Question

The following passage is adapted from an essay on epigraphy and Maya archaeology:

In the spring of 1960, epigrapher Tatiana Proskouriakoff published a groundbreaking study analyzing the carved stone stelae of Piedras Negras, an ancient Maya city situated along the Usumacinta River in Guatemala. Prior to her work, the prevailing consensus among scholars, championed by archaeologist J. Eric S. Thompson, maintained that Maya inscriptions were almost exclusively mathematical and astronomical computations, recording abstract calendrical cycles and religious rituals rather than historical accounts of human lives.

Proskouriakoff disrupted this long-held paradigm by focusing on the physical arrangement and chronological sequence of thirty-five stelae erected at Piedras Negras. She observed that these monuments were organized into distinct chronological groups, with each group associated with a specific architectural niche or structure. Within each group, the earliest stela invariably depicted a seated figure housed inside a niche, above which hung a carved celestial band. Proskouriakoff noticed a recurring motif carved beneath this initial image: a stylized footprint ascending a ladder toward the seated figure.

Crucially, Proskouriakoff examined the exact dates recorded on the monuments within each individual group. She discovered that the span of time covered between the earliest date on the first stela of a group and the latest date on the final stela never exceeded sixty-four years. Because no single group spanned more than a typical human lifespan, she postulated that each group recorded the reign of an individual Maya ruler. The earliest date marked the ruler's birth or accession to power, indicated by the 'niche motif' and the ascending footprints, while subsequent stelae in the group commemorated key events during that monarch's lifetime.

To verify her hypothesis, Proskouriakoff identified specific glyphic phrases that repeatedly accompanied these monumental dates. She demonstrated that a particular glyph, which scholars had previously classified as a mysterious ritual symbol, consistently appeared alongside the date marking the start of a group's timeframe. Proskouriakoff reasoned that this specific glyph represented the act of royal accession or enthronement. By proving that Maya texts recorded real political dynasties and historical human events, her 1960 paper fundamentally transformed the field of Mesoamerican studies.

According to the passage, what specific evidence directly led Proskouriakoff to conclude that each group of stelae recorded the lifetime or reign of an individual Maya ruler?