It seems to me, that chances are heavily against the emergence of macro-based life. The old Miller-Urey experiment shows that at least some of the most important amino acids can 'spontaneously' come to existence from non-organic compounds in a 'soup'. But there is no way, that molecules in such a soup could form a macroscopic part (like a gear) from themselves.
Therefore I suggest to use micro-based life as 'catalyst' or 'precursor'. The simplest way to do this would be intelligent design by the micro-based beings, like in my previous, quite inadequate answer. But there is a more evolutionary-style possibility:
Micro based life emerges, and flourishes on the planet in great variety. But then something happens, which exposes the planet to steady radiation (the soil is not contaminated by radioisotopes, the radiation probably comes from cosmic sources). Micro-based life sucks, since even a few lingering high-energy electrons or gamma-photons are capable to break its fine molecular structures.(for example mess up their DNA-equivalent, killing the cells or causing cancer.)
As the life resettles in the irradiated areas, the beings start to develop thick, metallic shells. These grow from inside, and protect the tissues from the radiation. But it renders the first Shellers almost immobile. As evolution goes on, the mechanichs of the shell becomes more complex. It allows the muscles to move the rigid external legs from the inside.
The classic photosynthesis requires the pigments to be exposed to direct sunlight. Under the new circumstances, this equals destroying them. More robust, macroscopic systems develop to harness energy from sunlight and radiation: the 'plants' grow solar panels, direct converters and electrothermal generators, while their whole metabolism slowly changes, to make use of their electricity.
After some millennia, the first macro-nerves appear: inorganic conductive lines in the legs, capable of transmitting basic information to the vulnerable brain from the outside. They are crude when compared to subtle micro-nerves, but instead of killing them, the radiation only causes some noise in their signals.
As the biomass of their food is in high percent made up from macrostructure shells, the digestion systems of the heterotrophic beings is in trouble. It is optimized for processing organic molecules, not for large pieces of shell. Crushing-jaws and smelter-stomaches emerge as they develop organs optimized for reshaping the ingested macroparts into useful shell-pieces.
At last, electromechanical systems start to substitute muscles. Initially, they are pretty clumsy, but there are plenty of electricity, and their usage allows significant reduction of micro-biomass, further reducing cancer risk.
It is hard to imagine that macroisation slowly takes over reproduction, but the evolution is capable of interesting tricks. Maybe, for example, the eggshell modifies into the macro-body of the offspring, premade by the mothers macro-manufacturer organs, and later continued by the hatchling.