The Paleozoic Period, which means “ancient life,” spanned from about 541 to 252 million years ago and marked a major expansion of life on Earth. One of its most significant events was the Cambrian Explosion during the Cambrian Period (541–525 million years ago), when a remarkable diversity of organisms suddenly emerged. Major animal phyla that make up modern animal life, such as crustaceans, starfish, sponges, mollusks, worms, chordates, and algae, appeared during this time. This burst of diversity was the product of millions of years of evolutionary development that began in the Precambrian Period.
During the Ordovician Period, around 470 million years ago, the first land plants began to appear. These early plants resembled mosses and simple ocean plants, lacking deep roots or leaves. Scientists believe that land plants evolved from algae, gradually developing new structures and reproductive mechanisms that allowed them to survive outside aquatic environments.
The Paleozoic also saw the rise of the first vertebrates, which originated in the Cambrian Period around 518 million years ago. These early animals belonged to the phylum Chordata. Over time, vertebrates continued to evolve: jawed vertebrates emerged in the Ordovician and Silurian Periods, bony fishes appeared in the Silurian, and they further diversified during the Devonian Period. Some of the earliest known vertebrates have been discovered in the Chengjiang fossil deposits.
The Paleozoic came to a dramatic end with the Permian–Triassic Extinction, the largest mass extinction event in Earth’s history. This catastrophic event was triggered by massive volcanic eruptions in Siberia, which released enormous amounts of greenhouse gases into the atmosphere. The resulting climate change devastated ecosystems and caused the extinction of a vast majority of Earth’s species, marking a major turning point in the planet’s biological history.