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We present deep Keck spectroscopy for a sample of I<22.5 field early-typegalaxies selected morphologically in the redshift range 0.56<z<1.02 in theHDF-N. Using velocity dispersions determined from the Keck spectra inconjunction with structural parameters measured from the deep WFPC2 images westudy the evolution of the M/L_B ratio and the fundamental plane (FP) withredshift. For the majority of galaxies the trends observed are very similar tothose determined earlier for rich clusters. The systematic offset between HDF-Ngalaxies and cluster galaxies is d(ln M/L_B) = -0.14 +- 0.13, corresponding toan age difference of only 16 +- 15 % at <z>=0.88. However, we find enhanced Hdelta absorption of equivalent width 4.0 Angstrom in the mean spectrum of theten galaxies, indicating the presence of young stars. We infer that thegalaxies have composite stellar populations, consisting of a low mass youngcomponent in addition to a dominating old component. As the bulk of the stellarmass must have formed at z >~ 2 our results argue against formation scenariosinvolving major mergers of gas-rich disk systems at 1<z<1.5, and we concludethat z~1 early-type galaxies were either assembled at higher redshift or inmergers involving little gas. The ubiquitous enhanced Balmer lines and thepresence of tidal features in two of the galaxies lend some support to thelatter hypothesis. The main uncertainty in the analysis is the small sample;larger samples are needed to study the interplay between the evolution ofstellar populations and morphology in detail.Comment: Accepted for publication in the Astrophysical Journal Letter

On the Assembly History of Early-Type Galaxies in the Hubble Deep Field-North