195 million years ago: earliest mammal fossil.
The earliest fossil of a mammal ancestor was found in China in 2000 and is dated 195 million years ago. It is a mouse-like tiny shrew called Hadrocodium wui.
The earliest fossil of a placental mammal is Juramaia sinensis, which was found in China in 2011 and dated 160 million years ago.
Mammals appeared 206 million years ago from the cynodonts, during the reign of the reptiles which lasted 100 million years. Cynodonts already had all the mammalian characteristics, but still laid eggs. From them descend all current mammals: placental, marsupials and monotremes.
Mammals had long existed before the mass extinction at the end of the Cretaceous, but they were small, insignificant and most likely were nocturnal. They benefitted from an explosive adaptive radiation at the beginning of the Tertiary. They were able to fill all the ecological niches that dinosaurs, which had previously been the dominant species, left empty after suffering the extinction.
Of all known species, mammals are the most familiar to us, but they are very few compared to other groups. There are 4,800 species of mammals, 9,800 species of birds, 7,150 species of reptiles and 400,000 species of beetles.