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The baryonic processes in galaxy evolution include gas infall onto galaxiesto form neutral atomic hydrogen (HI), the conversion of HI to the molecularstate (H$_2$), and, finally, the conversion of H$_2$ to stars. Understandinggalaxy evolution thus requires understanding the evolution of both the stars,and the neutral atomic and molecular gas, the primary fuel for star-formation,in galaxies. For the stars, the cosmic star-formation rate density is known topeak in the redshift range $z \approx 1-3$, and to decline by an order ofmagnitude over the next $\approx 10$ billion years; the causes of this declineare not known. For the gas, the weakness of the hyperfine HI 21cm transition,the main tracer of the HI content of galaxies, has meant that it has nothitherto been possible to measure the atomic gas mass of galaxies at redshiftshigher than $\approx 0.4$; this is a critical lacuna in our understanding ofgalaxy evolution. Here, we report a measurement of the average HI mass ofstar-forming galaxies at a redshift $z \approx 1$, by stacking their individualHI 21 cm emission signals. We obtain an average HI mass similar to the averagestellar mass of the sample. We also estimate the average star-formation rate ofthe same galaxies from the 1.4 GHz radio continuum, and find that the HI masscan fuel the observed star-formation rates for only $\approx 1-2$ billion yearsin the absence of fresh gas infall. This suggests that gas accretion ontogalaxies at $z < 1$ may have been insufficient to sustain high star-formationrates in star-forming galaxies. This is likely to be the cause of the declinein the cosmic star-formation rate density at redshifts below 1.

HI 21-centimetre emission from an ensemble of galaxies at an average redshift of one