The 16S tree of life shows that the streptococci and lactococci emerged from the enterococci, sharing a last common ancestor with Enterococcus hermanniensis. To understand the basic nature of this branch of microbial life, we investigated the origins of the enterococci and offspring. The enterococci are strongly associated with land animals, whereas ancestral lineages, the vagococci and carnobacteria, are associated with marine life. To determine when the enterococci arose, we sequenced and compared genomes of all major phylogenetic branches. Applying molecular clocks, we found that the enterococci originated between 600 and 400 MYA, around the time of the Cambrian explosion. The first fossil evidence of animal land life – the ecology in which enterococci are found – are of arthropods, dated to 425 MYA. All species radiations within enterococci date after this. After an initial burst of species radiation between 425 and 330 MYA, there was relative stability in the genus. Then, abruptly, after 250 MYA, a second wave of speciation occurs. This coincides perfectly with the radiation of new animal hosts following the Great Permian Extinction, 251 MYA. The enterococci possess about 100 traits that distinguish them from the ancestral marine vagococcal lineage, most encoding functions for hardening the cell wall to harsh conditions, precisely the traits needed for adapting to life on land. Discovering that terrestrialization led to the expansion of the Enterococcus genus, now ubiquitous in land animals, raises the fundamentally important question, what new ecologies gave rise to lactococcal and streptococcal genera?