We May Finally Know How Our Eyesight Evolved, And It's Not From Our Branch of Life

The development of the human eye has long been regarded as one of biology's most difficult riddles, sparking discussion about the processes necessary to transform primitive light sensitivity into a sophisticated photographic system.

According to recent findings, some aspects of vertebrate eyesight may not have evolved gradually as genes were transmitted down family lines, but rather may have been "borrowed" from completely other domains of life.

According to UCSD molecular biologist Matt Daugherty on Twitter, "at least one innovation that led to the current structure of vertebrate eyes did not occur from stepwise "tinkering" with genes that exist in other animals, but came from introduction of novel DNA from bacteria by horizontal gene transfer."

The term "horizontal gene transfer" refers to the movement of genetic material across various types of organisms, such as by bacteria or viruses that steal genes.

The division of light-sensing tissues from the cells in charge of recycling their light-reactive chemicals is one of the major characteristics that sets our precise focus, camera-like eyes apart from those of invertebrates.

This depends on processes that transfer chemicals known as retinoids across various cells. This new research shows that the transportation is carried out by the extremely conserved protein interphotoreceptor retinoid-binding protein (IRBP), which evolved from a bacterial gene that suddenly emerged in vertebrate-like eyes over 500 million years ago.

Invertebrates do not have IRBP, and neither do any other complex cells, such as yeast, amoebas, or trees. We only know of one instance of a gene sequence from bacteria that is comparable to the one that codes for IRBP.

Chinmay Kalluraya, a PhD student in molecular biology at Massachusetts Institute of Technology, and colleagues were able to identify the gene's occurrence across vertebrate lineages after analyzing over 900 genomes. That happened around the same time as vertebrate eyes first appeared more than 500 million years ago.

It appears that the progenitor of all animals with backbones stole the original gene from bacteria, copied it, and modified its function for retinoid transport over many generations of natural selection.

The ability of novel components to move between completely distinct regions of the biosphere offers new opportunities for understanding a variety of intricate biological processes, even if it only plays a minor role in vertebrate vision. Syncytin, a protein required for the development of the placenta in animals, is another illustration of this. Retroviruses are the source of the gene for this protein.

The team writes in its research that acquisition of foreign genetic material "has the ability to punctuate eukaryotic evolution by delivering instant functional innovation, unlike evolution of existing genes, or the so-called tinkering."

The researchers predict that when more genomes are sequenced, we will discover many more instances of horizontal gene transfer throughout the course of human evolution.

According to Daugherty, "DNA from microorganisms (including viruses) has affected vertebrate development in peculiar and unanticipated ways."

This research was published in PNAS.