We've already caused it. The overwhelming majority of land mammal mass is either humans, or farm animals raised for humans to eat. Almost half of the world's land surface is now farmland.
0.06 Gt C human biomass
0.1 Gt C livestock biomass
0.007 Gt C wild mammal biomass
0.002 Gt C wild bird biomass
[1]: https://www.pnas.org/doi/10.1073/pnas.1711842115
[2]: https://github.com/milo-lab/biomass_distribution/blob/master...
This is a BS statistic meant to shock and inappropriately counts the entire area of wilderness that might be used for grazing which is just wilderness that would have supported similar animals anyway.
Even grazing land rarely looks anything like it does before we've shaped it for our use. And even when it does, at first glance, it gets a monoculture of animals, instead. Farmers are not keen on seeing their herds deal with competition or predation from the wild.
That doesn't seem right. According to the FAO (United Nations), only 1.3 billion hectares of land are arable while nearly 5 billion hectares are used for agriculture. That's more like ~25%. And the latter figure only accounts for 38% of the world's land area, so the earlier half figure claim doesn't seem right either.
I suppose if the margin of error on those approximations is exceptionally large...