From Wikipedia: Yuri Knorozov:
In 1952 Knorozov published a paper which was later to prove to be a seminal work in the field (Drevnyaya pis’mennost’ Tsentral’noy Ameriki, or "Ancient Writing of Central America".) The general thesis of this paper put forward the observation that early scripts such as ancient Egyptian and Cuneiform which were generally or formerly thought to be predominantly logographic or even purely ideographic in nature, in fact contained a significant phonetic component. That is to say, rather than the symbols representing only or mainly whole words or concepts, many symbols in fact represented the sound elements of the language in which they were written, and had alphabetic or syllabic elements as well, which if understood could further their decipherment. By this time, this was largely known and accepted for several of these, such as Egyptian hieroglyphs (the decipherment of which was famously commenced by Jean-François Champollion in 1822 using the tri-lingual Rosetta Stone artefact); however the prevailing view was that Mayan did not have such features. Knorozov's studies in comparative linguistics drew him to the conclusion that the Mayan script should be no different from the others, and that purely logographic or ideographic scripts did not exist.