How complex and physiologically remarkable structures such as the human eye could evolve has long been a question that has puzzled biologists. But in research reported this week in Current Biology, the evolutionary history of a critical eye protein has revealed a previously unrecognized link between certain components of sophisticated vertebrate eyes - like those found in humans - and those of the primitive light-sensing systems of invertebrates. The findings, from researchers at the University of Oxford, the University of London and Radboud University in The Netherlands, put in place a conceptual framework for understanding how the vertebrate eye, as we know it, has emerged over evolutionary time. Human sight relies on the ability of our eye to form a clear, focused image on the retina. Critical to this function is the eye lens and the physical properties that underlie the transparency of the lens. The eye's ability to precisely refract light is because of high concentrations of special proteins called crystallins found in lens cells.
Vertebrates such as fish, frogs, birds, humans and other mammals all experience image-forming vision because our eyes express crystallins, which helps form the lens that is needed. But our invertebrate relatives, such as sea squirts, have only simple eyes that detect light but are incapable of forming an image.
This lead to the view that the lens evolved within vertebrates early in vertebrate evolution, raising the question: How could a complex organ with such remarkable physical properties have evolved in the first place?
Researcher Sebastian Shimeld from Oxford approached this question by examining the evolutionary origin of one crystallin protein family, known as the βγ-crystallins. Focusing on sea squirts, the researchers found that these creatures possess a single crystallin gene, which is expressed in its primitive light-sensing system. The identification of this single crystallin gene strongly suggests that it is the gene from which the more complex vertebrate βγ-crystallins evolved.
Perhaps even more remarkable is the finding that expression of the sea squirt crystallin gene is controlled by genetic elements that also respond to the factors that control lens development in vertebrates. This was demonstrated when regulatory regions of the sea squirt gene were transferred to frog embryos where they drove gene expression in the tadpoles' own visual system, including the lens.
The researchers say this suggests that prior to the evolution of the lens, there was a regulatory link between two tiers of genes, those that would later become responsible for controlling lens development, and those that would help give the lens its special physical properties. This combination of genes appears to have then been selected in an early vertebrate during the evolution of its visual system, giving rise to the lens.
The new findings deal a serious blow to the Intelligent Design movement which has long contended that the lack of an apparent evolutionary pathway for complex eye development indicated the presence of a supreme designer.
Ref: Current Biology, Vol. 15, pages 1684-1689, September 20, 2005. DOI 10.1016/j.cub.2005.08.046
