This allowed filling the nocturnal niche. Mammals evolved from small-sized reptiles that developed endothermic metabolism. OPN4M, mammalian-like melanopsin OPN4X, xenopus-like melanopsin RGR, retinal G-protein-coupled receptor peropsin, retinal pigment epithelial (RPE)-specific rhodopsin homologue (RRH) OPN5, neuropsin OPN3, panopsin/encephalopsin TMT, teleost multiple tissue opsin parapinopsin, parapineal gland-expressing opsin parietopsin, parietopsin-expressing opsin VA, vertebrate ancient opsin pinopsin, pineal gland-specific opsin LWS, long-wavelength-sensitive opsin SWS1, short-wavelength-sensitive opsin 1 SWS2, short-wavelength-sensitive opsin 2 RH2, middle-wavelength-sensitive rhodopsin 2 (cone) RH1, middle-wavelength-sensitive rhodopsin 1 (rod) exorhodopsin, extra-retinal rod-like opsin. The significant loss of opsin genes during this timeline forms part of the nocturnal bottleneck hypothesis. The presence (yes, Y) or absence (no, N) of a particular opsin class is shown, with a vertical red line indicating the reptilian – mammalian boundary. LWS, long-wavelength-sensitive opsin OPN3, panopsin/encephalopsin OPN5, neuropsin OPN4M, mammalian-like melanopsin OPN4X, xenopus-like melanopsin RGR, retinal G-protein-coupled receptor RH1, middle-wavelength-sensitive rhodopsin 1 (rod) RH2, middle-wavelength-sensitive rhodopsin 2 (cone) SWS1, shortwavelength-sensitive opsin 1 SWS2, short-wavelength-sensitive opsin 2 TMT, teleost multiple tissue opsin VA, vertebrate ancient opsin pinopsin, pineal gland-specific opsin parietopsin, parietopsin-expressing opsin parapinopsin, parapineal gland-expressing opsin peropsin, retinal pigment epithelial (RPE)-specific rhodopsin homologue (RRH).Ī summary table showing the presence of opsin orthologues in modern representatives of the main vertebrate classes from bony fishes to mammals. The nature of chromophore handling is highlighted, where determined: monostable, the ability of a photopigment to form a stable interaction with either 11-cis retinal or all-trans retinal in a light-dependent manner (pink) bistable, the ability of a photopigment to form stable interactions with both 11-cis retinal and all-trans retinal and the photo-conversion (isomerization) between them (orange). Our conclusion presently is that arguments in favour of the nocturnal bottleneck hypothesis in eutherians prevail.Ī phylogenetic tree showing the evolution of known visual (green) and non-visual (blue) opsin classes in gnathostome vertebrates. We also assess genes, relating to light detection (visual and non-visual systems) and the photolyase DNA protection system that were lost in the eutherian mammalian lineage. We review the ecological plausibility that the activity patterns of (early) eutherian mammals were restricted to the night, based on arguments relating to endothermia, energy balance, foraging and predation, taking into account recent palaeontological information. This review describes the status of what has become known as the nocturnal bottleneck hypothesis, giving an overview of the chronobiological patterns of activity. Walls based this concept of a longer episode of nocturnality in early eutherian mammals by comparing the visual systems of reptiles, birds and all three extant taxa of the mammalian lineage, namely the monotremes, marsupials (now included in the metatherians) and placentals (included in the eutherians).

In 1942, Walls described the concept of a 'nocturnal bottleneck' in placental mammals, where these species could survive only by avoiding daytime activity during times in which dinosaurs were the dominant taxon.

The nocturn