According to Gould and Eldredge, the reason why so many links are missing is that they simply do not exist. They take the view that biological evolution proceeds in successive stages of "punctuated equilibrium." Living species would remain unchanged for extremely long stretches of time, and then undergo profound changes in relatively short periods. To borrow a term from the quantum theory of atoms, evolution would occur in "quantum jumps." It is very likely that the spark of life appeared during the first such "jump."
Trinh Xuan Thuan, Chaos and Harmony (2001)
There is presumably an upper limit to the carrying capacity of humans on earth—of the numbers that agriculture can support—and that number is usually estimated at between 13-15 billion, though some people think the ultimate numbers might be much higher. (2001)Niles Eldredge