  A new view of disk evolution is emerging from self-consistent numerical
simulation modeling of the formation of circumstellar disks from the direct
collapse of prestellar cloud cores. This has implications for many aspects of
star and planet formation, including the growth of dust and high-temperature
processing of materials. A defining result is that the early evolution of a
disk is crucially affected by the continuing mass loading from the core
envelope, and is driven into recurrent phases of gravitational instability.
Nonlinear spiral arms formed during these episodes fragment to form gaseous
clumps in the disk. These clumps generally migrate inward due to gravitational
torques arising from their interaction with a trailing spiral arm.
Occasionally, a clump can open up a gap in the disk and settle into a stable
orbit, revealing a direct pathway to the formation of companion stars, brown
dwarfs, or giant planets. At other times, when multiple clumps are present, a
low mass clump may even be ejected from the system, providing a pathway to the
formation of free-floating brown dwarfs and giant planets in addition to low
mass stars. Finally, it has been suggested that the inward migration of gaseous
clumps can provide the proper conditions for the transport of high-temperature
processed solids from the outer disk to the inner disk, and even possibly
accelerate the formation of terrestrial planets in the inner disk. All of these
features arising from clump formation and migration can be tied together
conceptually in a Migrating Embryo model for disk evolution that can complement
the well-known Core Accretion model for planet formation.
