Pikaia - Unlikely Ancestor

Unlikely Ancestor

At first glance, Pikaia does not seem like a vertebrate ancestor, and in fact there is a lot of debate regarding the topic in scientific circles. It looks like a worm that has been flattened sideways. But in detail, the fossils compressed within the Burgess Shale clearly show chordate features such as traces of an elongate notochord, dorsal nerve cord and blocks of muscles (myotomes) down either side of the body – all critical features for the evolution of the vertebrates.

The notochord is a flexible rod that runs along the back of the animal, lengthening and stiffening the body so that it can be flexed from side to side by the muscle blocks for swimming. In the fish and all subsequent vertebrates, the notochord forms the backbone (or vertebral column). The backbone strengthens the body, supports strut-like limbs, and protects the vital dorsal nerve cord, while at the same time allowing the body to bend.

Surprisingly, a Pikaia lookalike still exists today, the lancelet Branchiostoma. This little animal was familiar to biologists long before the Pikaia fossil was discovered. With notochord and paired muscle blocks, the lancelet and Pikaia belong to the chordate group of animals from which the vertebrates have descended. Molecular studies have refuted earlier beliefs that lancelets might be the closest living relative to the vertebrates, and instead favor tunicates in this position. While the lancelet is a chordate, other living and fossil groups, such as acorn worms and graptolite, are more primitive. Called the hemichordates, they have only a notochord-like structure at an early stage of their lives.

The presence of a creature as complex as Pikaia some 530 million years ago reinforces the controversial view that the diversification of life must have extended back well beyond Cambrian times, deep into the Precambrian.