An ‘FYI’ update of what’s been happening over the past 50 million years or so…
Lifeless for some 5 million years before the then supernova we now call the sun came into existence, the first organic substance on earth was a primordial slime which in time evolved into a jelly and that, in turn (and appropriately), spawned jelly fish.
From whence came the Great Sea Scorpions.
The first ever vertebrates were the backboned fish. These gradually developed limb buds, and finally legs to exit the waters.
Yes, reptiles were the first land animals and they were primitive physically and socially, for once laid, they abandoned their eggs. Nonetheless, they thrived and grew in abundance (and size) into the dinosaurs that dominated for 180 million years before dying out at the ice-age 65 million years ago.
Birds were next, as scales gave way to feathers and smaller creatures that had adapted better to the lower temperature now inherited the earth; and they stayed to keep their eggs warm!
As feathers evolve into fur and two legs four, animals develop the ability to retain the baby inside until it is matured. Importantly, these primitive mammals have a social/educational relationship with their young as useful information is passed on and developed instead of every individual starting from instinctual scratch.
Reference: The No-Nonsense Guide to World History. New Edition, 2011. Chris Brazier. New Internationalist.