Mammals are warm-blooded (endothermic) vertebrates with hair.
They breath air and give birth to live young, which feed on milk
produced by their mothers. Some mammals in the vernal pool
prairie spend most of their time underground. The burrows they
dig create homes for many other animals.
For 2 billion years, bacteria were the only creatures on Earth.
Long before the dinosaurs, a special type of bacteria slowly
increased the level of oxygen in the Earth’s air to 20 percent.
Without this oxygen other plants and animals could not have
evolved, including us.
