Seems like the answer is total amount of human life. When you had 10K humans you could just walk around your cave and find enough food for everyone. When you ha

Seems like the answer is total amount of human life. When you had 10K humans you could just walk around your cave and find enough food for everyone. When you had 10M humans you started tilling fields and created organizations to manage collection and storage. now you need incredibly complex machines, that require incredibly complex supply chains to feed 10B. (food as an example. its the same for clothing to not freeze, shelter to rear offsprings, culture to not kill each other etc)