Reflectance anisotropy spectroscopy (RAS) in combination with reflection high-energy electron diffraction (RHEED) was used to study in situ the initial steps of molecular beam epitaxial growth of InAs on GaAs(001). Due to the large lattice mismatch InAs is known to grow in Stranski-Krastanov mode leading to the formation of quantum dots after the transition from two- to three-dimensional growth mode. In this article the precise determination of the growth mode transition and the subsequent development of the islands have been of particular interest. During the growth of the two-dimensional InAs layer, the RHEED-pattern changed from the c(4x4) of the clean GaAs to a (1x3) surface reconstruction. Accordingly, the RAS-spectra, taken every 0.2 ML, indicate changes of the As-dimer configuration. At 1.8 ML (spotty RHEED-pattern) a saturation of the intensity of the dimer related RAS-signal around 2.6 eV was found. The relaxation of the InAs layer and the formation of the quantum dots was followed by time-resolved RAS at 2.6 and 4 eV. It is shown here, that the time constant of this process, the thickness of the InAs wetting layer and the equilibrium morphology of the islands are strongly temperature dependent. The remaining equilibrium InAs wetting layer thickness at the surface was estimated to be about 1 ML (0.8 ML at 625 K and 1.2 ML at 725 K).