How are humans capable of altruism?
It is thought that humans can not be altruistic. Most social learning theories think that the only reason we do things to either get praised to make us feel good about ourselves or pass things on.
An example is caring for your own children. The fact that you look after them is thought to be because you want to pass on your genes and to help evolution. Also power in numbers is a large argument. You will look after them in the hope that if danger comes you are less likely you be attacked. It is thought that this is an innate behaviour that was developed to help the human species carry on surviving.
However, if you take what is thought to be a selfless act e.g. caring for the elderly it is thought that this is to be praised by others. It is well known that when you are complimented for anything hormones like serotonin are released making you feel happy and good about yourself. It is thought that this is the reason for doing things you just don't realise it as it is an unconscious behaviour.
To answer your question. Altruism doesn't exist. However, selfishness does. Humans are not hardwired to be selfless but are all unconsciously selfish but as they want to seem socially desirable perform what are thought to be selfless acts but with unconscious selfishness.