The Cambrian radiation represents a unique interval in Earth’s history, beginning approximately 539 million years ago. This period is characterized by the seemingly abrupt appearance of most major animal groups, or phyla, in the fossil record. Over a span of roughly 13 to 25 million years, the fundamental body plans that define modern animals came into existence. While often called an “explosion,” some research suggests the diversification unfolded more gradually over this timeframe.
Life Before the Cambrian Explosion
The world preceding the Cambrian was a dramatically different and simpler place, dominated by organisms known as the Ediacaran biota. These life forms, which thrived from about 575 to 541 million years ago, were among the first complex, multicellular organisms. They were typically soft-bodied, leaving behind impressions in ancient seafloor sediments. Their forms were often enigmatic, ranging from frond-like shapes anchored to the seabed to quilted, mattress-like organisms that lay flat on the ocean floor.
These early organisms lacked the complex structures that characterize later animals. Features like heads, limbs, and dedicated sensory organs appear to have been absent. Movement was likely limited, with most Ediacaran creatures being stationary or slow-moving crawlers. Their simple construction meant they were not active predators or burrowers in the way that later animals were. The ecosystems they inhabited were comparatively tranquil, lacking the intricate food webs and predator-prey dynamics that would later drive evolutionary innovation.
The Proliferation of New Animal Forms
The Cambrian period introduced a significant wave of evolutionary novelty. A primary development was the advent of biomineralization, the process by which organisms produce hard parts. This led to the appearance of animals with durable exoskeletons and shells, offering both physical support and protection. These new mineralized structures are a major reason why the Cambrian fossil record is so much richer than that of preceding eras.
Among the new arrivals were the trilobites, a group of arthropods that were among the first animals to develop complex, multi-lensed eyes. Alongside them were the first chordates, animals possessing a primitive spinal cord, which represent the earliest members of the phylum that would eventually include vertebrates. This period saw the emergence of nearly all modern animal body plans, from mollusks to echinoderms.
The era also produced a number of strange animals that have puzzled paleontologists. Anomalocaris, a large swimming predator up to a meter in length, was an apex predator of its time, equipped with formidable grasping appendages and a circular mouth for crushing prey. Another famous example is Hallucigenia, a worm-like creature with defensive spines on its back and slender, stilt-like legs.
Potential Triggers of the Radiation
Scientists hypothesize that a combination of environmental, ecological, and genetic factors created the conditions for this burst of evolution. A significant rise in oxygen levels in the atmosphere and oceans is a leading environmental explanation. Higher oxygen concentrations would have been necessary to support the metabolisms of larger, more active animals, allowing for the development of energy-intensive tissues and predatory lifestyles.
Ecological pressures also appear to have played a large part. The evolution of the first true predators likely ignited an “evolutionary arms race” between predators and prey. This dynamic would have exerted intense selective pressure on prey animals to develop new defenses, such as shells, spines, and improved mobility. Evidence of this can be seen in fossils from the period, such as bore holes in shells, which suggest predatory behavior.
Developments at the genetic level may have provided the biological blueprint for this diversification. The evolution and expansion of a group of genes called Hox genes are thought to have been important. These genes act as master controls, dictating the basic body plan of an organism from head to tail. The emergence of this more complex genetic “toolkit” could have provided the flexibility for new and varied body structures to evolve.
Evidence from Exceptional Fossil Sites
Our detailed understanding of Cambrian life is largely thanks to the discovery of rare fossil sites known as Lagerstätten. These are locations where unusual geological conditions led to the exceptional preservation of ancient organisms. Unlike typical fossil sites that only preserve hard parts like bones and shells, Lagerstätten can preserve the soft tissues of animals, offering a much more complete picture of past biodiversity.
Two of the most important Cambrian Lagerstätten are the Burgess Shale in the Canadian Rockies and the Chengjiang fossil site in China’s Yunnan Province. The fossils from these sites provide a “snapshot” of entire marine ecosystems. Discoveries at these locations have revealed the anatomy of early predators, the digestive systems of bottom-dwellers, and the delicate appendages of many bizarre creatures. This has allowed paleontologists to reconstruct the complex food webs and ecological relationships of the Cambrian period.