Oxygen paved the way for sharp animal vision
According to the researchers, the lack of oxygen supply to the retinal cells is what limited the development of the eye, not least the sharpness of vision, among animals that lived hundreds of millions of years ago.
“It is all about adaptations to improve the oxygen supply to the retinal cells. Such adaptations were a condition for the optimisation of vision in mammals, fish and birds”, says Professor Eric Warrant at Lund University in Sweden, one of the researchers behind the study.
Mutations and larger eyes
The study looks at the mechanisms in vertebrates that through history have developed with the common denominator of contributing to better oxygen supply to the cells in the eye.
Such a mechanism involves haemoglobin mutations in fish, something that made oxygen transport possible under high pressure. In the study, the researchers demonstrate that this mechanism emerged approximately 280 million years ago and coincided with dramatically larger eyes and a thicker retina.
Escaping the dinosaurs
Another mechanism that the researchers highlight is the emergence of fine-meshed networks of microscopic blood vessels in the retina of mammals approximately 100 million years ago. The background is that dinosaurs at the same time went from being cold-blooded to warm-blooded and were able to regulate their own body temperatures. This led to their ability to hunt at night, as they were no longer dependent on the sun to stay warm. To increase their chances of getting away from the dinosaurs, mammals, which until then had been largely nocturnal, were forced to become more active during the day. In the daylight, their need for better vision was greater than at night. Therefore, their eyes developed and their vision improved.
In the study, the researchers also demonstrate that the evolution led to some mammals that do not use their vision when hunting, for example, bats, losing their network of blood vessels, the retinal capillaries.
400 million years
In addition to Sweden, the team includes researchers from Denmark, Canada, the USA, the UK, Vietnam and Germany. Together, they have studied the anatomy and physiology of the eye in 87 different animal species. Researchers placed the species on a phylogenetic tree to establish the relationships between the species. In this way, they have successfully mapped the history of the development of the eye over 400 million years.
The results are published in an article in the journal eLife.