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The Cambrian explosion and eye evolution solved at one stroke
'In the Blink of an Eye' reviewed by Gert Korthof. 22 June 2003
In the Blink of an Eye.
by Andrew Parker.
Perseus Publishing, 2003, hardback, 316 pp.
(many black/white illustrations and color photographs,
In his book 'In the Blink of an Eye' Parker describes his solution for the famous Cambrian explosion. What is the Cambrian explosion and why is it a problem? The Cambrian explosion is the fact that 543 million of years ago there were 3 animal phyla and 538 million of years ago there were 38 animal phyla. So in 5 million years 35 phyla originated. Darwin already recognised this as a problem for his theory of gradual evolution. It seems too fast for gradual evolution. Religious critics routinely use it as an argument against evolution. But even some non-religious critics use it to refute gradual evolution. Critics often distort and exaggerate the problem, but hardly study it. There are several good popular science books describing the Cambrian Explosion, but Parker is the first who describes it and solves it.
Internal and externalParker introduces an important distinction between internal and external characteristics of organisms:
The Cambrian explosionHaving explained the differences between internal and external body parts, Parker states: "The Cambrian explosion is all about external body parts only". Evolution of hard external parts took place simultaneously in all phyla after a considerable period (120 million of years ago) during with nothing happened. "Now we know that the Cambrian explosion was the spontaneous evolution of external body parts in all phyla, where the internal body plans of all phyla are already in place." This is beautifully illustrated on pages 38,39 (one illustration tells us more than thousands words!). So what Parker does is in fact split up the Cambrian problem in two separate problems and focuses in the rest of his book on the second part. The first part, the evolution of internal body plans, is the most difficult one in terms of Darwinian processes because of the lack of viable intermediates (in Parker's view).
The solution: Light Switch theoryParker's Light Switch theory is based on the fact that organisms at the beginning of the Cambrian had hard parts, while in the Precambrian only soft bodied animals occurred. Palaeontologists interpreted hard parts (shells, spines) as a protection against predators. For example, Simon Conway Morris concluded that "one of our principal conclusions is that the role of predation was very important and had at least one direct feedback: the promotion of protective skeletons." (1). Parker adds predators with eyes. That is a very important addition to the explanation. It was a missing piece of the puzzle. The crucial evolutionary step was the development of eyes in predators. This is the Light Switch theory. The development of eyes in predators triggered the evolution of hard parts in prey. It must have been paradise for the first predators: soft bodied animals swimming all around: a free breakfast, a free lunch and a free dinner. No soft-bodied animal could escape from predators with eyes. Independent of this, Parker found that many organisms at the start of the Cambrian were obsessed with colour. They were colourful. He managed to reconstruct the original colours of fossils. Since colours are costly, they must have a function. That means colours were intended for eyes. I find this the most convincing part of Parker's Light Switch theory: "from the beginning of the Cambrian up to today, the world has been adapted to predators with vision." It neatly connects palaeontological observations with evolutionary theory and ecology, especially coevolution of prey and predator (evolutionary arms race).
TimingThe timing of the events (543 million years ago) that makes Parker's Light Switch theory an explanation of the Cambrian explosion (in Parker's definition). The eye did suddenly appear at the beginning of the Cambrian and it instantly became a common characteristic. The first eye on earth (compound eye) appeared in a Trilobite at the very beginning of the Cambrian, around 543 million years ago. It was a sophisticated designed eye. Not a single species of Trilobite before that time had eyes! (although light-sensitive patches existed for a long time). Thus 544 million years ago there were no eyes on earth.
CompletenessAccording to Parker, the appearance of eyes was the biggest change in the environment of all. Vision affects all animals unlike other senses. Only six of the 38 phyla today have eyes, but >95% of all animal species have eyes and 32 phyla did not evolve eyes in the Cambrian. Although this looks like a discrepancy or an incompleteness of his theory, the 95% suggest that eyeless phyla are not species rich, and the successful groups are those with eyes.
It would deepen our insight in the advantages and disadvantages of eyes, to learn which species can live without the benefit of eyes in the presence of predators with eyes. Why are there still blind marine animals (sea cucumber, starfish)? How can some be colourful? Do blind colourful predators exist? How could trilobites without eyes exist at all? What alternative lifestyles are available? Forget about eyes and produce toxins?
SpecificityParker's theory needs a reason that makes the evolution of the eyes faster than that of the other senses (hearing, taste, smell, touch). According to Parker, other senses show a gradual increase in performance during geological time, whereas the evolution of the eye is a nearly instantaneous process. Therefore, the evolution of the eye is the most important factor in the Cambrian explosion.
