Some time in the past 70 million years, the land mass that we now call India rammed into current-day Asia. Rocks were squeezed skyward to form the Himalayan Mountains and land-bound creatures from each continent expanded their ranges into the newly acquired territory. That much, scientists agree on. But most of the details of the continental merge remain controversial; estimates of when the collision occurred range from 66 to 20 million years ago. Some geologists—like van Hinsbergen et. al. in PNAS last year—have outlined evidence that part of India broke off and hit Asia before the rest of the subcontinent. Others think India, as it drifted from Madagascar toward Asia, collided with a chain of islands, leaving deceptive battle scars that have been confused with markers of the final collision with Asia. And some think it was more straight-forward but can’t agree on the speed of the impact.
In a new PNAS Early Edition paper, an international team of researchers turns to frogs—yes, those tail-less, hoppy amphibians—to shed some light on the matter. The frogs, all belonging to the phylogenetic family rhacophoridae (commonly known as Old World tree frogs or shrub frogs) are found across both Asia and India and their evolution had previously been traced back to a split from other frogs around 60 million years ago. So Li et. al. figured that the dispersal pattern of the frogs—which species are found where and when in history they evolved apart—could help explain when Asia and India had physically joined. After all, the frogs live in trees, avoid salt water, and certainly can’t survive a swim across any ocean. So to hop from one bit of land to another requires the two lands are touching (or there’s a conveniently placed bridge).
The team compared DNA from 114 different species of rhacophorid frogs collected from their native habitats across India, Asia, and Africa. By combining data on which frogs were most closely related and where they were found, the researchers could estimate when, and where, different branches of the Rhacophoridae family tree had originated. The new paper concludes that the frogs first spread from India to Asia around 50 million years ago. But from 46 to 39 million years ago there was a lack of movement of the frogs between the continents—those that were on India after the original dispersal remained there, evolving into new species on their own, and those that were in Asia likewise split into new species only within Asia. Then, 39 million years ago, the mixing of the frogs between the continents began again, and continued until 12 million years ago, when the Himalayans likely formed too large of a barrier.
The “traditional” model of the India-Asia collision (a relatively straight-forward joining) can’t account for the gap in dispersal of the frogs between 46 and 39 million years ago, the authors state. The climate at the time was ideal for frogs and they would have continuously expanded in range to any accessible land. So their evidence supports a more complex picture: either a collision of India with islands and a gradual merge of the continents that provided one brief land bridge for frogs (and other animals) to cross long before the final hit, or a model in which a small portion of India collided earlier than the remainder. While frogs’ DNA can’t tell the exact story of what happened, it can complement geological data to help put dates on the formation of the continents as we know them today. The dates provided by the Rhacophoridae, the authors write, contradict models in which the final collision of the continents didn’t occur until 25 to 20 million years ago. But by continuing to combine geology and evolutionary biology, scientists will get an ever-clearer picture of the past.