다트넬은 팔레오세에오세 극열기(기후온난화)가 생뭃학적 원인에 의해 발생했다고 말한다. 오늘날 인류세(자본세)의 기후온난화가 인간에 의해 발생되고 있다면 그 역시 생물학적 원인이라고 할 수 있을 것인데 이것이 의미하는 것은 무엇일까? 다트넬은 오늘날의 기후온난화에 대해 어떻게 바라보고 있을까?

"‘We would expect that the rapid heating of Earth’s atmosphere was the result of volcanic activity, as happened on numerous occasions in our planet’s history. But the curious thing is that the cause of most of this enormous and sudden release of carbon that triggered the temperature spike wasn’t volcanic–it was biological.§’

그 이후의 사태에 대해;
‘Although severe, the temperature spike was very brief in geological terms: the atmosphere and the global climate returned to their earlier levels again within 200,000 years or so.78 Yet this global warming–a short, intense fever of the world triggered by a great methane flatulence of the oceans–led to the emergence of the three orders of mammals most fundamental for all human history. Artiodactyls, perissodactyls and our own group the primates all appeared suddenly right at the beginning of the PETM, and then rapidly dispersed across Asia, Europe and North America.79’