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What are Species? —Tracing the Speciation Process with Mammalian Fossils—

Yuichiro Nishioka
Assistant Professor, Waseda Institute for Advanced Study, Waseda University

What are Species?

We humans have the species name Homo sapiens, and all living organisms on Earth are assigned these Latin scientific names. This practice is based on a biological classification system known as binomial nomenclature, which was proposed in the 18th century by Carl von Linné. Species are one of the units used in this classification system, and classifies different organisms based on appearance, genome, and other factors such as their reproductive ability.

How do Species Diverge?

The phenomenon of a species diverging into multiple species is called speciation. For example, if a part of a group of creatures inhabiting a specific region become geographically isolated, and changes in appearance or genetic makeup develop between the two groups, then it can be said that a different species has formed. Speciation comes in various shapes and forms, but in Southeast Asia, which is considered a hot spot of biodiversity, it has been discovered that mammals speciated due to past environmental changes.

Paleontological Excavations in Myanmar

Figure 1: Fossil locality in central Myanmar. As it is difficult to know where fossils could be hidden, researchers must search through the grass to find them.

A collaborative research team organized by the Primate Research Institute of Kyoto University and Burmese researchers has been conducting excavations of mammalian fossils in Myanmar. I joined the research team in 2009 and have been focusing on collecting and observing mammalian fossils from the Neogene Irrawaddy beds, which ranges from about 10 million to 3 million years ago. After 8 years of research, I finally have an understanding of how mammals at the time evolved.

A large number of mammalian fossils have been found in Myanmar, but we do not stay in one place and dig in a way like we would to archeological sites. We must walk across vast plains and pick up fossils that protrude from the surface (see Figure 1). Much of these plains are covered in dry grass and sediment, and temperatures reach over 30 degrees C under the blazing sun. Searching for small mammalian fossils under these conditions can be excruciating work.

Discovery of Fossils Belonging to Rats and Mice Endemic to Southeast Asia

Figure 2: Lower jawbone fossil of a rodent species (reported as the new species Maxomys pliosurifer in 2015) that was discovered during excavation.

Recently, at a fossil site (of about 4 million to 3 million years ago) near the village of Gwebin, a large quantity of fossils of rodent species was found. In order to find fossils of something as small as rodents, you have to basically sit on a hill of sand and have a staring contest with the ground. After several weeks in Myanmar, the fatigue from my research, in addition to the oily food, which was a bit much for my stomach to handle, often had me feeling ill and crouching on the ground in pain while I pretended to look for fossils in the shade. Then, by sheer chance, I made a lucky discovery of a small fossil that looked to be that of a rodent (see Figure 2). Small fossils often tend to crowd in a single place, and we have been able to excavate several hundred of these rodent fossils in Gwebin.

The fossils discovered in the Irrawaddy beds in Myanmar included the newly found species of Hystrix and Maxomys. Furthermore, these rodents were almost all from a variety endemic to Southeast Asia, which led us to hypothesize that the ancestral group of the rodents that inhabit this region today existed at least until about 4 million to 3 million years ago. Conventional wisdom stated that the mammalian species of Myanmar migrated from South Asia (regions of India and Pakistan), but the fossils of rodent species that were discovered became proof that a geographical barrier (such as a high-elevation mountain range) existed between Myanmar and South Asia.

The Rodents Were Not the Only Isolated Animals

Figure 3: Observing a large number of bovid fossils (horn cores and limb bones) at a fossil locality.

As we analyzed fossils of oxen and antelopes (= bovids) that were found in Myanmar, we learned something interesting (see Figure 3). After classifying bovid fossils from three time periods, specifically the early late Miocene (about 10 million to 8 million years ago), the latest Miocene to the early Pliocene (about 6 million to 4 million years ago), and the late Pliocene (4 million to 3 million years ago), it was discovered that the species from South Asia and Myanmar were identical up to about 8 million years ago, but around 6 million years ago, species endemic to Myanmar emerged. It became apparent that both of these species coexisted until about 4 million years ago, but after that, the species from South Asia became extinct, and only the species endemic to Myanmar survived. By tracing the changes in what species were living in each time period, it became clear that a major event that separated mammal species (not only small mammals such as rodents, but also groups of larger mammalian species) occurred in South Asia and Myanmar during the Pliocene. In western Myanmar, the Arakan Mountains and large rivers cut across the country and serve as borders. It is possible that these land formations were barriers that were the cause of speciation (see Figure 4).

Figure 4: The relationship between the speciation of mammalian species and the geographical transformation that occurred of the Southeast Asian region. The starred areas are major fossil localities.

Our Next Challenge

We have expanded the scope of our research to include neighboring Thailand, where we are continuing to excavate and analyze mammalian fossils from the same time periods. The mammalian fossils found in Thailand are very similar to those found in Myanmar, which is evidence that there were no geographical barriers between these regions that separated animal species. Our research of ancient mammalian animals in Thailand has only started. In order to discover new fossils, communicating and developing a relationship of trust with the local population, including collaborating researchers, is of utmost importance. Regardless of whether or not we are able to find fossils right away, I hope to continue searching for mammalian fossils during my travels to Thailand.

Yuichiro Nishioka
Assistant Professor, Waseda Institute for Advanced Study, Waseda University

Yuichiro Nishioka graduated from the Department of Earth and Planetary Sciences at the School of Science, Nagoya University in 2008. He continued his studies and earned his Doctor of Science degree from the Division of Biological Science at the Graduate School of Science, Kyoto University in 2013. Before assuming his current position in 2016, he worked as a researcher at the Primate Research Institute of Kyoto University and at the Museum of Osaka University. He is an expert on vertebrate paleontology. He is one of the authors of Mice and Rats in Japan: Their Diversity and Evolution [Nihon no Nezumi: Tayosei to Shinka] (University of Tokyo Press).