methanogenesis

The story

1. Proto-Indo-European*médʰu
2. Proto-Hellenic*métʰu
3. Ancient Greekμέθυ (méthu)
4. Proto-Indo-European*swel-der.?
5. Ancient Greekῡ̔́λη (hū́lē)
6. GermanMethy
7. Englishmethyl
8. Englishmethane
9. Proto-Indo-European*ǵenh₁-
10. Proto-Indo-European*ǵénh₁tis
11. Proto-Hellenic*génetis
12. Ancient Greekγένεσῐς (génesĭs)
13. English todaymethanogenesis

From methane + -o- + -genesis.