To some, it may seem obvious that humans are massively changing the environment, but what Zalasiewicz had to do was show that 10, 100, 500 million years down the line, if you were to slice through a chunk of sediment you would be able to identify a distinct layer that corresponds to our reign on Earth.
The group says there is enough evidence around to suggest this will be the case. Ocean acidification, if it continues, could bring an end to corals which will change the nature of ocean rocks. Humans activities have triggered huge amounts of erosion, generating a new layer of sediment.
Widespread agriculture is replacing natural vegetation with large expanses of single crops. Cutting down forests, draining marshlands and peat bogs, transforming the prairies have pushed out the animal and plant species that live there and caused them to go extinct. All of the above will mean that one day, the fossil record of our time will look very different to the pre-Anthropocene record.

Assuming there are any people alive millions of years from now. Assuming planet Earth still exists.