c. 550 Million BCE

Cambrian Explosion

The first life forms to emerge on Earth were prokaryotes—simple, single-celled organisms lacking either a distinct nucleus with a membrane or specialized internal cellular structures that are common in more complex eukaryotes. For the first 3 billion years or so of Earth’s history, life was dominated by such single-celled organisms. Even with the evolution of the first complex multicellular organisms about a billion years ago, it’s still very hard to find evidence for these early forms of “soft” life in the fossil record.

Around 550 million years ago, however, in what is often called the Cambrian Explosion because it so dramatically marks the beginning of the Cambrian geologic period, the diversity of life on Earth began to drastically increase. Specifically, many eukaryotic organisms began to develop hard-shelled exoskeletons and other body parts, structures that could be preserved in sediments after the organisms died and fell to the seafloor. As such, many of the ancestors of today’s modern plants and animals appear rather early in the fossil record, which really begins in earnest at the Cambrian Explosion. Biologists hypothesize that exoskeletons might have been an evolutionary response to adaptations by other competing organisms, such as the development of eyes and other advances in predation.

In addition to trying to understand the origin of large increases in species diversity, biologists are also trying to understand the reasons for at least five rather sudden, drastic mass extinctions of species found in the fossil record. The most dramatic of these occurred at the boundary between the Permian and Triassic geologic periods, around 250 million years ago. Across a span of perhaps only a million years, about 70 percent of all land species and 96 percent of all oceanic species died off, a period informally called the “great dying” and the “mother of all mass extinctions.”

What caused such massive and relatively sudden loss of life on Earth? Geologists have implicated climate change, massive impact events, and enormous volcanic outpourings. Whatever the cause, it took more than 100 million years for the diversity of life on Earth that had begun to flourish at the Cambrian Explosion to once again reach pre-Permian levels.

Fossilized remains (packed in lithified seafloor sediments) of the exoskeletons and other hard-shelled body parts from new organisms that began to appear around 550 million years ago. This “Cambrian explosion” marked an enormous increase in the diversity of life on Earth.