Skeletal remains recovered and analyzed from Archaic and Paleoindian periods demonstrate less pronounced Asiatic/Sinodont features that distinguish them from present day Amerindians. This paper describes the metric and nonmetric traits that link a Middle Plains Archaic male (radiocarbon dated to 2220-2500 B. C.), found near Sidney, Nebraska, to Sinodonts, Sundadonts, and Paleoindians. Metrically, the Sidney male differs from Late Prehistoric and Historic Mandan and Arikara males (1500 to 1830 A.D.) from the same region in cranial vault height (auricular height p ≤ .02 basion-porion height p ≤ .07). His cranium is longer and higher (acrocranic Cranial Breadth-Height Index) than that of the more highly derived Mandan and Arikara males. Several of the Sidney male’s cranial and femoral traits show a blend of Amerindian and earlier protomongoloid traits, distinguishable from recent Amerindian populations. These traits suggest affiliation to a common Eurasian progenitor for Sinodonts, Sundadonts, and Paleoindians, and support the hypothesis that Plains Amerindians descended from the earliest wave of Paleoindians who crossed the Bering Straits. Tracing microevolutionary changes across time is a challenging, incremental process, not yet resolved by the limited Paleoindian and Archaic skeletal remains discovered to date. However, the intermediate skeletal characteristics of the Sidney male indicate gradual adaptation and suggest that natural selection most strongly influenced the adaptation of Plains peoples. Information presented here increases the database needed for future investigations of microevolution, gene flow patterns and the cultural history that may someday link early Archaic populations and Paleoindians to specific tribes among the modern Plains Amerindians.