We present results from new deep HST/ACS photometry of I Zw 18 , the most metal-poor blue compact dwarf galaxy in the nearby universe .
It has been previously argued that this is a very young system that started forming stars only \lesssim 500 Myr ago , but other work has hinted that older ( \gtrsim 1 Gyr ) red giant branch ( RGB ) stars may exist in this galaxy .
Our deeper data indeed reveal evidence for an RGB .
Underlying old ( \gtrsim 1 Gyr ) populations are therefore present in even the most metal-poor systems , implying that star formation started at z \gtrsim 0.1 .
The RGB tip ( TRGB ) magnitude and the properties of Cepheid variables identified from our program indicate that I Zw 18 is farther away ( D = 19.0 \pm 1.8 Mpc ) than previously believed .