algae: A group of single-celled and multicellular organisms, once considered plants (they aren’t). As aquatic organisms, they grow in water. Like green plants, they depend on sunlight to make their food.

array: A broad and organized group of objects.

bilateral: An adjective that refers to something that involves two groups (such as nations) or two sides (such as the left and right side of the brain).

Cambrian: A period of Earth’s history that lasted from about 541 million to 510 million years ago. It is one of the earliest periods in which fossils can be used to date rocks.

Cambrian explosion: A term for a brief period of time, between 541 million and 530 million years ago, when the seas gave rise to an astounding number of new organisms. They included dozens of phyla that would evolve into modern animals. Many other phyla also emerged; most of these, however, would become extinct over the next tens of millions of years.

diversity: A broad spectrum of similar items, ideas or people. In a social context, it may refer to a diversity of experiences and cultural backgrounds. (in biology) A range of different life forms or a range of traits within the population of some species.

Ediacaran Period: A geologically defined period that spanned from 635 million to 538 million years ago. Most life from that time had soft bodies, which meant few species left fossils with well-defined details.

fossil: Any preserved remains or traces of ancient life. There are many different types of fossils: The bones and other body parts of dinosaurs are called “body fossils.” Things like footprints are called “trace fossils.” Even specimens of dinosaur poop are fossils. The process of forming fossils is called fossilization.

insight: The ability to gain an accurate and deep understanding of a situation just by thinking about it, instead of working out a solution through experimentation.

paleontologist: A scientist who specializes in studying fossils, the remains of ancient organisms.

radial: An adjective to describe things that spread out — radiate — from one central point.

sea: An ocean (or region that is part of an ocean). Unlike lakes and streams, seawater — or ocean water — is salty.

species: A group of similar organisms capable of producing offspring that can survive and reproduce.

symmetry: In geometry, the property of being indistinguishable from a shifted, rotated or reflected image of the same object. For example, the letter X looks the same whether reflected in a mirror or turned upside down — two different kinds of symmetry.