contingent opportunity

Eukaryotes were swift to radiate
exploiting niches no archaean can
where complex organisms dominate

they dabbled in all sorts of body plan
Precambrian! a world before the jaw
or teeth, but somehow predator and prey
obeyed the evolutionary law
the winners passed their genes down to this day
whole phyla flourished, but were doomed to fail
genetic innovations all laid waste
remembered only in the Burgess shale
uncounted others gone without a trace
'til mitochondria were brought in-house

one can't conceive a grouse, a mouse, a louse