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We explore the complete history of the 21-cm signal in the redshift range z =7-40. This redshift range includes various epochs of cosmic evolution relatedto primordial star formation, and should be accessible to existing or plannedlow-frequency radio telescopes. We use semi-numerical computational methods toexplore the fluctuation signal over wavenumbers between 0.03 and 1 Mpc$^{-1}$,accounting for the inhomogeneous backgrounds of Ly-$\alpha$, X-ray,Lyman-Werner and ionizing radiation. We focus on the recently noted expectationof heating dominated by a hard X-ray spectrum from high-mass X-ray binaries. Westudy the resulting delayed cosmic heating and suppression of gas temperaturefluctuations, allowing for large variations in the minimum halo mass thatcontributes to star formation. We show that the wavenumbers at which theheating peak is detected in observations should tell us about thecharacteristic mean free path and spectrum of the emitted photons, thus givingkey clues as to the character of the sources that heated the primordialUniverse. We also consider the line-of-sight anisotropy, which allowsadditional information to be extracted from the 21-cm signal. For example, theheating transition at which the cosmic gas is heated to the temperature of thecosmic microwave background should be clearly marked by an especially isotropicpower spectrum. More generally, an additional cross-power component $P_X$directly probes which sources dominate 21-cm fluctuations. In particular,during cosmic reionization (and after the just-mentioned heating transition),$P_X$ is negative on scales dominated by ionization fluctuations and positiveon those dominated by temperature fluctuations.

The rich complexity of 21-cm fluctuations produced by the first stars