The Science of Life

active already alveoli animals appear bacteria become begin birds blood body bones brain branches called carried cells changes chemical close contains continue creatures digestive direction early embryo Evolution example existence fact female fertilized fish flowers forms fossils genes give green grow growth hand higher human important individual insects kinds known land later layer leaves less light living male mammals matter means method microscopic million move muscles nature never normal organs original oxygen pass period plants Pleistocene possible present produced remains reproduction rest result round seen selection separate side simple single skin species spores stage structure substances surface things tissue true turn types variety various vertebrates whole