The logic of holding it to be objectively moral while adhering to atheism falls apart however.
Why?
Objectively, a human zygote is a human (homo sapiens) in its zygotic stage of being, a human embryo is a human in its embryonic stage of being, a human fetus is a human in its fetal stage of being, a human neonate is a recently born human in its newborn stage of being, a human in their teens is a human in the adolescence stage of being, and a young adult is a human being in early adulthood. What do they all have in common? All or parts of their central nervous systems are developing.
At some point in early adulthood, the human being comes into their own and stops growing and developing physiologically, becoming an adult. By the time a woman reaches 35 years, she is considered advanced maternal age, or a senior mother, if she gets pregnant. At 55 to 60 years, we consider these adults to be geriatric adults or senior citizens, with the human body and mental capacities like memory and reflexes beginning to decline. But if we’re from the species homo sapiens, we’re human beings.
Atheists accept objective morality in the form of ethics that
acknowledge accepted standards of right and wrong or a moral code of right and wrong, especially based on the human dignity and human experience. Being a human atheist doesn’t exclude adherence to objective morality. If anything, it can strengthen it exponentially, because unlike other belief systems, for an atheist, we get one life and this is it. I’ve met many atheists who are more principled than others of different beliefs because there is no afterlife that will make it all better, no devil that makes anybody do something bad, no God that makes someone good, no karma, no dharma, no reincarnation to give more chances to get it right from their perspective. We have this life and if our species and the earth is going to survive, it must be approached with respectful concern and not treated like a wasteland.
This is what I observed from other atheists and it was my own approach to life when I was an atheist.