It may surprise you to learn that both you and a sea urchin are deuterostomes. Adult sea stars, sea urchins, and sea cucumbers—the most familiar echinoderms—look so different from adult vertebrates (fishes, frogs, lizards, birds, and mammals) that it may be difficult to believe all these animals are closely related. The evidence that all deuterostomes share a common ancestor that is not shared with the protostomes includes early developmental patterns and phylogenetic analysis of gene sequences, factors that are not apparent in the forms of the adult animals.
The major groups of living deuterostomes comprise three distinct clades. These are the echinoderms, which are the sea stars, sea urchins, and their relatives; hemichordates, which are acorn worms and pterobranchs; and chordates. The chordates include lancelets, sea squirts and other tunicates, and vertebrates.
Scientists are learning a lot about the ancestors of modern deuterostomes from fossils discovered in China, dating from the Cambrian period (about 500 million years ago). This particular fossil animal is an ancestral deuterostome with bilateral symmetry, external gills, and a segmented posterior body. Features such as these of fossil animals, together with the phylogenetic analyses of living species, show that the earliest deuterostomes were bilaterally symmetrical, segmented animals with pharyngeal slits. Pharyngeal slits are gill-like openings between the throat (or pharynx) and the outside of the body.
If we look at adult echinoderms, it is easy to wonder why they are grouped with other deuterostomes. Adult echinoderms have radial symmetry, not bilateral symmetry. However, their young, the larvae, are actually bilaterally symmetrical. Echinoderms undergo a radical change during body development into radially symmetrical animals. Radial symmetry is a derived trait in echinoderms that is absent in the deuterostome common ancestor. Another derived trait of echinoderms is the presence of calcified internal plates, which serve as an internal skeleton.
As is typical of animals with radial symmetry, echinoderms have no head, and they move slowly and equally well in many directions. Most echinoderms have an oral side (containing the mouth) and an opposite aboral side (containing the anus).
It turns out that living echinoderms also lack pharyngeal slits, a feature found sometime during the development of all other deuterostomes. However, gill-like structures have been found in fossilized ancestral echinoderms, so echinoderms did at one time have pharyngeal slits, but lost them during their evolution.
Hemichordates are wormlike marine deuterostomes. Hemichordates, unlike echinoderms, remain bilaterally symmetrical into adulthood. Hemichordates are acorn worms, such as this one, and pterobranchs, with bodies organized in three major parts: a proboscis, a collar (which bears the mouth), and a trunk (which contains the other body parts, including pharyngeal slits). Hemichordates are more closely related to echinoderms than to other deuterostomes. In fact, they both have ciliated larvae that in some cases appear very similar to each other.
Chordates are also deuterostomes. As adults the three groups of chordates couldn't appear more different. Yet their relatedness is evident, in many cases by comparing larval or embryo stages. At some stage in their development, all chordates display several derived structures: a dorsal hollow nerve cord, a tail that extends beyond the anus, and a dorsal supporting rod, the notochord.
A sea squirt is a marine filter-feeder in the group of tunicates. Tunicates lose their notochord as they develop into adults, but their larval forms display all the chordate features: a notochord, a dorsal hollow nerve cord, and a tail that extends beyond the anus. You can also observe the pharyngeal slits that mark tunicates as members of the deuterostome group. In tunicates and lancelets, the pharynx, with its slits, functions as a straining device to filter small food particles.
Lancelets are fishlike marine chordates. The adult lancelet is unique in displaying all three derived structures of chordates. The notochord is the most distinctive derived chordate trait. It is composed of a core of large cells with turgid fluid-filled vacuoles, which make the notochord rigid but flexible.
The vertebrate group takes its name from the jointed, dorsal vertebral column that replaces the notochord during early development. The pharyngeal slits found in the common ancestor of deuterostomes are present at some developmental stage in all chordates but are often lost or greatly modified in adults.
Four other key features, in addition to the vertebral column, characterize the vertebrates. They have an anterior skull enclosing a large brain. The vertebral column supports a rigid internal skeleton. Internal organs are suspended in a body cavity called a coelom. Vertebrates also have a well-developed circulatory system, driven by contractions of a ventral heart. Specialized structures for locomotion (such as fins) and feeding (such as jaws and teeth) evolved among the vertebrates. The evolution of these features allowed many vertebrates to become large, active predators, which in turn allowed the vertebrates to diversify widely, including to living on land.
The three major clades of deuterostomes are the echinoderms, the hemichordates, and the chordates. The common ancestry of these groups is supported by early developmental similarities and by phylogenetic analyses of DNA sequences. The earliest deuterostomes had bilateral symmetry, pharyngeal slits, and a segmented body.
Echinoderms are characterized by pentaradial symmetry, but they are bilaterally symmetrical as larvae. Echinoderms also have an internal skeleton of calcified plates. Also unique to this group is a water vascular system, a network of water-filled canals leading to extensions called tube feet that functions in gas exchange, locomotion, and feeding.
Hemichordates have a bilaterally symmetrical body divided into three parts: proboscis, collar, and trunk.
Chordates are characterized by a dorsal hollow nerve chord, a post-anal tail, and a dorsal supporting rod called a notochord at some point during the life cycle. Specialized structures for support (a vertebral column), locomotion (such as fins), and feeding (jaws and teeth) evolved among the vertebrates.