skillful (good); wouldn't that be subjective to say?

Section 2: Biology

How can there have been past Buddhas if civilization/Homo sapiens evolved recently (archeologically speaking)?
How do the planes of existence and rebirth make sense amid the heavily (as far as I understand) supported theory of evolution?

Thank you for your time and may you be well.
OP: How is it possible that those who achieve Nibbana are morally infallible?

Once ignorance and the rest of the ten fetters have been uprooted, the enlightened are free from mental defilements, causing them to not have any thoughts based on greed/ lust, anger/ aversion or delusion.
Please read the Buddhism part of this answer for more details.

OP: How is it that certain mind-states are objectively unskillful (evil) or skillful (good); wouldn't that be subjective to say?

Objective morality in Buddhism is based on compassion and the notion that you will feel remorse hurting others, because you know how it would feel if others hurt you.
Removal of remorse is needed to brighten the mind, which is needed for the rest of the path to enlightenment.
Please read this answer for details.

OP: How can there have been past Buddhas if civilization/homo sapiens evolved recently (archeologically speaking)?

It's not clearly answerable.
Some texts imply the Big Bounce model of the universe where the universe expands, then contracts and becomes destroyed, then expands again. So, past Buddhas may even have existed in past universes. Please see this answer for details. But this is only a conjecture.
Some people see this only as an exaggeration added to the texts.
Either way, this does not make any difference to the Buddha's message and teachings. It is not important compared to the path to the end of suffering.

OP: How do the planes of existence and rebirth make sense amid the heavily (as far as I understand) supported theory of evolution?

Please read this answer and this answer to understand what rebirth is in Buddhism. It's the rebirth of individuality and not really the rebirth of the individual, since there is no soul or permanent consciousness in Buddhism.