Complex sentence: This instinct was salutary in pre-agricultural civilization because opportunities to eat were far more rare, and since fat is more nutrient-dense than other sources of food, its presence in the diet and as stored adipose tissue aided survival in periods of food scarcity.
Split into simpler sentences: This instinct was salutary in pre-agricultural civilization because opportunities to eat were far more rare. Since fat is more nutrient-dense than other sources of food, its presence in the diet as well as its presence as stored adipose tissue aided survival in periods of food scarcity.