That's curious that you first say that spirituality is needed for altruism, but in the very next paragraph you admit that altruism can arise out of biological processes alone.
Isn't that curious? You see, many atheists in history who were in the Sciences or the Humanities came to the conclusion that altruism is innate within not only humans, but even in common insects.
And yet, the average person who defines themselves as an atheist...doesn't donate time, talent, and treasure. This is a real problem to solve for the nonprofits who would love to get new funding, have atheists volunteer, and have atheists donate their talents to the organization.
So my theory is that the average person who claims to be an atheist, really doesn't understand atheism at all. If they only read about evolutionary biology AND altruism within it, then it would be self-evident that all of those species are born with an instinct to act altruistically.
Then it naturally follows to volunteer.
My guess is that some atheists who are vocal and trying to get atheist churches formed and who are already volunteers have intuited this. But they're very rare birds based upon the philantropy studies. The studies show the higher the level of religiousity (devotion to their spiritual system, be that devout Buddhists or Muslims, or Christians, etc), the more they volunteer, donate, and offer their specialized services. The lower the score of religiousity, the lower the amount of giving.
This is why I maintain that atheism is not defined as a lack of belief in god. atheism is defined as an active disbelief in GOD. But it doesn't stop there. The natural unfolding of atheism is to seek an alternative philosophy due to the absence of GOD. If there is no Supreme Creator, then each human is responsible for each individual. But then it follows that humans are just another higher species, the apex predator, and so there is a responsibility to care for the other species to manage the ecosystem.
That altruism. It's imprinted on the mind of each human being within atheism.
If one only mantains that atheism is the absence of belief in god, then this form of pop atheism is worthless and meaningless.
Pop atheism attempt to defy Christians and defines itself as "not them". Well, instead a believer, and especially a passionate believer develops their own philosophy. Otherwise one's idea of Self is defined through the lens of Christianity. How weird and lame for atheists to do that.