Tarantulas, everyone's favorite hairy spiders, are found worldwide, inhabiting all continents except Antarctica. But how did they become so widespread? Females rarely leave their burrows, spiderlings stick close to where they hatch, and mature males only travel when they're searching for a mate.
To answer this question, researchers went looking for the origins of the tarantula group more than 100 million years ago, building a tarantula family tree based on molecular clues from existing databases of spiders' transcriptomes — the protein-coding portion of the genome, found in ribonucleic acid, or RNA.
Once they created the tree, they mapped it to a timeline of spider fossils, to estimate when — and where — tarantulas appeared and dispersed.
The scientists discovered that tarantulas first emerged during the Cretaceous period in what is now the Americas. But at the time, the Americas were part of the massive supercontinent Gondwana. Ancient tarantula relatives, even if they were homebodies like tarantulas today, likely spread across the joined landmasses, dispersing from the Americas into Africa, Australia and India. Then, after Gondwana broke apart, India separated from Madagascar and collided with Asia — and brought the hairy spiders to that continent, too, he researchers reported.
There are only two known tarantula fossils, both preserved in amber: One is from Mexico, and is thought to be around 16 million years old, and the other is from Myanmar and is about 100 million years old, the study authors reported. Because tarantula fossils are so rare, the researchers also collected data from related mygalomorphs — the arachnid group that includes tarantulas and other big, ground-dwelling spiders — that are better represented in the fossil record than are tarantulas.
After constructing a family tree for tarantulas from transcriptome data, representing 29 tarantula species and 18 other mygalomorphs, the scientists time-calibrated the tree using data from fossils. This enabled the researchers to calculate the ages of tarantula lineages, and to approximate when the ancestors of modern tarantulas spread over the world.
According to this new timeline, tarantulas first appeared in the Americas about 120 million years ago. There, the spiders that were ancestors to Africa's tarantulas emerged around 112 million to 108 million years ago. By about 108 million years ago, tarantulas were established in what is now India. India separated from Madagascar between 95 million and 84 million years ago, and drifted toward Asia; that slow-motion collision, which began between 58 million and 35 million years ago, brought tarantulas to the Asian continent.
However, before that happened, India's tarantulas diverged into two lineages with different lifestyles: One group of tarantulas was predominantly tree-dwellers, and the other mostly preferred life in burrows. Both lineages eventually spread into Asia, but the arboreal group (Ornithoctoninae, also known as "Earth tigers") did so 20 million years after their burrowing cousins.
This second, later wave of tarantula dispersal into Asia suggests that the spiders were able to fill ecological niches and adapt to new habitats more effectively than once thought.
"Previously, we did not consider tarantulas to be good dispersers," lead study author Saoirse Foley, an evolutionary biologist at Carnegie Mellon University in Pittsburgh, said in a statement. "While continental drift certainly played its part in their history, the two Asian colonization events encourage us to reconsider this narrative," Foley said.
The findings were published online April 6 in the journal PeerJ.
Originally published on Live Science.