The Unlikely Cousin of Vertebrates: Tunicates and Their Evolutionary Journey

The question of which invertebrate is most closely related to vertebrates has puzzled biologists for years. The answer lies in the two invertebrate chordate groups, Tunicata and Cephalochordata (Lancelets). While Lancelets share features similar to some early vertebrates, it is the Tunicates that appear to be closely related to our more familiar Olfactores. However, the controversy surrounding the Olfactores theory suggests that Lancelets may have simply lost their advanced olfactory system.

Understanding Tunicates

Most biologists consider tunicates to be invertebrates, despite their resemblance to plants in their adult forms. Tunicates, also known as sea squirts, are fascinating creatures that start their lives as tadpole-like larva. These larvae possess a notochord, which is a characteristic shared with vertebrates. As they mature, they transition into sessile, filter-feeding adults. The tadpole stage is particularly interesting because these creatures exhibit features remarkable enough to be mistaken for sperm cells at a glance. This early stage possesses rudimentary sensory organs and a bundle of nerves that help direct the larva to find a suitable spot to settle down.

Phylum Echinoderms: Closest Relatives Among Invertebrates

The echinoderms, such as starfish and sea urchins, are the closest related invertebrate phylum to the chordate phylum (e.g., vertebrates). The main morphological similarity is that both echinoderms and chordates are deuterostomes. Deuterostomia, a subtaxon of the Bilateria, are distinguished from protostomes by their deuterostomic embryonic development. In deuterostomes, the blastopore becomes the anus, whereas in protostomes, it becomes the mouth. This classification highlights the close evolutionary relationship between these phyla.

The Evolutionary Journey of Tunicates

The fascinating evolutionary journey of tunicates offers insights into the origins of vertebrates. At one point, these larval stages that resemble sperm cells evolved into the adult form. Instead of settling down and becoming stationary filter feeders, these creatures developed advanced sensory cells, making more complex decisions to better exploit their environment. Over time, the notochord strengthened and became a true notochord, protected by rudimentary cartilage. This transition also led to the development of a mouth and a functioning digestive system. The brain, which was initially a simple nerve bundle, evolved into a more complex structure capable of making decisions.

The evolution of tunicates did not stop there. By attaching to large animals or plants and becoming filter feeders, these organisms evolved into lampreys. Lampreys, with their fin development and peculiar nerve branching, were the first fish to have a nervous system extending to the shoulders and hips. The brain and sensory organs became more advanced, preparing the way for the development of vertebrates.Over time, the notochord became a spine, cartilage evolved into bone, and the bony fish eventually led to the development of land-dwelling animals.

In conclusion, tunicates, with their intriguing larval stages and evolutionary journey, have much to offer in understanding the close relationship between invertebrates and vertebrates.