Vertebrates have complex, camera-type eyes which have been a source of interest to evolutionary biologists since the nineteenth century, when this seemed an unusually complex system, which it was difficult to imagine arising through a series of gradual steps. Modern evolutionary biologists are less concerned by this, recognising that even a very simple eye is better than no-eye-at-all, and that therefore a complex eye could arise step-wise from the simplest cluster of light-sensitive cells, but beyond this have been able to give no real explanation of what the eyes of our earliest Chordate ancestors looked like.
Camera eyes comprise a comprise of a spherical lens, a retina, an iris, and a set of muscles exterior to the main eye structure, which can be used to alter the shape of the lens, enabling it to focus an image on the hemispherical retina, which are detected by the optic nerve, and transmitted to the brain.
Almost all modern Vertebrates have two lateral camera eyes, although some groups have lost these, and, curiously, some Lizards have a third such eye on the top or back of their heads, which is derived from the pineal complex of the brain.
Eyes in Vertebrate fossils are often identified by the preservation of the pigmentation from the retinal epithelium, which is rich in melanin, as dark stains, and/or by impressions left by the hard lens. The oldest purportative fossil Vertebrate eyes are seen in Metasprigginna walcotti, a probable Chordate from the Burgess Shale of Canada, dated to about 505 million years before the present. In these fossils a hemispherical shape has been interpreted as the retina, and an associated circular area as the lens. The earliest known example of melanostomes (the cells which contain the pigment melanin) being preserved in the eye of a Vertebrate is the Devonian Jawless Fish Euphanerops longaevus, from the Escuminac Formation of Canada, which has lateral eyes with abundant such cells, inferring the presence of a retina.
No non-Vertebrate Chordates possess a camera eye. The Lancets, or Amphioxi, have four clusters of photoreceptor cells, but are not thought to be able to produce an image (unsurprising since they also lack a brain). Salps, which are planktonic Tunicates, have a multiple stage life cycle, with an colonial adult phase which reproduces sexually, and a solitary adult phase which reproduces asexually. The larval form of the colonial Salp has three pigment cup eyes, while the larval form of the solitary stage has a single eye. During the embryonic development of Vertebrates, the paired eyes arise from a section of the anterior neural plate which also gives rise to the pineal organ, leading some biologists to speculate that these three organs are analogous to the three eyes of the Salp larvae.
In a paper published in the journal Nature on 21 January 2026, Xiangtong Lei of the Center for Vertebrate Evolutionary Biology and Institute of Palaeontology at Yunnan University, Sihang Zhang, also of the Center for Vertebrate Evolutionary Biology, and of the State Key Laboratory for Vegetation Structure, Functions and Construction at Yunnan University, Peiyun Cong, also of the Center for Vertebrate Evolutionary Biology, and State Key Laboratory for Vegetation Structure, Functions and Construction at Yunnan University, as well as the Oxford University Museum of Natural History, Jakob Vinther of the Palaeobiology Research Group and School of Biological Sciences at the University of Bristol, Sarah Gabbott of the Centre for Palaeobiology & Biosphere Evolution at the University of Leicester, Fan Wei again of the Center for Vertebrate Evolutionary Biology, and State Key Laboratory for Vegetation Structure, Functions and Construction at Yunnan University, and Xing Xu, once again of the Center for Vertebrate Evolutionary Biology and Institute of Palaeontology at Yunnan University, and of the Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences, identify organs which they believe are eyes in two species of Myllokunmingids (Early Chordate Animals which may be ancestral Vertebrates) from the approximately 518-million-year-old Chengjiang Biota of Yunnan Province.
Lei et al. consider Myllokunmingids such as Haikouichthys ercaicunensis and Myllokunmingia fengjiaoa to be the earliest known Vertebrates. For their study they examined six specimens of Haikouichthys ercaicunensis and four slabs which each contained multiple specimens of an as yet unnamed new Myllokunmingid. In both species they found that the head region typically has four black spots, two larger spots being placed laterally on the head, and two smaller spots facing forward. Previous studies have identified the larger of these spots as eyes, while the forward-pointing spots have been identified as nasal sacs.
Energy dispersive X-ray, Raman spectroscopy, and X-ray photoelectron spectroscopic analysis of the lateral eyes and the forward facing spots are enriched in organic carbon. Examination under a scanning electron microscope revealed that these organic patches are made up of oblong or cylindrical microbodies, which measure 200-1200 nm in length, and 200-900 nm in width. Most of these microbodies appear deformed or fused together, and they are associated with pyrite minerals and a clay matrix.
