jats:titleAbstract</jats:title> jats:pWe report the first direct measurement of the helium isotope ratio, jats:sup3</jats:sup>He/jats:sup4</jats:sup>He, outside of the Local Interstellar Cloud, as part of science-verification observations with the upgraded CRyogenic InfraRed Echelle Spectrograph. Our determination of jats:sup3</jats:sup>He/jats:sup4</jats:sup>He is based on metastable He jats:sci</jats:sc>* absorption along the line of sight toward Θjats:sup2</jats:sup>A Ori in the Orion Nebula. We measure a value jats:sup3</jats:sup>He/jats:sup4</jats:sup>He = (1.77 ± 0.13) × 10jats:sup−4</jats:sup>, which is just ∼40% above the primordial relative abundance of these isotopes, assuming the Standard Model of particle physics and cosmology, (jats:sup3</jats:sup>He/jats:sup4</jats:sup>He)jats:subp</jats:sub> = (1.257 ± 0.017) × 10jats:sup−4</jats:sup>. We calculate a suite of galactic chemical evolution simulations to study the Galactic build up of these isotopes, using the yields from Limongi & Chieffi for stars in the mass range jats:italicM</jats:italic> = 8–100 jats:italicM</jats:italic> jats:sub⊙</jats:sub> and Lagarde et al. for jats:italicM</jats:italic> = 0.8–8 jats:italicM</jats:italic> jats:sub⊙</jats:sub>. We find that these simulations simultaneously reproduce the Orion and protosolar jats:sup3</jats:sup>He/jats:sup4</jats:sup>He values if the calculations are initialized with a primordial ratio jats:inline-formula jats:tex-math

</jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> mml:msub mml:mrow <mml:mfenced close=")" open="("> mml:mrow mml:msup <mml:mrow /> mml:mrow mml:mn3</mml:mn> </mml:mrow> </mml:msup> mml:miHe</mml:mi> mml:msup mml:mrow <mml:mo stretchy="true">/</mml:mo> </mml:mrow> mml:mrow mml:mn4</mml:mn> </mml:mrow> </mml:msup> mml:miHe</mml:mi> </mml:mrow> </mml:mfenced> </mml:mrow> mml:mrow <mml:mi mathvariant="normal">p</mml:mi> </mml:mrow> </mml:msub> mml:mo=</mml:mo> <mml:mo stretchy="false">(</mml:mo> mml:mn1.043</mml:mn> mml:mo±</mml:mo> mml:mn0.089</mml:mn> <mml:mo stretchy="false">)</mml:mo> mml:mo×</mml:mo> mml:msup mml:mrow mml:mn10</mml:mn> </mml:mrow> mml:mrow mml:mo−</mml:mo> mml:mn4</mml:mn> </mml:mrow> </mml:msup> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjac6503ieqn1.gif" xlink:type="simple" /> </jats:inline-formula>. Even though the quoted error does not include the model uncertainty, this determination agrees with the Standard Model value to within ∼2jats:italicσ</jats:italic>. We also use the present-day Galactic abundance of deuterium (D/H), helium (He/H), and jats:sup3</jats:sup>He/jats:sup4</jats:sup>He to infer an empirical limit on the primordial jats:sup3</jats:sup>He abundance, jats:inline-formula jats:tex-math

</jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> mml:msub mml:mrow <mml:mfenced close=")" open="("> mml:mrow mml:msup <mml:mrow /> mml:mrow mml:mn3</mml:mn> </mml:mrow> </mml:msup> mml:miHe</mml:mi> mml:mrow <mml:mo stretchy="true">/</mml:mo> </mml:mrow> <mml:mi mathvariant="normal">H</mml:mi> </mml:mrow> </mml:mfenced> </mml:mrow> mml:mrow <mml:mi mathvariant="normal">p</mml:mi> </mml:mrow> </mml:msub> mml:mo≤</mml:mo> <mml:mo stretchy="false">(</mml:mo> mml:mn1.09</mml:mn> mml:mo±</mml:mo> mml:mn0.18</mml:mn> <mml:mo stretchy="false">)</mml:mo> mml:mo×</mml:mo> mml:msup mml:mrow mml:mn10</mml:mn> </mml:mrow> mml:mrow mml:mo−</mml:mo> mml:mn5</mml:mn> </mml:mrow> </mml:msup> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjac6503ieqn2.gif" xlink:type="simple" /> </jats:inline-formula>, which also agrees with the Standard Model value. We point out that it is becoming increasingly difficult to explain the discrepant primordial jats:sup7</jats:sup>Li/H abundance with nonstandard physics, without breaking the remarkable simultaneous agreement of three primordial element ratios (D/H, jats:sup4</jats:sup>He/H, and jats:sup3</jats:sup>He/jats:sup4</jats:sup>He) with the Standard Model values.</jats:p>