The earliest animals on Earth

Life on Earth is believed to have begun about 1.2 billion years ago, but it took a long time to advance beyond the single-celled stage. The age of the earliest multicelled creatures has hovered somewhere around 530MY, perhaps 575. Rocks in Oman, however, have yielded chemical traces left by animals called demosponges. Demosponges of 635 MYA may have been only a millimeter or two across (their modern descendants can be over a meter high), but they were definitely multicellular creatures. This discovery has a host of implications for the early development of more complex life and the now-questioned picture of what the Earth's oceans were like in the days when mammals were a half-billion years in the future.