In the lateral eyes of Haikouichthys ercaicunensis these microbodies are consistently oval in shape, ranging from 250 to 900 nm in length and from 200 to 800 nm in width. Element mapping suggests that these objects are carbonaceous structures with a small central hole. In the unnamed Myllokunmingid, there are two morphotypes of microstructures present, the first similar to those seen in Haikouichthys ercaicunensis, and the second being cylindrical in shape and between 400 and 1200 nm in length and between 200 nm and 550 nm in width. These structures also have a central hole. Transverse sections of the melanosomes of some living Vertebrates have also shown such a central hole.
Lei et al. next investigated the molecular composition of the microstructures using Time-of-Flight Secondary Ion Mass Spectrometry. This revealed that in both species the microstructures contained the pigments eumelanin and phaeomelanin, both of which are found in living Vertebrates, confirming that these structures are in fact melanosomes.
The melanosomes in the lateral eyes of Haikouichthys ercaicunensis appear to be largely distributed on the horizontal axis, while those of the unnamed Myllokunmingid are spread along a diagonal axis, with the two types of melanosomes present having different distributions and pigment contents; the cylindrical cells have a higher eumelanin content (which would have made them browner in colour) while the ovoid cells have a higher phaeomelanin content (which would have made them oranger in colour).
In living Vertebrates, melanosomes are found in the iris, choroid and retinal pigment epithelium, but layers of ovoid and cylindrical melanosomes are found only in the retinal pigment epithelium. The observed structures in the eyes of the unnamed Myllokunmingid are consistent with a retinal pigment epithelium with a similar structure. However, in the six specimens of Haikouichthys ercaicunensis examined only ovoid melanosomes could be observed. However, rather than interpreting this as a more primitive state, Lei et al. note that in the Lamprey Mayomyzon pieckoensis and the Cartilaginous Fish Bandringa rayi from the Carboniferous Mazon Creek Fauna of Illinois, a preponderance of ovoid melanosomes have also been observed in eye structures, and that relatively few living Vertebrates have have been investigated to determine what forms of melanosomes are present in their retinas.
In both Chengjiang Myllokunmingids, the central spots are smaller than the lateral spots, about 160-240 µm in diameter in Haikouichthys, and about 90-120 µm in diameter in the unnamed Myllokunmingid. These were also found to be carbonaceous in composition, and to contain microbodies which appeared to be melanosomes; in each species these were consistent with the bodies found in the larger lateral eyes, with only oval melanosomes in Haikouichthys and both cylindrical and oval forms in the unnamed Myllokunmingid. Based upon this, Lei et al. conclude that these medial organs are also preserved retinas.
Carbonaceous preservation of Myllokunmingids eyes and median dark s pots (a-h). (a)-(b) Haikouichthys ercaicunensis (YNGIP-90285) showing lateral eyes (grey) and pineal eyes (green) with lens (blue). (c) Carbon element map of Haikouichthys ercaicunensis (YNGIP-90285) head. (d)-(e) Haikouichthys ercaicunensis (YNGIP-90296) showing lateral eyes (grey) and pineal eyes (green) with lens (blue). (f) Carbon element map of Haikouichthys ercaicunensis (YNGIP-90296), arrows indicating left pineal eye. (g)-(i) Eyes of Haikouichthys ercaicunensis showing lens (arrows). (g) YNGIP-90283. (h) YNGIP-90284. (i) YNGIP-90289. (j), (m) lens in Elonichthys peltigerus (ROM56794). (k), (n) Lens in Platysomus circularis (PF7333). (l), (o) Lens in Bandringa rayi (ROM56789). Scale bars are 200 μm (a)-(f); 50 μm (g)-(i); and 500 mm (m)-(o). Lei et al. (2026).As well as melanosomes within their retinas, both species show preserved lenses, which are ovoid in structure, and about one fifth of the size of the associated retinas. These structures are preserved as impressions with some relief, suggesting that they represent an original structure which was somewhat decay resistant. This placement, size, and composition is consistent with the interpretation of these structures as eye lenses, which are harder tissue than other components of the eyes, and have been found in other Vertebrate fossils, including the Middle Cambrian vertebrate Metaspriginna walcotti.
