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EYE ON CHINA
Tracing Evolutionary History of the Himalayas
Scientists study samples of amphibians and reptiles collected from the Himalaya to gather information of the evolutionary prosses among the species within the region.

The Himalayan region is among the highest mountains in the world, however questions regarding the exact timing of their uplift and origins of their biodiversity are still in debate. Generally, there are two hypotheses about the uplift process of the Himalayas. The "Stepwise Hypothesis" states that the Himalaya rose slowly from 1000-2500 metres during 56 to 23 million years ago, before an additional rapid uplift to 4000 metres during 23 to 19 million years ago, and a final rise to the current average elevations – approximately 5000 metres – at around 15 million years ago. Recent hydrological and thermal evidence supported that this region was probably not elevated to current elevation till mid-Pliocene – the Late Orogeny Hypothesis.

Time-based records of biological processes can provide information about montane histories and environmental changes. Various hypotheses about Himalayan origins can be tested using phylogenetic information and estimates of the timing of biological speciation events. To address the question about the timing of the Himalaya uplift, a team of researchers carried out field work across the Himalayas to collect samples of amphibians and reptiles. The Himalayan region encompasses multiple countries, accessing each area proved to be challenging. Sampling across the entire region is difficult, thus inhibiting integrative studies of the origin of the Himalayan biota.

Combining 14 time-calibrated phylogenies of Himalayan-associated amphibian and reptile families involving 85 genera and 1628 species, the team estimated times of divergence among 183 species that occur in the Himalaya. 230 biogeographic events were identified to be related to the Himalayan species. The dynamics of in-situ diversification and dispersal rates remained essentially parallel across the Cenozoic. Both the in-situ diversification rate, as well as the dispersal rate into the Himalaya, fit the Stepwise Hypothesis for the origin of this mountain range. In contrast, the research team’s estimates of origination and peak diversification are not consistent with the late-uplift hypothesis.

The rapid Himalayan uplift and associated intensified South Asia Monsoon not only promoted a pulse of uplift-driven in situ diversification, but also affected the rates of biotic interchange. Biotic interchange was restricted by the lack of a moist environment that is required by many reptiles and amphibians. In contrast, an expanded tropical forest belt is thought to have persisted between the Himalaya and Southeast Asia since the middle Miocene, which likely accounts for the high dispersal rates between these two regions.

The research team highlighted that this work has important implications about the assembly process of Himalayan herpetofauna and its conservation. Their analyses demonstrated a deep-rooted origin of Himalayan herpetofauna originating in the Palaeocene, but with rapid diversification in the Miocene.

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