Cancer cells are cells that keep growing even when they are told to stop. They continue to grow even when they are a detriment to the body. They consume a large amount of resources and they have no regard for the future even though their unsustainable growth will always lead to the death of their host and to themselves.
Humans have a lot of these similarities. Nature has a lot of mechanisms to prevent the unbounded growth of any population either through finite resources (ie. Carnivores die when they are too numerous because they will run out of prey to hunt) or disease and crazy fungi that spread much easier when there is over population. Humans pretty much learned to circumvent these mechanisms, leading to essentially unbounded growth. Humans over consume resources and we don’t stop even when we learn of our over consumption. And despite learning that our way will lead to either mass deaths or extinction we do almost nothing and keep going.
Does capitalism exist without humans?
Depends on how low you define it. Many have monetary like transactions for procreation (e.g male insects offering food, some wasps offering service to mix her eggs into the colony eggs, penguins offering stones to mate), so the idea of offering a good/currency for a service is not innately human.
Paying for goods isn’t capitalism in the same way breathing isn’t speech. Currency makes capitalism easier but using currency does not make you capitalist.