We have established by experiments and thermodynamical calculations that a large amount of hydrogen can be dissolved in iron under high hydrogen pressure, causing appreciable reduction of density and melting point. More recent experiments have shown that the reaction of Fe with hydrous silicates under high pressure and temperatures (≲5 GPa, ≲1200¿ C, 20 min) yields iron hydride (FeHx), olivine and pyroxene, and that coagulation of iron particles is greatly facilitated in the partially molten state of the silicates. On the basis of these experimental findings on Fe-H and Fe-H2O systems, we propose that the Fe-H20 reaction should have played a crucial role in the evolution of the earth, including in particular the coremantle separation and dissolution of hydrogen in the earth's core. A tentative calculation on the mass flow in the evolution process was made by assuming a 10:90 mixture of low-temperature (LT) and high-temperature (HT) components for the primordial material and Fe/(Fe+Mg)≂0.11 for the whole mantle. The composition of the core thus deduced, together with compressibility data at high pressures, is found to give reasonable values for the density deficit of the outer core. If, conversely, we adopt the density reduction as input, the composition of the outer core is estimated to be FeH0.25C0.05O0.13S0.03. Finally, some implications of the present picture on other terrestrial planets are examined briefly.