Light switch hypothesis
The light switch hypothesis is the proposal by the zoologist Andrew Parker that the appearance of the first image-forming eyes, around 543 million years ago, set off the Cambrian explosion. On this account a predator that could see its prey started an arms race in which prey acquired armor, speed and eyes of their own, and the animal body plans of the Cambrian followed in a geological moment. Parker set the idea out in a paper of 1998 and in a book for general readers, In the Blink of an Eye: How Vision Sparked the Big Bang of Evolution (2003).[1][2] It is one of several explanations of the explosion, and the objections to it concern mostly timing.
The proposal
The 1998 paper, "Colour in Burgess Shale animals and the effect of light on evolution in the Cambrian", reported diffraction gratings on the external hard parts of three animals from the Burgess Shale, the scaly Wiwaxia corrugata, the bristle worm Canadia spinosa and the arthropod Marrella splendens, which would have made them iridescent. Parker found similar gratings on living marine animals of a comparable depth.[1] Iridescence is useful only to an animal that is seen, and Parker took the gratings as evidence that vision was already part of the ecology of the Cambrian sea.
In a review of 2005 Parker dated the first image-forming eye to around 543 million years ago, which he took as the start of the Cambrian. The International Commission on Stratigraphy now puts the base of the Cambrian at 538.8 million years ago.[3] The book developed the argument. In Parker's account an animal without eyes found its food and its enemies by touch and chemistry, and its world had a short horizon. Once a predator could pick a target at a distance, and prey could be seen, armor, hard shells, burrowing and swimming each had a new use. The sudden diversification of hard parts in the fossil record would then record a change in the rules of survival.[2]
Evidence cited in support
The strongest evidence is the eyes themselves. In the Emu Bay Shale of South Australia, about 515 million years old, arthropods had eyes with more than 3,000 lenses each and a specialized bright zone, the pattern of an active predator that could see in dim light.[4] From the same deposit come the eyes of Anomalocaris, each with at least 16,000 lenses. The authors of the description concluded that a large predator with sharp vision "probably helped to accelerate the escalatory 'arms race' that began over half a billion years ago."[5] Trilobites, which appear soon after the base of the Cambrian, had compound eyes with lenses of calcite.[6] And the theory of eye evolution allows an eye to form fast: Dan-Eric Nilsson and Susanne Pelger calculated in 1994 that a light-sensitive patch could become a focusing lens eye in a few hundred thousand years.[7]
Objections
The objection raised most often is about timing: when vision arose, compared with when predation and skeletons did.
Predation is also recorded before the Cambrian. Cloudina, a tube-forming animal of the latest Precambrian and the earliest known to build a mineralized exoskeleton, was attacked by shell-borers. Of more than 500 tubes from Shaanxi Province in China, 2.7 percent have rounded holes 40 to 400 micrometers across, and the relation between the size of a hole and the width of its tube suggests a predator choosing prey by size. Stefan Bengtson and Yue Zhao proposed in 1992 that the selection pressure of predation was a significant factor in the evolution of skeletons around the Precambrian-Cambrian boundary.[8] Whether the predators could see is not known.
Animals also appear to be older than their fossils. Douglas Erwin and colleagues argued in 2011 that the developmental toolkits of animals were in place in the Cryogenian, 850 to 635 million years ago, and that the Cambrian records the ecological success of animals that already existed. If so, the date of the Cambrian fossils needs some other explanation, and the eye is one candidate among several.[9]
Eyes may be older as well. Nilsson proposed in 2013 that the sequence from the first opsin to high-resolution vision took about 170 million years and was largely complete by the onset of the Cambrian, about 530 million years ago, with directional photoreception already present in the last common ancestor of the bilaterian animals.[10]
Roy Plotnick, Stephen Dornbos and Junyuan Chen reversed the direction of causation in 2010. They argued that the Ediacaran to Cambrian transition made the marine environment spatially far more complex, through macroscopic predation, larger organisms and the burrowing that turned flat microbial mats into mixed sediment, and that this complexity drove the evolution of macroscopic sense organs, eyes and antennae among them. They called the coevolution of environmental information and the ability to use it the "Cambrian Information Revolution".[11]
Other candidates do without vision. Charles Marshall's review of 2006 sorted the explanations into environmental, developmental and ecological groups.[12] Shanan Peters and Robert Gaines proposed in 2012 that the erosion that produced the Great Unconformity changed the chemistry of the sea and may have triggered the evolution of mineralized skeletons.[13]
See also
References
- ^ a b Parker, Andrew R. (1998). "Colour in Burgess Shale animals and the effect of light on evolution in the Cambrian". Proceedings of the Royal Society B 265 (1400): 967-972. doi:10.1098/rspb.1998.0385.
- ^ a b Parker, Andrew (2003). In the Blink of an Eye: How Vision Sparked the Big Bang of Evolution. Cambridge, Massachusetts: Perseus. ISBN 0-7382-0607-5.
- ^ Parker, Andrew R. (2005). "A geological history of reflecting optics". Journal of the Royal Society Interface 2 (2): 1-17. doi:10.1098/rsif.2004.0026.
- ^ Lee, Michael S. Y.; Jago, James B.; García-Bellido, Diego C.; Edgecombe, Gregory D.; Gehling, James G.; Paterson, John R. (2011). "Modern optics in exceptionally preserved eyes of Early Cambrian arthropods from Australia". Nature 474 (7353): 631-634. doi:10.1038/nature10097.
- ^ Paterson, John R.; García-Bellido, Diego C.; Lee, Michael S. Y.; Brock, Glenn A.; Jago, James B.; Edgecombe, Gregory D. (2011). "Acute vision in the giant Cambrian predator Anomalocaris and the origin of compound eyes". Nature 480 (7376): 237-240. doi:10.1038/nature10689.
- ^ Clarkson, E. N. K.; Levi-Setti, R. (1975). "Trilobite eyes and the optics of Des Cartes and Huygens". Nature 254 (5502): 663-667. doi:10.1038/254663a0.
- ^ Nilsson, Dan-Eric; Pelger, Susanne (1994). "A pessimistic estimate of the time required for an eye to evolve". Proceedings of the Royal Society B 256 (1345): 53-58. doi:10.1098/rspb.1994.0048.
- ^ Bengtson, Stefan; Zhao, Yue (1992). "Predatorial borings in late Precambrian mineralized exoskeletons". Science 257 (5068): 367-369. doi:10.1126/science.257.5068.367.
- ^ Erwin, Douglas H.; Laflamme, Marc; Tweedt, Sarah M.; Sperling, Erik A.; Pisani, Davide; Peterson, Kevin J. (2011). "The Cambrian conundrum: early divergence and later ecological success in the early history of animals". Science 334 (6059): 1091-1097. doi:10.1126/science.1206375.
- ^ Nilsson, Dan-Eric (2013). "Eye evolution and its functional basis". Visual Neuroscience 30 (1-2): 5-20. doi:10.1017/S0952523813000035.
- ^ Plotnick, Roy E.; Dornbos, Stephen Q.; Chen, Junyuan (2010). "Information landscapes and sensory ecology of the Cambrian Radiation". Paleobiology 36 (2): 303-317. doi:10.1666/08062.1.
- ^ Marshall, Charles R. (2006). "Explaining the Cambrian "explosion" of animals". Annual Review of Earth and Planetary Sciences 34: 355-384. doi:10.1146/annurev.earth.33.031504.103001.
- ^ Peters, Shanan E.; Gaines, Robert R. (2012). "Formation of the "Great Unconformity" as a trigger for the Cambrian explosion". Nature 484 (7394): 363-366. doi:10.1038/nature10969.