The similarity of the lateral eyes of the two Myllokunmingid species from the Chengjiang Fauna to those found in later Vertebrate fossils is taken by Lei et al. to indicate that camera eyes had appeared by the Early Cambrian. The combination of a large retinal pigment epithelium and smaller lens is consistent with a fluid-filled retinal sphere with an iris opening within which the lens is suspended, as seen in living Vertebrates. Such eyes would almost certainly have been capable of image formation, although the quality of such images is impossible to know.
The median, forward-facing spots on Myllokunmingids have previously been interpreted as nasal sacs, or possibly pineal organs. The former explanation seems unlikely, as nasal sacs otherwise appear to have been quite a late development, not found in many later stem Vertebrates, and probably first evolving in Galeaspids (probable stem Gnathostomes) between 435 and 370 million years ago. Lei et al. report the discovery of melanosome-bearing tissues and lenses in these spots, which are again inconsistent with an interpretation as nasal sacs. They instead interpret them as paired pineal organs functioning as a second pair of camera eyes.
Lei et al. also note that the Middle Cambrian stem Vertebrate Metaspriginna walcotti also has a pair of dark spots between the lateral eyes, preserved as carbonaceous films, and that these also appear to have associated spherical objects, which may also have been lenses, suggesting that this species may also have had a second pair of median eyes.
In Lampreys, the pineal organ is photosensitive, helping the Animal to respond to changes in light levels within the environment. In Mammals, the pineal organ is entirely internal, but it is associated with aligning the neuroendocrine system with the day/night cycle. In Lizards, the pineal organ is also associated with the neuroendocrine system, but in some species retains a photoreceptive capacity. It has therefore previously been suggested that the pineal organ may have developed from some sort of precursor eye, something that has entered popular culture as the 'third-eye' theory. Lei et al. suggest that the pineal organ may have begun as a pair of photosensitive organs acting as additional camera eyes.
The presence of complex visual systems in the earliest Vertebrates suggests that this sense was of key importance to the success of the group from very early in its history. Both the photoreceptive cells of Vertebrates and the cells of the retinal ganglion arise from nurosensory cell precursors also present in Tunicates. A theoretical model has previously been developed in which the camera eye developed via two rounds of whole-genome duplication, the first allowing for a divergence between the photoreceptor cells and the optical ganglion cells, the second between the pineal complex and the lateral eyes. The apparent presence of a second pair of camera eyes associated with the pineal complex in Early Cambrian Myllokunmingids may represent a transitional stage, in which the genes associated with the development of the eyes have been duplicated, but only just started to evolve towards the modern pineal complex.
Euphanerops longaevus, an anaspid-like fossil from the Devonian Escuminac Formation of Canada, which has been suggested as a stem-Agnathan (jawless Fish) also has paired median dark patches which have been shown to be carbonaceous films with structures identical to the melanosomes of its lateral eyes. Living Lampreys have a pineal eye and a smaller parapineal eye, both of which have functioning retinas (but not lenses) and are used to detect changes in light conditions. The stem Gnathostome (jawed Fish) Sacabambaspis has two pineal openings, which Lei et al. suggest are analageous to the pineal and parapineal eyes of Lampreys. Later stem Gnathostomes, such as the Galeaspids, only have a single such opening, suggesting a progressive loss of this system. Crown Gnathostomes have lost this opening completely, but some have a preserved pineal window, with an area of thin, semitransparent skull overlaying a pigmented area associated with the pineal complex. Thus an image-forming pineal complex was slowly replaced with a light sensitive organ regulating the production of the hormone melatonin, which regulates sleep patterns. Most crown Vertebrates possess both pineal and parapineal organs, sugesing that this complex was originally paired.
During the Cambrian Explosion, early Animals went through a phase of remarkable morphological innovation, with each new development changing the ecological environment in which all Animals lived, particularly as predation became more common. It has been suggested that higher levels of ultraviolet radiation in shallow waters during the Cambrian may have made the rapid evolution of vision more important, although it is likely that the evolution of predator-prey relationships would have been sufficient to drive this. The appearance of large (for the Cambrian) predators such as Radiodonts, gilled Lobopods, and stem Chaetognaths, all of which developed complex visual systems, would have made it important for smaller, non-predatory Animals such as Myllokunmingids to develop equivalent systems to evade predation and survive.
See also...
.png)
.png)
.png)
.png)
.png)
.png)
.png)
.png)
