Left unexplained is this rather large mystery: "For the long period from the appearance of modern Homo sapiens up to the starting point of this chart in 10,000

Left unexplained is this rather large mystery: "For the long period from the appearance of modern Homo sapiens up to the starting point of this chart in 10,000 BCE it is estimated that the total world population was often well under one million." As others have often pointed out, as a large bodied mammal, we would expect humans to be roughly as successful as other widely dispersed large body mammals. And yet its thought that there were 100 million bison on the planet before human civilization arose. So there should have been 100 million humans on the planet. Why would there be so few humans, if other large body mammals were so plentiful? I'm inclined to think they are underestimating the number of humans on the planet in 10,000 BC, or there must be a good reason why there were so few humans.