Cambrian fossils reveal earliest vertebrates had 'four camera-type eyes'

A research team at Yunnan University has resolved a longstanding mystery regarding Earth's earliest vertebrates, revealing that primitive fish from the Cambrian Period likely possessed four high-functioning, "camera-type" eyes.

The study, published in the journal Nature, focuses on myllokunmingids — tiny, jawless vertebrates that swam the oceans roughly 518 million years ago. The findings provide an unprecedented look at the early evolution of the vertebrate visual system.

The research team, led by Xu Xing, a professor at the Chinese Academy of Sciences, found that these animals had a dorsal pineal complex on top of their heads with imaging capabilities similar to those of their lateral eyes, offering new insights into the early evolution of the vertebrate visual system.

The research project initially sought to investigate how the nervous system could be preserved in early fossils, but unexpectedly revealed an additional pair of camera-type eyes in the Cambrian vertebrates, according to Cong Peiyun, a researcher at the Institute of Paleontology at Yunnan University.

In 2022, team member and co-first author Lei Xiangtong discovered abundant fossilized melanosomes within a smaller pair of round, dark structures located between the lateral eyes of a myllokunmingid fossil. Because melanosomes are typically responsible for light-reception and pigment in eyes, this was a significant breakthrough.

Lei and another co-first author Zhang Sihang, had differing opinions on whether this pair of dark structures were nasal sacs or pineal complexes. They employed a variety of techniques to test these two competing identifications.

The research team started by using techniques to analyze the fossil specimens of two myllokunmingid species from the Chengjiang biota in Yunnan. They revealed that the two lateral eyes of myllokunmingids, along with the dark structures between them, were preserved as organic carbon films primarily composed of microbodies.

The morphology and size of these microbodies closely matched the melanosomes — organelles that synthesize and store melanin — found in the retinas and skin of both fossil and living vertebrates.

The team further analyzed the composition of the organic carbon films. The results revealed that the molecular fingerprints of the microbodies samples closely matched the melanin signals found in the eyes of living amphibians and showed no overlap with samples from other potential contamination sources.

This molecular evidence confirmed that the microbodies in the lateral eyes and the median dark structures of the myllokunmingids were indeed melaninrich melanosomes.

Another piece of evidence was the discovery of the lens. In the lateral eyes of the myllokunmingids and the black structures in between, there were regular, well-defined circular structures. The team confirmed that the shape, size and relative position of these structures were consistent with the lenses preserved in the eyes of other vertebrate fossils dated to the Phanerozoic Eon.

The evidence implied that myllokunmingids from the Chengjiang biota had a pair of camera-type eyes in addition to their lateral eyes. Cong noted that the evolution of animal sensory systems is often driven by predatory pressure.

Early vertebrates such as myllokunmingids were small and lacked effective defense mechanisms compared to the large predators of the Cambrian Period. Consequently, possessing "four camera-type eyes" likely provided them with a significant advantage.

Zhang compared the morphology of the pineal gland across the vertebrate evolutionary tree and found its gradual specialization from an organ with visual functions into an endocrine organ that plays a crucial role in regulating circadian rhythms and sleep in living vertebrates.

Professor Xu said the discovery expands the understanding of the origin and early evolution of the vertebrate visual system and offers a fresh perspective on animal survival strategies during the Cambrian explosion.

Contact the writers at yanyj@chinadaily.com.cn