 Zuckerkandl and Pauling then assumed that amino acid changes were roughly linearly proportional to time, and thus the time between the divergence of humans and horses was nine times greater than that between humans and gorillas. Using paleontological estimates for calibration, Zuckerkandl and Pauling concluded that the humans and gorillas diverged a mere 14.5 million years ago—much more recent than traditionally was thought. This line of inference is not limited to molecular phylogenetics, but has been imported for application to other data types, e.g., similar reasoning from phylogenetic techniques has been applied to language-tree divergence times to estimate Indo-European and Polynesian human migration (Gray &amp; Jordan 2000; Gray &amp; Atkinson 2003; Gray, Drummond, &amp; Greenhill 2009). (For more on the early development of the molecular clock, see Morgan 1998.)