<P> The evolution of mammals has passed through many stages since the first appearance of their synapsid ancestors in the late Carboniferous period . The most ancestral forms in the class Mammalia are the egg - laying mammals in the subclass Prototheria . This class first started out as something close to the platypus and evolved to modern day mammals . By the mid-Triassic , there were many synapsid species that looked like mammals . The lineage leading to today 's mammals split up in the Jurassic ; synapsids from this period include Dryolestes , more closely related to extant placentals and marsupials than to monotremes , as well as Ambondro , more closely related to monotremes . Later on , the eutherian and metatherian lineages separated ; the metatherians are the animals more closely related to the marsupials , while the eutherians are those more closely related to the placentals . Since Juramaia , the earliest known eutherian , lived 160 million years ago in the Jurassic , this divergence must have occurred in the same period . </P>
<P> The evolution of mammals has passed through many stages since the first appearance of their synapsid ancestors in the late Carboniferous period . The most ancestral forms in the class Mammalia are the egg - laying mammals in the subclass Prototheria . This class first started out as something close to the platypus and evolved to modern day mammals . By the mid-Triassic , there were many synapsid species that looked like mammals . The lineage leading to today 's mammals split up in the Jurassic ; synapsids from this period include Dryolestes , more closely related to extant placentals and marsupials than to monotremes , as well as Ambondro , more closely related to monotremes . Later on , the eutherian and metatherian lineages separated ; the metatherians are the animals more closely related to the marsupials , while the eutherians are those more closely related to the placentals . Since Juramaia , the earliest known eutherian , lived 160 million years ago in the Jurassic , this divergence must have occurred in the same period . </P>
when did the first mammal appear on earth
when did the first mammal appear on earth
late Carboniferous period
late Carboniferous period