The entire evolutionary sequence, is defined by a series of lonely accidents of time and place, and stuff...
These led onward to multiple independent organized systems of energy conversion; to wee small clots of Auto: Hetero: and Auxo-trophic organisms that comprised the first autonomous living units - leading to a theory of symbiontic evolution - onward to the first little bags of invasive, and hungry proto-organelles that were fully enclosed within a semipermeable goo - that eventually led to an organized cell membrane - containing all the instructions, parts, and processing capabilities for self organization, sometimes a little chlorophyll -
and LIFE was on its way....
I would like to stimulate more discussion of the role of metal ions, chlorophylls, and the generation of oxygen, although that subject has often been described. Oxygen levels rose to levels that were poisonous to early life forms, but without which very large, complex organisms would not have reliable sources of storable reactive resources. (We will come back to this subject, once my colleague, Daniel Pauly, gets his oxygen related tale in order.)
Eventually, more complex multicellular organisms emerged, whose diverse parts performed ever more sophisticated roles, including both formation of and "encoding" the instructions for meiosis, mitosis, and self-replication.
Ponder the concepts and design issues that underlie the development of and maintenance of haploid, unfertilized "eggs"- and sperm - eggs to be fertilized and nurtured internally - then egg-laying -
Speaking of miracles! - and eventually the placental vertebrates... Whew!
The general situation that evolved is clearly toward assuring a more certain, secure early developmental environment - certain costs accrued. For example, there is a school of thought that argues that sexual reproduction was the precursor to mortality, i.e., prior to the first meiosis, all cell lines led from the primordial cell(s), by direct linear ascent. Once haploid fission/fusion processes began, it was possible to lose entire genetic lineages, through accidents, or selection.