Humans are late arrivals on Earth. For nearly 75% of Earth’s history, life consisted of single-celled microbes without a nucleus (prokaryotes). Volcanoes and erosion sculpted Earth 3.5 billion years ago. Life consisted only of cyanobacteria, which trapped sediments in intertidal zones to form mound-like structures (stromatolites). Cells with a nucleus (eukaryotes) evolved about two billion years ago, after oxygen had accumulated in the ocean. Over time, they developed greater complexity, with different parts taking on specialized functions—allowing evolution of sexual reproduction.

Earth’s first animals had soft bodies. This illustration shows a community of soft-bodied Ediacaran (edi-A-karan) animals. Some species resemble living ocean creatures. Others are unlike any known organisms and cannot be classified. Scientists have found fossils of these fauna in sedimentary rocks worldwide.

The basic body plans of all modern animals were set during the Cambrian Period, 542 - 488 million years ago. Your friends, family, and pet turtle may not look much like the creatures here. But we and our fellow animals are heirs of these ancient ocean dwellers. Changes in Earth’s climate and ocean oxygen levels may have fueled rapid diversification during this period. Another possible cause may have been the interactions among the increasingly complex animals. Competition and predation often spark innovation. Not every Cambrian body plan was successful. But those that did succeed set the pattern for every animal that followed—in the water and on land.

Crinoids (echinoderms related to sea stars and sea urchins) dominate the Paleozoic shallow water habitat in this illustration. They evolved a variety of stalk heights, which enabled them to capture food at different levels above the sea floor. The base of their stalks was modified to anchor the animal securely in the soft sediment. Crinoids were relative skyscrapers in the community, sometimes towering up to two meters (6.5 feet). Lacy bryozoans occupied a lower level. Below them, huge numbers of brachiopods monopolized the muddy bottom. Sharks cruised above these crinoid forests, while smaller bony fishes weaved among the crinoid stalks.

This illustration shows the edge of a warm inland sea during the Cretaceous Period, heyday of the dinosaurs. Constantly shifting sediment supported new groups of organisms, including rudist clams—major molluscan reef builders. Various organisms have taken a turn as the dominant tropical reef builders. Visit a tropical reef today and you’ll see coral. But if you visited during the Cretaceous Period, you might have seen reefs built by mollusks: rudist clams. Rudists enjoyed a brief tenure as the ocean’s reef chiefs. They died out about 65 million years ago, the same time dinosaurs became extinct.

This rendering shows life at the end of the Cretaceous Period, before the impact of a 10 km (6.2 mi) asteroid triggered mass extinctions on land and sea. Dinosaurs are the most famous victims of the extinction at the end of the Cretaceous, 65 million years ago. But they weren't alone. Nearly half of Earth's species disappeared -- including almost 75 percent of ocean species. Evidence indicates an asteroid was to blame. Dust and smoke from the impact blocked sunlight, disrupted photosynthesis and altered Earth's climate.