photoadaptational

The story

1. Proto-Indo-European*bʰeh₂-
2. Proto-Indo-European*bʰéh₂os
3. Proto-Hellenic*pʰáwos
4. Ancient Greekφᾰ́ος (phắos)
5. Ancient Greekφῶς (phôs)
6. Ancient Greekφωτο- (phōto-)
7. Englishphoto-
8. Proto-Indo-European*h₂éd
9. Proto-Italic*ad
10. Proto-Italic*ad-
11. Latinad-
12. Proto-Indo-European*h₂ep-
13. Proto-Indo-European*h₂ptós
14. Proto-Italic*aptos
15. Latinaptus
16. Proto-Indo-European[Term?]
17. Proto-Italic*apjō
18. Latinapiō
19. Latinaptō
20. Latinadaptō
21. Medieval Latinadaptātiō
22. Frenchadaptation
23. Englishadaptation
24. Proto-Indo-European*h₂el-
25. Proto-Indo-European*h₂élisder.?
26. Englishadaptational
27. English todayphotoadaptational

From photo- + adaptational.