India is home to around 62 species of Skinks, a type of lizard that is commonly found in rocky areas throughout India. For years, a puzzling mystery has surrounded the Sphenomorphus genus of skinks in India. A total of 5 Sphenomorphus species are found in India. While four species are found in the country’s northeastern region, nestled close to Southeast Asia, a fifth species, Sphenomorphus dussumieri, is found far away in the southwestern part of Kerala, in peninsular India.
This disjunct distribution, where species of the same group are found in widely separated areas, begged a crucial question as to how they came to live so far apart. Did these skinks once live across a continuous stretch of land that later broke apart, or did they arrive in these different locations through separate, long-distance journeys?
A new study from the National Institute of Science Education and Research (NISER), Homi Bhabha National Institute (HBNI), University of Florida, USA and San Diego Natural History Museum, USA, sheds new light on the fascinating dispersal history of the Sphenomorphus skinks. The researchers found that the peninsular Indian skink, Sphenomorphus dussumieri, isn’t closely related to its Indian cousins in the Northeast. Instead, its closest relatives are found primarily in the Sunda region of Southeast Asia, a vast area that includes islands like Borneo, Sumatra, and Java.
Did You Know? Skinks are everywhere! The genus Sphenomorphus is one of the most diverse groups of lizards, with over 115 recognised species found across tropical Asia, making them one of the most successful lizard groups on the planet! |
This surprising family connection suggests that S. dussumieri’s ancestors didn’t walk across a land bridge that once connected Northeast India to the peninsula. Instead, they likely made an incredible transoceanic journey, essentially rafting across the sea from the Sunda shelf to peninsular India. This epic voyage, the study estimates, occurred around 21 million years ago, during the early Miocene epoch, a time when global sea levels were significantly lower, making such a journey more feasible. In contrast, the Sphenomorphus species in Northeast India, like S. maculatus and S. indicus, appear to have arrived through a more conventional geodispersal route, likely via ancient land connections from the Indo-Chinese and Sunda regions.
To study their ancient dispersal route, the researchers employed a suite of genetic and computational techniques. First, they collected samples of Sphenomorphus dussumieri from Kerala, India, and also gathered samples of other Sphenomorphus species from across Indo-China and Southeast Asia. From these samples, they carefully extracted DNA, focusing on six specific genetic markers – four from the mitochondria and two from the nucleus. These genetic markers act like unique barcodes, allowing scientists to compare the genetic makeup of different species.
Once they had the DNA sequences, they used computer programs to build a phylogenetic tree, which is essentially a family tree showing the evolutionary relationships between all the Sphenomorphus species. They used two main methods for this: Maximum Likelihood (ML) and Bayesian Inference (BI), both of which help determine the most probable evolutionary relationships. To figure out when these evolutionary branches occurred, they performed divergence dating using a software called BEAST2. This process uses known geological events or fossil records as calibration points to estimate the timing of evolutionary splits. For instance, they used previously established dates for the common ancestors of other lizard groups to set a timescale for their skink family tree.
Finally, to understand where these ancient dispersal events happened, they used a program called BioGeoBEARS. This tool allowed them to reconstruct the ancestral geographic ranges of the skinks, testing different scenarios like land bridge dispersal versus transoceanic rafting, and identifying the most likely historical pathways. By comparing the likelihood of various models, they concluded that the jump dispersal model, which includes long-distance movements, best explained the current distribution of S. dussumieri.
While previous studies have shown evidence of faunal exchange between Southeast Asia and India, many of these events, such as the dispersal of certain agamids (another type of lizard) or frogs, occurred much earlier, during the Eocene epoch (around 48-56 million years ago). The discovery of Sphenomorphus dussumieri‘s transoceanic dispersal in the early Miocene (around 21 million years ago) highlights that such long-distance movements are a recurring theme in the region’s biogeographic history, happening at different geological times. This work also provides a clearer picture of the Sphenomorphus genus specifically, distinguishing between two distinct dispersal events into India.
However, the researchers noted that there might be missing species in their sampling, particularly between the Sunda region and India, which could potentially refine the estimated divergence dates. Additionally, the complex evolutionary history of the broader Sphenomorphinae group and the lack of genetic data for some markers in certain species meant that some parts of their evolutionary tree had weaker support. Despite these challenges, the study’s core findings regarding S. dussumieri‘s unique journey remain robust.
In an era of rapid climate change and habitat loss, knowing how species have historically responded to environmental shifts, whether by migrating across land or by rafting across oceans, provides crucial insights into their resilience and adaptability. More importantly, by piecing together the evolutionary history of Earth’s biodiversity, we gain a deeper appreciation for the intricate web of life and the dynamic forces that have shaped it.
This article was written with the help of generative AI and edited by an editor at Research Matters.