Humans have always eaten meat, and for good reason. There’s enough archaeological evidence to suggest that ancient humans were eating meat on a regular basis over two million years ago. Some scientists even believe that meat is the reason why humans have the cognitive capacities that we do today. Human brains require about twenty percent of the body’s energy to function, which is a relatively large amount compared to the three to four percent that dogs and cats expend on brain activity. With all its calories and protein, meat is a much more energy-dense food than the plants and fruits that early humans scavenged for. Meat could have provided the necessary energy to develop such a complex brain. Meat is not unique in that it is an energy-dense food; the science journalist Marta Zaraska points out that peanut butter could have worked just as well. The difference is that meat was highly available in the environment of early humans, as there were many large predators , such as sabre-toothed cats, that killed large prey. These predators would often leave behind some of their kills, and humans would scavenge from the left behind carcasses. Humans also learned to hunt and kill their own food, but many scientists hold that scavenging was the original method of acquiring meat for humans. Either way, meat played a large role in the diet of early humans and provided highly necessary nutrients for the complex cognitive functions that early humans were developing. Thus, one can see how meat has always been a key component of the natural human diet.  One can also make a case for the fact that eating meat is natural based on human anatomy. The human digestive tract is vastly different from that of obligatory herbivores, as it is much shorter with enzymes adapted to breaking down meat. Our teeth are also indicative of a naturally omnivorous diet, as we have flat molars for eating plants, fruits, and seeds, but also sharp incisors for tearing meat. 
The way that many people eat meat is not natural at all, and these unnatural food practices are the biggest threats to the climate. The way that humans eat meat in the twenty-first century is vastly different than the way that primitive humans ate meat. Factory farms, which are where 99% of meat in the United States comes from, are completely artificial human constructs that have no basis in natural meat consumption. Animals are kept in extremely close quarters in foul, unhygienic, cruel cages and are fed unnatural diets that are packed with antibiotics because the animals would fall ill from their horrible living conditions if they weren't fed antibiotics. Even the animals themselves are much different than the wild animals that primitive humans ate and kept, even once primitive humans domesticated livestock. The animals that we eat today have been selectively bred and genetically altered for decades to grow freakishly fast and to pack on more fat and muscle than wild animals. Primitive humans killed or scavenged the meat that they ate, with that meat coming from wild animals. There is nothing natural about factory farming, which is the biggest offender when it comes to pollution from meat production. Also, just because something is "natural" doesn't mean that we should continue to practice it in the twenty-first century. It's also natural for humans to forage for the plants that they eat and to not wear shoes, but we don't follow those practices. On the other hand, using modern technology, like phones and toilets, is completely unnatural, but that doesn't mean we shouldn't use it. Artificial coloring in food, heavily processed food, microwave meals, eating off of plates, grocery stores, and cheese are all unnatural foods/food practices that have become part of our every day life. There is no reason to eat factory farm meat, something extremely unnatural, because humans in the past ate meat from wild animals. One could also point out that arguing for something because "it's natural," when the consequence is climate change, may not be something worth arguing for.