AdequacyNow the Cambrian explosion, a process so fast that it was called an explosion, is explained by the sudden evolution of eyes. It seems that sudden evolution is explained by sudden evolution. How fast can eyes evolve? Can evolution produce eyes in one million years? Parker invokes Nilsson and Pelger (2) to deliver the answer. A camera-type eye can evolve from light sensitive cells in 364,000 generations. If there is one generation per year, the evolution of an eye is theoretically possible within half a million years. Now that really is 'In the Blink of an Eye' (book title!). That is a pessimistic estimate. It could be faster if selection pressure was higher or the steps bigger. Parker further points out how the nerves and the brain of the animal could have evolved the capacity to do something with the newly acquired visual information. The theoretical time calculated for the evolution of the eye fits neatly in what palaeontology tells us: between 544 and 543 million years ago. So it seems Parker has a solution for two old Darwinian enigmas: the evolution of the eye and the Cambrian explosion at one stroke.
IntermediatesParker did not emphasise one condition and that is each step in the evolution of an eye must be advantageous to be positively selected. If neutral or disadvantageous steps are involved, the evolution of the eye fails. Remarkably, Parker forgets the importance of this condition when he argues for the unparalleled speed of the evolution of eyes, when compared with that of other senses (smell, taste, touch, hearing). "The evolution of receptors for stimuli other than vision can theoretically show a linear progression, but a light perceiver with an inadequate lens has little advantage over one with no lens." (p. 284). This seems to imply that only big jumps (saltationism) or an unknown non-Darwinian process (a miracle?) could produce eyes. However, the whole point of Nilsson and Pelger was that all intermediates are useful and 'a little advantage' does drive the process forward. Consequently, there are no insurmountable obstacles on the road to the eye. Furthermore, if Parker accepts half a million years for the evolution of eyes, then necessarily only intermediates existed during that period. Intermediate eyes could not survive for such a long period if they did not have a selective advantage. Clearly, a fast process does not require that intermediate eyes are useless. A fast process could be caused by huge benefits. Parker himself pointed out those benefits: better predation. I think Parker is still right about the specificity of the eye factor because of the huge benefits of eyes.
Handicap principleParker remarks about courtship of the notched seed-shrimp: 'it had to be really efficient to outweigh the disadvantages inherent in making oneself so conspicuous to predators'. This is 'The Handicap Principle' in short (Parker does not mention it by name). There is a beautiful and popular exposition of the Handicap Principle in the book of Zahavi and Zahavi (3). It is supported by an increasing amount of evidence. Remarkably, Parker denies that it applies to birds, because they can escape predators by flying away! His example, the peacock, is certainly handicapped by his long and heavy tail. The peacock is an outspoken example of the Handicap Principle.
QuestionsA few questions remain. Nilsson's calculations are about phenotypes (optics of an eye) and need to be supplemented by genetic data. How many genes are involved in the construction of a light sensitive cell and a camera-type eye? How many and which mutations are necessary to transform one into the other? It seems possible to design experiments to test the hypothesis that imperfect eyes have an advantage.
Light-sensitive cells existed for 100 million years before the Cambrian explosion without producing camera-type eyes. Why the delay? This delay in combination with the sudden appearance of eyes is the next problem any Cambrian explosion hypothesis needs to solve.
Another question is, how did the basic body plans arise? If internal body plans cannot be built up gradually, as Parker claims, then how? What does Parker mean by '38 monumental genetic events'? Parker writes a lot about palaeontology, optics and marine ecology, but not much about genomics and evo-devo. However, neither palaeontologists nor evolutionary biologists can solve evolutionary problems in isolation. They need to work together. Evo-devo seems promising in this respect (4).
Whatever questions remain and whatever their importance, one cannot reasonably demand from the inventor of a new hypothesis that he solves all the remaining problems in evolutionary biology. There is always a next problem lurking around the corner.
Parker's hypothesis does more than solving an old palaeontological problem. His Light Switch hypothesis is an eye-opener. It throws light upon all creatures swimming or crawling in the depths of the sea. When I see an exotic marine predator in another episode of "Built for the kill", I now ask questions, I never asked before: do prey and predator have eyes, light sensitive cells or are they blind? Do they have eye-catching colours? By the way, "Built for the kill" is certainly misleading because, for every species built for the kill, we need a species "Built for the escape".
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|Copyright ©G. Korthof 2003||First published: 1 June 2003||Updated: 22 June 2003 Links/F.R.: 1 Jul 2013|