Masatoshi Nei is a very smart man who knows a lot about how living things (like plants and animals) change and evolve over time. He studied lots of different animals, like birds and monkeys, to figure out how they are related to each other and how they might have changed over millions of years.
Nei used a special technique called molecular evolution to study these animals. This means that he looked at their DNA (which is like a set of instructions for how to build and live a certain way) to see how similar it is to other animals' DNA. This helped him figure out which animals are more closely related to each other and which ones are more distant.
By studying DNA, Nei was able to learn some really cool things about how animals change and adapt over time. He also helped scientists understand more about how different species are related to each other, which is important for things like understanding human evolution or how diseases are spread between different animals.