That 60% is an average btw. It's doesn't mean humans have wiped out exactly 60% of animals.
Here's a link to an article in The Atlantic which explains it.
Quote:
Ultimately, they found that from 1970 to 2014, the size of vertebrate populations has declined by 60 percent on average. That is absolutely not the same as saying that humans have culled 60 percent of animals—a distinction that the report’s technical supplement explicitly states. “It is not a census of all wildlife but reports how wildlife populations have changed in size,” the authors write.
To understand the distinction, imagine you have three populations: 5,000 lions, 500 tigers, and 50 bears. Four decades later, you have just 4,500 lions, 100 tigers, and five bears (oh my). Those three populations have declined by 10 percent, 80 percent, and 90 percent, respectively—which means an average decline of 60 percent. But the total number of actual animals has gone down from 5,550 to 4,605, which is a decline of just 17 percent.
Have we really killed 60% of animals