Trilobite fossil, Cordania falcata Whittington (National Museum of Natural History)
When Charles Darwin wrote The Origin of Species, the oldest known fossils were trilobites preserved in rocks deposited during the Cambrian Period. That the oldest animals should have such complex morphology troubled Darwin, prompting him to argue that metazoans must have existed earlier, even though their records had evidently been destroyed or remained undiscovered.
Many decades and countless discoveries later, Darwin’s intuition has proven to be correct. Fossils from six continents now extend the animal record backward into the Ediacaran Period, some 50 million years before the first trilobites. The oldest are widely thought to record extinct animals without active locomotion, with no mouth or digestive system, and without specialized organs for gas exchange.
Soon thereafter slightly more familiar animals emerged, with a distinct front and back, left and right, top and bottom, as well as complex organs. Some of these creatures moved along the seafloor, as evidenced by simple trackways on bed surfaces. Mineralized skeletons evolved, too, suggesting a growing need for protection against predators. Despite this earlier record of animal life, however, Cambrian fossils are still seen to record the initial diversification of the major body plans found among animals to this day.
In an illustrated lecture, Andrew H. Knoll, Fisher Research Professor of Natural History at Harvard University, traces the fossil record of Earth’s earliest known animals, asking how these remains illuminate the early evolution of our own kingdom. He also looks at geological evidence that animals diversified in a rapidly changing world, where increasing availability of food and oxygen may have facilitated biological change. Set within the framework of Earth’s entire history, the initial diversification of animals can be seen as both the culmination of more than three billion years of evolution and a radical departure from all that came before.