During the Eocene period, tiny ‘early drafts’ of mammals related to species you know today first evolved. Studying these miniature animals feels like peeking at someone’s baby photos. Most modern mammals are descendants of species tiny enough outlast the extinction event which wiped out the dinosaurs, and then grew in size and diversity to fill the ecological niches the dinosaurs left in their wake. As result of this shared origin, most early mammals — with some notable exceptions — were on the smaller side for several million years. Most of these diminutive ancestors of the species we know today emerged during the Eocene period, a geological epoch which lasted from about 56 to 33.9 mya. So, let’s examine the fossils of the species which populated this period at different points, and explain their connection to the mammals we know today.