 Since the 1960s , it has been hypothesized that severe glacial action between 750 and 580 mya , during the Neoproterozoic , covered much of Earth in ice . This hypothesis has been termed " Snowball Earth " , and it is of particular interest because it preceded the Cambrian explosion , when multicellular life forms began to proliferate . Following the Cambrian explosion , about 535 mya , there have been five major mass extinctions . The most recent such event was 66 mya , when an asteroid impact triggered the extinction of the non @-@ avian dinosaurs and other large reptiles , but spared some small animals such as mammals , which then resembled shrews . Over the past 66 Ma , mammalian life has diversified , and several million years ago an African ape @-@ like animal such as Orrorin tugenensis gained the ability to stand upright . This facilitated tool use and encouraged communication that provided the nutrition and stimulation needed for a larger brain , which allowed the evolution of the human race . The development of agriculture , and then civilization , led to humans having an influence on Earth and the nature and quantity of other life forms as no other species ever has . 
