Scorpions tell a fascinating story of India’s repeated brush-ups with Southeast Asia
doi:10.1038/nindia.2021.16 Published online 4 February 2021
Tracking the evolutionary history of Asian forest scorpions, zoologists suggest that the arthropods moved out of India three times to colonise Southeast Asia1. This new evidence supports the theory that India brushed past Southeast Asia multiple times before finally fusing into it and becoming one landmass – Eurasia – several million years ago.
Geologists believe that an ancient southern supercontinent – Gondwana – broke into pieces during the Jurassic era (about 180 million years ago) and spread out at different times to form a landmass mosaic that is the present day Southeast Asia.
“Although today we see whole continents and islands, many landmasses such as Borneo, are actually composite, made up of different parts that came together at different periods of time,” says Stephanie F. Loria, one of the zoologists involved in the study from the American Museum of Natural History (AMNH) in New York.
Scorpions, she says, are great storytellers because of the age of their lineage, as well as their diversity and limited distributions. Besides their ecological importance, scorpions also help us to understand the history of the world, she says of the poorly studied organisms.
Scorpiologists have wondered how these scorpions reached Southeast Asia – did they travel south from Eurasia or did they move towards Southeast Asia on fragments of Gondwana? The Indian landmass, with its large size, makes for an important living laboratory, given organisms in India, Africa and South America – all once part of Gondwana – are closely related.
Loria and her PhD supervisor Lorenzo Prendini from AMNH's Arachnology Laboratory and Scorpion Systematics Research Group drew from earlier studies which suggested that scorpions rode on India to reach Southeast Asia. The information missing from these studies – just how many times did these scorpions colonize Southeast Asia from India and when that happened – was of interest to them.
Analysing the evolutionary relationships among various species of the Asian forest scorpion subfamily, Heterometrinae, measured by the number of genetic changes they accumulated over time, Loria and Prendini found evidence that these scorpions colonized Southeast Asia three times from India. In the Cretaceous period (between 145 to 66 million years ago), the Heterometrinae diverged from another subfamily, Pandininae, on the African continent when the Indian subcontinent separated from it.
During the Cretaceous–Tertiary (KT) mass extinction, environmental stresses associated with Deccan volcanism caused the range of this subfamily to contract, restricting one clade of Heterometrinae to refuges in the Western Ghats of India and in the Central Highlands of Sri Lanka, according to Prendini.
Heterometrinae dispersed to Southeast Asia three times during India’s collision with Eurasia – first when the Indian subcontinent brushed up against the western side of Sumatra, and twice later as India moved closer to Eurasia. The AMNH scorpiologists say that the Indian Heterometrinae, confined to southern India and Sri Lanka, recolonized the Deccan Plateau and northern India, and diversified into the increasingly arid habitats there after environmental conditions stabilized.
By reconstructing evolutionary relationships among living scorpions, scientists offer a peek into the geological and climatic past of life on earth. The oldest scorpion fossils date to roughly 440 million years ago, much older than most vertebrate groups.
Loria says scorpions offer a window to how life evolved deep in time. Scorpions are also diverse and most species have small ranges. This allows scientists to reconstruct regional biogeographical histories. “Range size is like the number of pixels in an image,” Loria says. More pixels mean clearer image resolution. Vertebrates inhabit larger ranges and therefore offer fewer pixels whereas many arthropod groups have smaller ranges and more pixels.Sankar Chatterjee, a professor of paleontology at the Texas Tech University (TTU) says the study gives a broader context of rifting, drifting and collisions of India by combining evidence from multiple disciplines. A curator of paleontology and director of TTU’s Antarctic Research Center at the Museum of Texas Tech University, Chatterjee says the study refines the ‘Out of India’ hypothesis using scorpions. The work, he says, is bolstered by the sophisticated diversification analysis using molecular clock calibration for reconstructing the biogeographic dispersal of scorpions from the Cretaceous period till today.
1. Loria, S. F. & Prendini, L. Out of India, thrice: diversification of Asian forest scorpions reveals three colonizations of Southeast Asia. Sci. Rep. 10, 22301 (2020) doi: 10.1038/s41598-020-78183-8