Martins compared the results of existing studies into the Maku languages and for many years he also worked with speakers of these various languages. The Brazilian researcher established that the modern Maku languages can be divided into two groups, an eastern Brazilian branch and a western Colombian branch. Furthermore he reconstructed the sound structure as well as part of the lexicon of the 'ancestor' of the Brazilian Maku languages, Proto-East-Maku.
The Brazilian branch of the language family contains the Indian languages Nadëb (300 speakers), Kuyawi (130), Hupda (1900), Yuhup (400, on the Colombian/Brazilian border), and Dâw (65). The Colombian branch contains the languages Nukak (700), Kakua (125), and the more widely distributed Puinave (2000).
Most of the older classifications of the languages spoken in the Amazon area are based on incomplete reports of European explorers made at the start of the last century. These mostly learned travellers used the phonetic similarity of words to group languages. However the Brazilian Maku languages evolved in a misleading manner. As a result of this, the earlier comparative linguists could not discover the relationship between the Maku languages and the other language families which Martins discovered.
The words of the eastern Maku languages have gradually become shorter during their history because, for example, the unstressed syllables within words and at the at the end of words have disappeared.
Now the majority of words contain just one syllable. Perhaps as a compensation for the increasingly shorter words, the speakers of these languages increased the number of vowels. The languages also developed a system of tonal oppositions, with which words could be distinguished from each other in terms of phonetic form and meaning. As a result of these developments the Maku languages sound nothing like the other Amazonian languages.
Due to the somewhat exceptional structure of their languages, some researchers claimed that the Maku Indians were the first inhabitants of the Amazon region. However, the proto-language of the Brazilian Maku languages constructed by Martins is so similar to the Arawak languages that in the researcher's opinion, these language families must be genetically related. Accordingly, the question as to when the language family arrived in the region has become irrelevant.
Valteir Martins' research was funded by the Netherlands Foundation for the Advancement of Tropical Research (NWO-WOTRO).