This is a silly question and heres why. ALl life started out a single cells. The seeds have evolved in the very early stages whilst still in cellular scale. The microscopic organism instead of multiplying A sexually would instead leave a type of egg with a replica of itself to be born later for some reason I would have to think longer. This grandually evolved into a seed.

Eggs are the same but the microscopic organism of course evolved differently to house its duplicate inside itself in an external pod to be layed and preserved until a short time later.

So we have organisms which spread their seed, others who A sexually duplicate later evolving into birthing, and some organisms that would duplicate itself inside an exoskeleton inside its own body.

This is the brief answer and time frames are much shorter perhaps less than seconds. with the seed type being longest to develop of all.

To summarize plants werent always plants and they could well be walking around like types will be on other planets. They started out as single cell and evolved eventually into a stationary lifeform not needing to find minerals because it evolved to feed on the sun and moisture in the air and ground.