Your Cousin in the Mud
In the brackish mud of coastal estuaries across England, a small, unassuming creature lives buried up to its tentacles. This is the starlet sea anemone, Nematostella vectensis. Reaching only a few centimeters in length, this translucent animal with up to 20 delicate tentacles looks simple. Its internal biology, however, tells a different story that connects it directly to us. Sequencing of the Nematostella genome revealed a surprising complexity. It has about 20,000 genes, a number very close to the human genome.
This genetic blueprint shows that the last common ancestor of humans and cnidarians (the group including anemones, corals, and jellyfish) was more complex than previously thought. This ancestor lived over 600 million years ago, before the Cambrian explosion of life. The starlet sea anemone's genome has retained many of the ancestral genes that more-studied model organisms, like fruit flies and nematode worms, have lost over time. This makes Nematostella a way to see the deep evolutionary past, allowing scientists to reconstruct the genetic toolkit of the first animals.
A Shared Inheritance
The genetic link between humans and Nematostella is more than the number of genes. Many of these genes are arranged in similar patterns on the chromosomes, a conservation that has persisted across hundreds of millions of years of evolution. This includes genes responsible for forming the nervous system, head, and sensory organs.
One of the most direct connections is the pou-iv gene. In mammals, this gene is essential for the development of the mechanical-sensing "hair cells" in the inner ear that allow us to hear; mice born without it are deaf. Researchers discovered that Nematostella possesses this same gene. In the anemone, pou-iv controls the development of its own mechanosensory hair cells located on its tentacles, which it uses to sense touch and movement in the water. When the gene is disabled using CRISPR-Cas9 gene editing, the anemone cannot respond to touch. This finding suggests the genetic building blocks for our sense of hearing originated in a common ancestor that lived in Precambrian seas.
Because it is easy to culture in a laboratory, has a short generation time of two to three months, and its genome is fully mapped, Nematostella vectensis has become an important model organism for developmental and evolutionary biology. It provides a unique look at how a shared set of ancient genes can be used to build animals with vastly different body plans.