The dog genome compared to the wolf genome shows signs of having undergone positive selection, these include genes relating to brain function and behavior, and to lipid metabolism. This ability to process lipids indicates a dietary target of selection that was important when proto-dogs hunted and fed alongside hunter-gatherers. The evolution of the dietary metabolism genes may have helped process the increased lipid content of early dog diets as they scavenged on the remains of carcasses left by hunter-gatherers. Prey capture rates may have increased in comparison to wolves and with it the amount of lipid consumed by the assisting proto-dogs. A unique dietary selection pressure may have evolved both from the amount consumed, and the shifting composition of, tissues that were available to proto-dogs once humans had removed the most desirable parts of the carcass for themselves. A study of the mammal biomass during modern human expansion into the northern Mammoth steppe found that it had occurred under conditions of unlimited resources, and that many of the animals were killed with only a small part consumed or left unused.