The authors report the detection of a flattened absorption profile in the sky-averaged radio spectrum, centered at 78 MHz with a full-width-at-half-maximum of 19 MHz and an amplitude of 0.5 K. This profile is consistent with the expected 21-cm signal from early stars, but the observed amplitude is more than twice the largest predictions. The discrepancy suggests either that the primordial gas was colder than expected or that the background radiation temperature was hotter. The low-frequency edge of the profile indicates that stars existed and produced Lyman-α photons by 180 million years after the Big Bang, while the high-frequency edge suggests the gas was heated to above the radiation temperature by 270 million years later. The detection was made using the Experiment to Detect the Global EoR Signature (EDGES) low-band instruments, which observed spectra between 50 and 100 MHz. The authors performed extensive tests to verify the detection, including calibration checks and foreground model validation, and conclude that the observed profile is robust to instrumental artifacts. The result supports ongoing efforts to detect the statistical properties of spatial fluctuations in the 21 cm signal using interferometric arrays.The authors report the detection of a flattened absorption profile in the sky-averaged radio spectrum, centered at 78 MHz with a full-width-at-half-maximum of 19 MHz and an amplitude of 0.5 K. This profile is consistent with the expected 21-cm signal from early stars, but the observed amplitude is more than twice the largest predictions. The discrepancy suggests either that the primordial gas was colder than expected or that the background radiation temperature was hotter. The low-frequency edge of the profile indicates that stars existed and produced Lyman-α photons by 180 million years after the Big Bang, while the high-frequency edge suggests the gas was heated to above the radiation temperature by 270 million years later. The detection was made using the Experiment to Detect the Global EoR Signature (EDGES) low-band instruments, which observed spectra between 50 and 100 MHz. The authors performed extensive tests to verify the detection, including calibration checks and foreground model validation, and conclude that the observed profile is robust to instrumental artifacts. The result supports ongoing efforts to detect the statistical properties of spatial fluctuations in the 21 cm signal using interferometric arrays.