The golden frog of Panama is an early symbol in the book, as is the fungus that almost causes the species to die out. The frog and fungus are the first of many motifs that Kolbert presents concerning the scientific studies that show how ill-equipped some species are to handle invasive species and illnesses caused by agents foreign to their native environments. In all of these circumstances, man is the cause of the transfer of fungi, diseases, and invasive species.

Symbolic of the current era and harbinger of the Sixth Extinction, Anthropocene is the term coined by Danish chemist Paul Crutzen. While scientists argued about what name to give the current era of the planet, Crutzen suggested Anthropocene because the word represents an era dominated and irrevocably changed by the spread of humanity. Of particular note in the acceptance of this name is the acknowledgment of the impact human activities have had on the environment and on species endangerment and extinction.

These two species are representative of organisms driven extinct by the rise of mankind. The extinction of the ancient American mastodon and the somewhat more modern great auk happened because of human activity, specifically “overkill.” As an invasive species, humans spread around the globe, adapting to different territories and terrains, and completely decimating other species.