Human eye showing the iris and pupil
Despite the claims of creationists frauds who cite Charles Darwin, whom they falsely claim admitted the structure of the eye couldn't be explained, the evolution of the mammalian eye from a simple patch of light-sensitive cells is well known to biological science.
Darwin, of course, was setting out the problem before explaining the solution in a typical literary technique of his time. The paragraph that creationists often try to get away with citing is:
To suppose that the eye with all its inimitable contrivances for adjusting the focus to different distances, for admitting different amounts of light, and for the correction of spherical and chromatic aberration, could have been formed by natural selection, seems, I freely confess, absurd in the highest degree.
'Origin of Species', page 100
What they always fail to cite is the very next paragraph:
Yet reason tells me, that if numerous gradations from a perfect and complex eye to one very imperfect and simple, each grade being useful to its possessor, can be shown to exist; if further, the eye does vary ever so slightly, and the variations be inherited, which is certainly the case; and if any variation or modification in the organ be ever useful to an animal under changing conditions of life, then the difficulty of believing that a perfect and complex eye could be formed by natural selection, though insuperable by our imagination, can hardly be considered real.
'Origin of Species', page 100
This refutation of creationism deals with the evolution of the mammalian eye and in particular the different specialist cells of the retina. These have the origins in the eyes of the first jawed vertebrates over 400 million years ago. It is the work of Karthik Shekhar and colleagues from the University of California, Berkeley and a group led by Joshua Sanes at Harvard University. Their work is published open access in Nature as part of a 10-paper series reporting the latest results of the BRAIN Initiative Cell Census Network's efforts to create a cell-type atlas of the adult mouse brain.
By examining the eyes from 17 separate species, including humans, the team showed that certain cell types are highly conserved and must have been present in the stem mammal from about 200 million years ago, which had a retina as complex as that of modern mammals, and some had their origins over 400 million years ago in the common ancestors of all mammals, birds, reptiles amphibians and jawed fish.
From the UC Berkley news release:
The findings were a surprise, since vertebrate vision varies so widely from species to species. Fish need to see underwater, mice and cats require good night vision, monkeys and humans evolved very sharp daytime eyesight for hunting and foraging. Some animals see vivid colors, while others are content with seeing the world in black and white.
Yet, numerous cell types are shared across a range of vertebrate species, suggesting that the gene expression programs that define these types likely trace back to the common ancestor of jawed vertebrates, the researchers concluded.
The team found, for example, that one cell type — the "midget" retinal ganglion cell — that is responsible for our ability to see fine detail, is not unique to primates, as it was thought to be. By analyzing large-scale gene expression data using statistical inference approaches, the researchers discovered evolutionary counterparts of midget cells in all other mammals, though these counterparts occurred in much smaller proportions.
"What we are seeing is that something thought to be unique to primates is clearly not unique. It’s a remodeled version of a cell type that is probably very ancient," said Shekhar, a UC Berkeley assistant professor of chemical and biomolecular engineering. "The early vertebrate retina was probably extremely sophisticated, but the parts list has been used, expanded, repurposed or refurbished in all the species that have descended since then."
[…]
The discoveries are, in a sense, not a total surprise, since the eyes of vertebrates have a similar plan: Light is detected by photoreceptors, which relay the signal to bipolar, horizontal and amacrine cells, which in turn connect with retinal ganglion cells, which then relay the results to the brain's visual cortex. Shekhar uses new technologies, in particular single-cell genomics, to assay the molecular composition of thousands to tens of thousands of neurons at once within the visual system, from the retina to the visual cortex.
Because the number of identified retinal cell types varies widely in vertebrates — about 70 in humans, but 130 in mice, based on previous studies by Shekhar and his colleagues — the origins of these diverse cell types were a mystery.
One possibility that emerged from the new research, Shekhar said, is that as the primate brain became more complex, primates began to rely less on signal processing within the eye — which is key to reflexive actions, such as reacting to an approaching predator — and more on analysis within the visual cortex. Hence the apparent decrease in molecularly distinct cell types in the human eye.
"Our study is saying that the human retina may have evolved to trade cell types that perform sophisticated visual computations for cell types that basically just transmit a relatively unprocessed image of the visual world with the brain so that we can do a lot more sophisticated things with that," Shekhar said. "We are giving up speed for finesse."
AbstractNot only is the tired old creationists lie that Darwin admitted the evolution of the eye couldn't be explained refuted, but the evolution of the eye has its origins way back in that 'pre-Creation Week' history when all the major events in Earth's long history occurred.
The basic plan of the retina is conserved across vertebrates, yet species differ profoundly in their visual needs1. Retinal cell types may have evolved to accommodate these varied needs, but this has not been systematically studied. Here we generated and integrated single-cell transcriptomic atlases of the retina from 17 species: humans, two non-human primates, four rodents, three ungulates, opossum, ferret, tree shrew, a bird, a reptile, a teleost fish and a lamprey. We found high molecular conservation of the six retinal cell classes (photoreceptors, horizontal cells, bipolar cells, amacrine cells, retinal ganglion cells (RGCs) and Müller glia), with transcriptomic variation across species related to evolutionary distance. Major subclasses were also conserved, whereas variation among cell types within classes or subclasses was more pronounced. However, an integrative analysis revealed that numerous cell types are shared across species, based on conserved gene expression programmes that are likely to trace back to an early ancestral vertebrate. The degree of variation among cell types increased from the outer retina (photoreceptors) to the inner retina (RGCs), suggesting that evolution acts preferentially to shape the retinal output. Finally, we identified rodent orthologues of midget RGCs, which comprise more than 80% of RGCs in the human retina, subserve high-acuity vision, and were previously believed to be restricted to primates2. By contrast, the mouse orthologues have large receptive fields and comprise around 2% of mouse RGCs. Projections of both primate and mouse orthologous types are overrepresented in the thalamus, which supplies the primary visual cortex. We suggest that midget RGCs are not primate innovations, but are descendants of evolutionarily ancient types that decreased in size and increased in number as primates evolved, thereby facilitating high visual acuity and increased cortical processing of visual information.
Hahn, J., Monavarfeshani, A., Qiao, M. et al.
Evolution of neuronal cell classes and types in the vertebrate retina.
Nature 624, 415–424 (2023). https://doi.org/10.1038/s41586-023-06638-9
Copyright: © 2023 The authors.
Published by Springer Nature Ltd. Open access.
Reprinted under a Creative Commons Attribution 4.0 International license (CC BY 4.0)
No wonder so many former creationists realised that a cult that needs to lie to its members to keep them in the cult is not a cult worth belonging to.
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