UF study suggests spiders, scorpions, ticks, mites far older than fossil record indicates
GAINESVILLE, Fla. — Halloween is the only holiday when spiders and other arachnids get a little respect from humans, and a new University of Florida study suggests they deserve more, because they’ve apparently managed to survive a very, very long time.
By analyzing gene sequences in modern-day spiders, scorpions, ticks and mites, researchers have estimated that these invertebrates first appeared on Earth roughly 400 million to 450 million years ago.
The study, published online this month by the journal Experimental and Applied Acarology, is the first large-scale attempt to use genetic analysis to make projections suggesting when various arachnids evolved, said entomologist Marjorie Hoy, an eminent scholar with UF’s Institute of Food and Agricultural Sciences.
“This is a first estimate, it’s not the be-all and end-all,” said Hoy, who co-authored the paper with Ayyamperumal Jeyaprakash, a senior biological scientist with the entomology department.
“It suggests for the first time that these creatures are much older than the fossil record indicates,” Hoy said.
Fossil arachnids are rare because their bodies contain no bones and typically decompose altogether, so many questions remain unanswered about their early history.
The oldest known fossil spider is 125 million to 135 million years old; the oldest fossil scorpion is about 200 million years old, Hoy said.
Jeyaprakash and Hoy used a relatively new type of computer software in their study, analyzing all the genetic sequences from the mitochondria, cell components sometimes called “cellular power plants” because they produce chemical energy. Mitochondria provide a good yardstick for calculating when related organisms diverged from a common ancestor.
“Not too many people are using this strategy,” Hoy said. “It’s been used on fish but not on invertebrates.”
The UF researchers obtained complete genetic sequences for the mitochondria of 25 different invertebrates, including four spiders, three scorpions, 10 ticks and four mites. Then Jeyaprakash identified genetic sequences common to the organisms and used two sophisticated computer models to calculate how much time had passed since the sequences evolved from a common origin.
The results also supported the hypothesis that spiders, scorpions, ticks and mites all descended from a common ancestor, something scientists have generally believed for decades, Hoy said. The study didn’t provide any hints regarding that creature’s appearance, but it probably lived in the ocean, she said.
Although the findings are subject to debate, the UF study will be helpful to researchers making similar estimates in the future, said Hans Klompen, an associate professor with The Ohio State University’s entomology department.
“I think this is a great start,” said Klompen, who studies mite evolution. “Now we have something specific to test, and that’s always a good thing.”
The computer models Hoy and Jeyaprakash used may not fully compensate for variations in the speed of mitochondrial evolution, he said. So the results may indicate that some arachnids appeared on Earth earlier or later than they actually did.
“It might work better if the sampling size was bigger,” Klompen said. “But we can’t wait for the perfect data set. We have to work with what’s available.”