Few spiders elicit as much reaction from humans as the famous and feared tarantula. These giant, hairy arachnids are known for their remarkable size, brilliant colors, and distinctive physical attributes.
But it's not just the tarantula itself that is so impressively (albeit unsettlingly) large. So is the creature's footprint on the globe – which is surprising since tarantulas are relatively sedentary spiders; females and juveniles in particular rarely wander away from their burrows, if they do at all.
Nonetheless, tarantulas (the Theraphosidae family of spiders) are to be found virtually everywhere, living on all Earth's continents except for Antarctica.
"They are quite widespread and are found throughout the subtropical regions of every continent," a research team led by bioinformatician Saoirse Foley from Carnegie Mellon University explains in a new study.
"[Their] behaviors do not portend that tarantulas would be successful dispersers, yet they have spread across the globe and have colonized strikingly different ecological niches."
What can explain the successful migration of tarantula spiders to so many different corners of the globe?
In their new study, Foley and fellow researchers investigated the biogeographic patterns of tarantulas throughout history, analyzing messenger RNA in tarantula transcriptome databases, and modeling how the tarantula family tree could have developed over a hypothesized 120 million years of evolution.
That ancestry looks to be the key to the tarantula's wide dispersion, with early tarantula ancestors journeying around the world via continental drift, as the world's most fundamental landmasses roamed and collided over millions of years after the breakup of the supercontinent Gondwana.
"Previous studies estimate that tarantulas emerged between 150 Ma-71 Ma or ~107 Ma, which is compatible with a Gondwanan origin," the researchers explain.
"Indeed, some tarantulas (Selenocosmiinae) are suggested to be North Gondwanan taxa."
In their own analysis, the team found evidence for two separate 'out of India' dispersals of ancient tarantulas into Asia, when the Indian subcontinent collided with Asia roughly 55 to 35 million years ago.
"Interestingly, despite our analyses suggesting a Gondwanan origin for Theraphosidae, this pattern suggests that tarantulas were not always present in Oceania, and instead is consistent with Selenocosmiinae having diversified across Asia, eventually crossing the Wallace line sometime after the India/Asia collision (possibly as early as 47 Ma, Fig. S3), while the terrestrial Thrigmopoeinae remained in India," the researchers write.
"Our results indicate that both of these Asian lineages diverged while the Indian Plate was still rafting towards Asia… Interestingly, the two lineages also appear to be ecologically divergent."
The results ultimately suggest that while continental drift played a pivotal role in helping these ancient spiders colonize new continents, their own evolutionary adaptations were also important to geographic spread, with species seizing the opportunity to capitalize on ecological and environmental circumstances.
"Perhaps these radiations can be attributed to an ancient switch in lifestyle that each ecologically distinct subfamily to become successful by exploiting different ecological niches," the authors explain.
"Ancient tarantulas appear to have undergone several diversifications on India while it was still rafting, affirming the evolutionary significance of the subcontinent."
The findings are reported in PeerJ.