Complex geological processes within the lowermost mantle are keys to our understanding of Earth's evolution and present-day inner composition. Among such processes, partial melting has been proposed to explain mantle regions with ultralow seismic velocity near the core-mantle boundary (CMB). To test this hypothesis, the solidus curve of a natural fertile peridotite, approximating the composition of the primitive mantle, was determined in laser-heated diamond-anvil cells between 36 and 140 GPa. Melting at CMB pressure occurs at 4180 ± 150 K, a value matching estimated mantle geotherms. Molten regions may therefore exist at the base of the present-day mantle. Together with melting phase relations and element partitioning data, our results are also used to discuss the geochemical implications for early mantle differentiation.