hough Martin claims the lack of evidence actually supports his model—the evidence is sparse because the spread of humans and the extinction of animals occurred so quickly—this argument seems weak. And how could we ever disprove it?As archaeologist Donald Grayson points out, in other cases where extinction resulted from the quick spread of human hunters—for example, the extinction of the moa, the large flightless bird of New Zealand—archaeological evidence in the form of remains is abundant. Grayson has also shown that the evidence is not so clear that all or even most of the large herbivores in late Pleistocene America became extinct after the appearance of Clovis. Of the 35 extinct genera, only 8 can be confidently assigned an extinction date of between 12,000 and 10,000 years ago.Many of the older genera, Grayson argues, may have succumbed before 12,000 B.C., at least half a century before the Clovis showed up in the American West. 

请问老师，这里的at least half a century before the Clovis showed up in the American West.  是做什么成分？

我的理解，Many of the older genera, Grayson argues, may have succumbed before 12,000 B.C., at least half a century before the Clovis showed up in the American West. 等价于Many of the older genera, Grayson argues, may have succumbed before 12,000 B.C., which is at least half a century before the Clovis showed up in the American West. 这里的which指代公元前12,000年。所以at least half a century before the Clovis showed up in the American West.是12,000 B.C.的同位语，第二个before是连词。如果上述理解正确，请问老师，这里的before the Clovis showed up in the American West.充当什么成分？