New Insight Into the Evolution of Sight From 54 Million Year-Old Fossil

 


The story of how insects developed their eyes has taken a surprising turn thanks to fossilized flies from 54 million years ago. These crane flies, which were published in Nature today, demonstrate that insect eyes use the pigment melanin to capture light in the same manner as human eyes do. This is yet another illustration of how evolution finds solutions to similar problems. 


The study of evolution has always captivated evolutionary biologists. In anticipation of the doubters, Charles Darwin gave a lengthy explanation of how random mutation and natural selection could quickly produce "organs of extreme perfection". It should not come as a surprise that these useful adaptations have evolved repeatedly across the animal kingdom; for instance, octopuses and squids have independently developed eyes that are eerily similar to our own. 


The majority of animals today have photoreceptors of some kind because vision is so important. Creatures that live in complete darkness, such as in caves or the deep ocean, are notable exceptions.  


However, there is very little eye fossil evidence. Hard parts like bones and shells are typically preserved in the rock record. Only in exceptional circumstances can soft tissues like the eyes, nerves, veins, and intestines be preserved


Exceptionally preserved insect fossils

In Denmark, perfectly preserved eye fossils from 54 million-year-old insects have been found. In sediments rich in fine-grained volcanic ash, the fossils were superbly preserved. Significantly, the fossilized eyes were surprising similar to our own


The fossil crane fly eyes, on the other hand, contained melanin that was human-like, according to a comprehensive chemical analysis. Surprised, the researchers were able to confirm the presence of melanin and many chromes when they examined the eyes of living crane flies once more. Another illustration of convergent evolution was the discovery by fossils that the shielding pigments (melanin) that insects and humans use in their eyes are the same.


Interestingly, calcite, the mineral that makes up the majority of limestone, was abundant in the outer layers of the fossilized eyes. In addition, the calcite's crystals were aligned to efficiently deliver light to the eye. However, since the eyes of living crane flies are not mineralized, this apparent feat of fine engineering (a mineralized outer eye layer optimized to transmit light) was almost certainly caused by the fossilization process. 


If not carefully interpreted, the fossil record can reveal as well as erroneously. Mineralized, light-transmitting outer eye layers are frequently discovered in Trilobites, the hard-shelled crab-like creatures that are among the most abundant and diverse animal fossils. Typically, these have been assumed to accurately reflect their living circumstances: Trilobites even encased their eyeballs in armor because of the intense predation in ancient oceans


The following is a cautionary tale from Lindgren and colleagues: Like the crane flies, the trilobite's "protective goggles" may have only emerged after fossilization. However, this interpretation is likely to be the subject of debate. Because they are preserved in three dimensions much more frequently than the eyes of other animals, trilobite eyes appear to have been exceptionally rigid and resilient in real life. When the rigid outer layer is accepted as real, it also has certain optical properties that make more sense.


Although disagreements among a few paleontologists may appear obscure, they can be relevant to the real world. The discussion of how the dinosaurs went extinct, when a meteorite impact engulfed the planet in a cloud of dust and deep-froze the entire biosphere, was the most well-known source of inspiration for the idea of nuclear winter. 


Although it is unlikely that the debate over how trilobites and insect eyes work will affect world peace, it might still have applications that are beneficial. For instance, bioengineers have developed high-performance optical devices for applications ranging from laser physics to microscopy after being inspired by the way trilobite lenses appear to provide constant acuity while remaining completely rigid


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