550 Million BCE

Cambrian Explosion

The first life forms to emerge on Earth were simple, single-celled microbes that were able to tap into chemical and thermal energy sources in the environment of early Earth. For the first 3 billion years of Earth’s history, in fact, life was apparently dominated by single-celled organisms, occasionally organized into colonies like those found in stromatolites. Around 550 million years ago, however, in what is often called the Cambrian explosion, the diversity of life on Earth began to dramatically increase. As such, many of the ancestors of today’s modern plants and animals appear rather early in the fossil record, geologically speaking. Biologists are actively debating possible reasons for the rather sudden and dramatic diversification of life on Earth at the Precambrian/Cambrian boundary.

Biologists are also trying to understand the reasons for many rather sudden, drastic decreases in the number of species and species diversity 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.” It took more than 100 million years for the diversity of life on Earth to once again reach pre-Permian levels.

Such a massive loss of life on Earth could have been triggered by climate change, although many believe that the rapidity is difficult to explain that way. Alternatively, a catastrophe such as a large impact event or an enormous volcanic outpouring, which may also have triggered other climatic and geologic catastrophes, could be responsible. Biologists, geologists, and astronomers are still searching for clues.

The Permian extinction 250 million years ago saw Earth’s largest decrease in the number of species and diversity of life. The view from Yavapai Point across the Grand Canyon exposes nearly 2 billion years of Earth’s geologic record for detailed study.