Selfishness is the default setting, and is easy to explain in evolutionary terms. Unselfishness is what usually surprises us when we see it in animals.
Surprises you perhaps. Over 90% of plants rely on cooperation with mycorrhizal fungal networks. The evolution of lichen, a partnership between fungi and lichen, has happened independently tens of thousands of times. Our very cells rely on a once-alien cell with its own DNA that is now called the "mitochondria" for our very survival. Even still, the majority of cells in the human body are usually non-human. We literally couldn't eat a single thing without the complex multi-species microbes we rely on.
Cooperation is at the basis of all of nature. The difference between "competition" and "cooperation" is usually a matter of perspective and human bias. They are two sides of the same coin. We fit details about nature into one of those two frameworks to uphold a specific worldview we find useful for organizing information. Historically we've favored the competition framework, but take a look at the Annual Reviews journals for a field like Ecology, Mycology, Biology, etc and I think you'll see this is quickly changing
Cooperation is at the basis of all of nature. The difference between "competition" and "cooperation" is usually a matter of perspective and human bias. They are two sides of the same coin. We fit details about nature into one of those two frameworks to uphold a specific worldview we find useful for organizing information. Historically we've favored the competition framework, but take a look at the Annual Reviews journals for a field like Ecology, Mycology, Biology, etc and I think you'll see this is quickly changing