|Fossil cyclostomes from the Mazon Creek Lagerstätte|
(For caption see original paper)
No sooner do we have a paper on the evolution of mammalian eyes that shows any hypothetical 'intelligent (sic) designer' as an incompetent fool with no plan, we have another paper which shows pretty much the same with regards to the evolution of eyes in the early vertebrates.
This time, creationist have to explain why their supposed designer gave hagfish perfectly good eyes then set about making them go blind with just remnants of their former functional eyes. It's revealing that creationist regard this as evidence of intelligence!
Using scanning electron microscopes, the research team led by Professor Sarah Gabbott from the Department of Geology, University of Leicester, UK, examined the 300 million year-old fossils of lampreys (Mayomyzon) and hagfish (Myxinikela) from Carboniferous rocks in Mazon Creek fossil bed, Illinois, USA. What they found has changed our understanding of the pace of the evolution of the vertebrate eye.
The success of vertebrates is linked to the evolution of a camera-style eye and sophisticated visual system. In the absence of useful data from fossils, scenarios for evolutionary assembly of the vertebrate eye have been based necessarily on evidence from development, molecular genetics and comparative anatomy in living vertebrates. Unfortunately, steps in the transition from a light-sensitive ‘eye spot’ in invertebrate chordates to an image-forming camera-style eye in jawed vertebrates are constrained only by hagfish and lampreys (cyclostomes), which are interpreted to reflect either an intermediate or degenerate condition. Here, we report—based on evidence of size, shape, preservation mode and localized occurrence—the presence of melanosomes (pigment-bearing organelles) in fossil cyclostome eyes. Time of flight secondary ion mass spectrometry analyses reveal secondary ions with a relative intensity characteristic of melanin as revealed through principal components analyses. Our data support the hypotheses that extant hagfish eyes are degenerate, not rudimentary, that cyclostomes are monophyletic, and that the ancestral vertebrate had a functional visual system. We also demonstrate integument pigmentation in fossil lampreys, opening up the exciting possibility of investigating colour patterning in Palaeozoic vertebrates. The examples we report add to the record of melanosome preservation in Carboniferous fossils and attest to surprising durability of melanosomes and biomolecular melanin.
Lampreys and hagfish are the only living jawless vertebrates; they occupy crucial intermediate phylogenetic positions between the nearest invertebrate relatives of vertebrates—urochordates and cephalochordates—and gnathostomes. The condition of hagfish eyes has proved particularly influential in scenarios of eye evolution. In contrast to lampreys, which possess a sophisticated eye with a lens, iris and eye muscles, hagfish eyes lack such structures and, unlike almost all other vertebrates, including lampreys, the retinal epithelium of hagfish is devoid of pigment granules. This condition has been interpreted to reflect a rudimentary intermediate evolutionary grade in the gradual assembly of the vertebrate eye. However, extant cyclostomes cannot be taken as accurate proxies for their last common ancestor. Hagfish and lampreys differ significantly in their morphology, and like all living representatives of deep-branching clades, they have acquired, lost and transformed characteristics compared with their last common ancestor. Consequently, the hypothesis that hagfish eyes reflect an evolutionary intermediate of invertebrate eyespots and vertebrate camera eyes, or a faithful vestige of such an ancestral state, is open to question. Fossil evidence of the condition of eyes in ancient cyclostomes has the potential to resolve this important issue in understanding the evolution of the vertebrate eye.*
*Citation links removed for clarity. See original paper.
Sarah E. Gabbott, Philip C. J. Donoghue, Robert S. Sansom, Jakob Vinther, Andrei Dolocan, Mark A. Purnell.
Pigmented anatomy in Carboniferous cyclostomes and the evolution of the vertebrate eye.
Proceedings of the Royal Society B: Biological Sciences, 2016; 283 (1836): 20161151 DOI: 10.1098/rspb.2016.1151
© 2016 The Authors. Reprinted under the terms of the Creative Commons Attribution License 4.0 (CC BY 4)
The surprise was that hagfish eyes were regarded as examples of primitive eyes, giving the lie to creationist claims that the eye couldn't have evolved gradually because 'half an eye' would be useless, therefore it must have been designed fully formed. There are of course very many other primitive eyes but we now know the modern hagfish eye is not a primitive precursor to the vertebrate eye but a degenerate eye.
Sight is perhaps our most cherished sense but its evolution in vertebrates is enigmatic and a cause célèbre for creationists. We bring new fossil evidence to bear on an iconic evolutionary problem: the early evolution of the vertebrate eye. We will now scrutinize the eyes of other ancient vertebrate fossils to see if we can finally build a picture of the sequence of events that took place in early vertebrate eye evolution.Now, creationists might be tempted to rejoice at this 'loss' of evolutionary biology's refutation of their superstition but what they now need to explain is just why an intelligent designer would design a perfectly good eye for a hagfish 300 million years ago and then redesign it so it doesn't work any more and make it look just as though it had degenerated over time from a fully functional eye, complete with nonfunctional remnant structures.
(quoted in University of Leicester press release).
(quoted in University of Leicester press release).
And of course, there is the problem of explaining how evolution had led in this case to a loss of complexity when it is axiomatic to creationism that, because evolution as described by the scientific theory of evolution involves increasing complexity this is somehow impossible due to some mysterious operation of the second law of thermodynamics. Evolutionary theory makes no such claims of course because there are very many examples of loss of complexity in redundant structure due to evolution.
What this finding suggests is that we now need to revise our understanding, not of the stepwise evolution of the vertebrate eye but simply the timescale in this particular species. It would seem that relatively complex eyes evolved sooner than we thought.
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