As humans evolved to hunt, gather and share food, cooperation provided a key to our success as a species. While chimpanzees and other primates sometimes share food, humans stand out. As hunter-gatherers—the subsistence strategy that all humans followed until the invention of agriculture—our survival depended on daily sharing of food between unrelated adult males and females.
I’m not an expert but if I recall correctly, fully bipedal locomotion, erect posture and later the development of agriculture were all possible thanks to a series of changes in environment. So in my understanding that paragraph is suggesting that pair bonding provided advantages obtaining food resources which allowed early hominids to thrive in more areas.