Monday, April 14, 2014

Four-eyed harvestmen

Three-dimensional scan of a fossil harvestman, Hastocularis argus.  Credit:
Russell Garwood, Prashant Sharma, Manchester University.  Image source.
Members of the order Opiliones, known as harvestmen or "daddy-long-legs," may closely resemble spiders, but they form their own distinct group of arachnids.  Harvestmen generally have eight long legs, a single body segment, and two tiny eyes.  Apparently, this has not always been the case.  A remarkably preserved fossil of a harvestman named Hastocularis argus has been studied using microtomography, a technology similar to CT scans that reveal cross-sections of the human brain.  Despite being dated at 305 million years, the fossil very closely resembled modern harvestmen except for one major difference: it had four eyes instead of two.

Evolutionary biologists have struggled to figure out the exact arrangement of Opiliones among themselves and among other arachnids in the evolutionary tree.  Because most other arachnids have multiple pairs of eyes, the four-eyed condition of Hastocularis is seen as a primitive trait, allowing biologists to place it closer to the node that unites Opiliones with other arachnids.  The authors of the study even note that modern harvestman embryos can show hints of an undeveloped second pair of eyes.

Although this find is certainly interesting, it does not at all prove that Opiliones and other arachnids share a single common ancestor.  This fossil only demonstrates that at least some ancient Opiliones had four eyes.  Either this lineage went extinct, or it lost its second pair of eyes.  The embryonic evidence, if valid, would support the second scenario, but I am not entirely convinced by it.  The authors did not find a second pair of eyes in the embryos but rather the potential to form scent glands near the area where the ancient forms had its second pair of eyes.  The authors apparently closely associate these glands with the eyes, but I am not sure this is necessary.

If this fossil was the ancestor of modern harvestmen, does that mean evolution took place?  As I have discussed numerous times previously, it depends on your definition of "evolution."  Simply losing a pair of eyes, as we have seen, is not particularly difficult and could potentially only involve the inactivation of a single gene.  Admittedly, such an inactivation could be termed "evolution."  However, such evolution could never increase the amount of genetic information in the way necessary to unite all organisms in a single tree of life.

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