The humans have shown up many hundreds of thousands of years before the extinction of the megafauna, so these 2 events did not have the same cause.
While in the first part of the coexistence between hunting humans and big animals the extinction of various species was more gradual, in Africa and Eurasia, in more recent times the big animals have disappeared almost immediately (at geologic time scales) after the arrival of humans, e.g. in Australia, America and various big islands.
During hundreds of millions of years, there have been many extinction events, and the biggest animals have always been the most affected by them.
Nevertheless, soon after some big species became extinct, other previously smaller species have grown in size, to fill their place.
There has never been a time with a so low abundance of big animals as since the humans have spread all over the world, and it is obvious that no other big animals will emerge naturally, as the cultivated lands have left too little space for them.
The only way for some big terrestrial animals to appear again would be through genetic engineering and they could live only in reservations, like in Jurassic Park.
While in the first part of the coexistence between hunting humans and big animals the extinction of various species was more gradual, in Africa and Eurasia, in more recent times the big animals have disappeared almost immediately (at geologic time scales) after the arrival of humans, e.g. in Australia, America and various big islands.
During hundreds of millions of years, there have been many extinction events, and the biggest animals have always been the most affected by them.
Nevertheless, soon after some big species became extinct, other previously smaller species have grown in size, to fill their place.
There has never been a time with a so low abundance of big animals as since the humans have spread all over the world, and it is obvious that no other big animals will emerge naturally, as the cultivated lands have left too little space for them.
The only way for some big terrestrial animals to appear again would be through genetic engineering and they could live only in reservations, like in Jurassic Park.