The Life on Kerbin is among the most underrated aspects of the planet. A lot of organisms detailed here are my creations, but some are other people's creations, and may be adapted into this project. I apologise in advance if you feel I've bastardised your particular creation 

Life on Kerbin has been around for about 3.5 billion years, starting off as simple, unicellular organisms. Over time, they have given rise to many lineages, many of which have gone extinct. Still, lots of other lineages are still alive and well, and some are even thriving. There are around 100 phyla across all kingdoms of life on Kerbin, minus the 23 phyla of mystical, exotic organisms, and the lifeforms that call Laythe, the first moon of Jool, home. Those other 100 phyla are split between 9 kingdoms, which are themselves divided into 3 domains, the Eucyta, the Procyta, and the Simplovita.

Many creatures on Kerbin are created as homages to headcanons of other people. For example, the 'Kerbals evolved from plants' theory is represented by the fact that K-Animalia and K-Plantae are more closely related to each other than to all other Eucytes, Kerbinian analogues for Eukaryotes. 

Anyway, life on Kerbin started the same way life began on Earth, or on all other planets with this peculiar characteristic. Various simple molecules combined to form more complex structures, under the influence of heat. Over millions of years, this pattern continues until one gets to protocells, the simplest living things. This is an gross oversimplification, but it should suffice.