I understood your point. If we are going to assume that altruism is a result of Darwinian evolution, there is no reason to assume it only began in the human lineage. Hunter-gathering is just one of the phases of man's lifestyle. The fact that it was probably the first phase doesn't really mean it would be more important evolutionarily than the 2nd phase or third phase, etc. But every time someone comes up with a human personality trait that they want to explain through evolution-you can guarantee the first thing they will say is..."well, but when we were hunter gatherers, it would have been an advantage to have the middle finger be slightly longer than the ring finger, because the middle finger was used for dislodging seeds from the Savannah grasslands..." Hunter-gathering is the magic explanation for everything.
To clarify graphically why people focus on the hunter-gatherer lifestyle, as opposed to the other stages of human existence, look at the time line below. This covers the approximately 6 million years since our ancestors split from the ancestors of chimpanzees. The yellow portion is the time we spent as foragers or hunter-gatherers. The red portion is everything that's happened since the invention of agriculture.
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