Studying 305-million-year-old spider fossil, researchers found the ancestors of modern day daddy long legs to have had two sets of eyes instead of one.

Known today as a "harvestman," the daddy long legs is a spider-like arachnid that is known to exist on every continent except Antarctica. The fossil of its ancient ancestor was discovered in eastern France, LiveScience reported, and should provide the researchers with evolutionary details of the harvestman.

The ancient arachnid, named Hastocularis argus, was not actually a spider, but was still related. The research team used X-ray to create an image of the 305-million-year-old fossil and found it had eyes on the front and on opposite sides of its head.

"Although they have eight legs, harvestmen are not spiders; they are more closely related to another arachnid, the scorpion," study author Dr. Russell Garwood, a paleontologist in the University of Manchester's School of Earth, Atmospheric and Environmental Sciences, said in a press release. "Arachnids can have both median and lateral eyes, but modern harvestmen only possess a single set of median eyes -- and no lateral ones. These findings represent a significant leap in our understanding of the evolution of this group."

He said the team was lucky to examine a fossil like this one.

"Harvestmen fossils preserved in three dimensions are quite rare and our X-ray techniques have allowed us to reveal this exceptional fossil in more detail than we would have dreamed possible just a couple of decades ago," Garwood said.

Modern arachnids - spiders, harvestman and scorpions - only have one set of eyes close to the middle of their bodies. However, the researchers found they posses genes to have a second set. Studying the "eye stalk" gene in modern harvestmen embryo, the researchers found a dormant sign of lateral eyes.

Terrestrial arthropods like harvestmen have a sparse fossil record because their exoskeletons don't preserve well. As a result, some fundamental questions in the evolutionary history of these organisms remain unsolved," study co-author Prashant Sharma, a postdoctoral researcher at the American Museum of Natural History, said in the release. "This exceptional fossil has given us a rare and detailed look at the anatomy of harvestmen that lived hundreds of millions of years ago. What we were also able to establish is that developing modern harvestmen embryos retain vestiges of eye-growth structures seen only in the fossil."

The new study is published in the journal Current Biology.