For the first 3 billion years of Earth’s history, life was predominantly composed of prokaryotes—simple, single-celled organisms without a distinct nucleus or specialized internal structures. Even after complex multicellular organisms evolved about a billion years ago, evidence of early “soft” life forms remained scarce in the fossil record.
However, around 550 million years ago, during the Cambrian Explosion, the diversity of life on Earth began to increase significantly. This period, marking the beginning of the Cambrian geologic period, saw many eukaryotic organisms develop hard-shelled exoskeletons and other durable body parts, which could be preserved in sediments after death. Consequently, the fossil record from this era includes many ancestors of modern plants and animals. Biologists hypothesize that the development of exoskeletons was an evolutionary response to predation, including the emergence of eyes and other predatory adaptations.
In addition to exploring the origins of this surge in species diversity, biologists are also investigating the reasons behind at least five major mass extinctions recorded in the fossil history. The most catastrophic of these occurred at the Permian-Triassic boundary, around 250 million years ago. During this “great dying,” approximately 70 percent of land species and 96 percent of oceanic species disappeared, making it the most severe mass extinction event. The causes of this massive loss of life have been attributed to various factors, including climate change, massive impact events, and enormous volcanic eruptions.
The aftermath of this catastrophic event saw a slow recovery of life’s diversity. It took over 100 million years for the variety of life that had blossomed during the Cambrian Explosion to return to pre-Permian levels, highlighting the profound impact these mass extinctions had on the course of evolutionary history.
