The quest to uncover the origins of vertebrates—animals with a backbone—leads scientists deep into the fossil record. This archive provides direct evidence of when major animal groups appeared on Earth. An important chapter in this story is the Cambrian Period, a time of rapid evolutionary development often called the “Cambrian Explosion.” During this interval, the blueprints for nearly all modern animal phyla were established.
Identifying the Earliest Known Vertebrate
The most widely accepted candidate for the oldest known vertebrate is a small, fish-like creature named Myllokunmingia. Its fossilized remains were discovered in the Maotianshan shales of the Chengjiang fossil beds in China’s Yunnan Province. These fossils date back approximately 518 million years, placing them in the early Cambrian Period and offering a window into the dawn of vertebrate life.
Barely an inch long, Myllokunmingia possessed a combination of features that paleontologists recognize as foundational to the vertebrate body plan. It had a distinct head with what appear to be paired eyes, a skull made of cartilage, and gill pouches for breathing. Its body was segmented into muscle blocks called myomeres, arranged in a zig-zag pattern characteristic of fish, which would have allowed it to swim with an efficient undulating motion.
Although it lacked a true bony skeleton, Myllokunmingia displayed precursors to a backbone. It had a supportive rod-like structure called a notochord, a defining feature of all chordates, the broader phylum to which vertebrates belong. Fossils also show evidence of rudimentary vertebral elements, cartilaginous structures that hint at the beginning of a spinal column’s evolution. These anatomical details identify Myllokunmingia as one of the first animals to possess foundational vertebrate traits.
The Scientific Debate and Other Contenders
Identifying the first vertebrate is a complex task, as the interpretation of soft-bodied fossils from the Cambrian period is often difficult. The age and delicate nature of these specimens mean that definitive evidence can be elusive, leading to ongoing scientific discussion and re-evaluation as new analytical techniques are developed.
One contender in this debate is Haikouichthys, another fossil creature found in the same Chengjiang deposits as Myllokunmingia. Some researchers have argued that the two may belong to the same genus or are at least very closely related. However, detailed studies point to subtle but distinct differences, suggesting they are separate taxa. Both provide a glimpse into what the earliest vertebrates looked like.
Another creature, Yunnanozoon, has also been at the center of this scientific conversation. Discovered in the same fossil beds, its classification has been particularly challenging. Some studies have interpreted Yunnanozoon as a true vertebrate, while others suggest it may represent a more primitive chordate or a stem-vertebrate—an animal that belongs to the vertebrate lineage but branched off before the emergence of true vertebrates.
Life During the Cambrian Explosion
The world Myllokunmingia inhabited was a shallow marine environment with a diverse array of newly evolved life. The Chengjiang fossil beds reveal an entire ecosystem established during the Cambrian Explosion, a period of significant biological innovation. The sediments of this ancient sea floor preserved not just our distant ancestors but also the complex community they were a part of.
This ancient sea was home to a variety of organisms that would seem alien today. Trilobites, one of the most successful early arthropods, crawled along the seafloor, while various worms burrowed into the sediment. The water column was occupied by creatures like the jellyfish-like Eldonia.
Among the residents of this ecosystem was Anomalocaris, a large swimming predator considered one of the first apex predators on Earth. The existence of such a creature indicates that the Cambrian seas were a competitive and dangerous environment. In this world, the small, agile Myllokunmingia would have needed its advanced musculature and sensory organs to navigate, find food, and evade capture.
The Evolutionary Significance of Early Vertebrates
The discovery of Myllokunmingia and its contemporaries is significant because it anchors the vertebrate story deep in geological time. These fossils demonstrate that the foundational vertebrate body plan—including a protected brain, advanced sensory organs, and precursors to a spine—was established very early in animal life. This confirms that the innovations defining this group emerged during the Cambrian period.
The appearance of these early vertebrates marks a turning point in the history of life. The development of a head with centralized sensory organs and a brain set the stage for more active lifestyles. Features like a cartilaginous skull and myomeres provided the structural and locomotive advantages for these animals to compete in complex ecosystems.
From these jawless beginnings, a diverse lineage would unfold over the next 500 million years. The basic blueprint in creatures like Myllokunmingia was the starting point for the evolution of all vertebrates that followed. This lineage includes the evolution of jaws, bony skeletons, and paired fins, leading to the rise of:
- Sharks and bony fish
- Amphibians that ventured onto land
- Reptiles, dinosaurs, and birds
- Mammals, including humans
This path connects this small Cambrian swimmer directly to our own evolutionary heritage.