Within the animal kingdom, species are organized into groups based on shared evolutionary history, a process known as phylogeny. This creates a “family tree” tracing lineages to common ancestors. One of the largest of these supergroups is Deuterostomia, a branch of the animal tree that includes creatures from starfish to humans. Understanding deuterostomes involves exploring the unique developmental path they share and the evidence scientists use to piece together their ancient connections.
Defining Deuterostome Development
The classification of an animal as a deuterostome is rooted in its embryonic development. All deuterostomes share developmental characteristics that separate them from other animal groups, like protostomes. These traits emerge after fertilization, once the embryo forms a hollow ball of cells called a blastula. During gastrulation, an indentation known as the blastopore forms, which deepens to create the rudimentary gut.
In all deuterostomes, this initial opening, the blastopore, is fated to become the organism’s anus. The mouth develops later at a different site as the gut tube tunnels through the embryo. This “second mouth” origin is the literal meaning of the term deuterostome and stands in contrast to protostomes, where the blastopore forms the mouth. This difference is a defining feature of the deuterostome lineage.
Another unifying characteristic is the pattern of cell division, or cleavage. Deuterostomes exhibit radial cleavage, where cells divide in planes parallel or perpendicular to the embryo’s main axis, creating neatly stacked layers. This contrasts with the spiral cleavage seen in many protostomes, where cell divisions are oriented at an angle, resulting in a misaligned arrangement.
Deuterostome development is also characterized by indeterminate cleavage, meaning the developmental fate of the earliest embryonic cells is not fixed. If a cell from an early embryo is separated, it can grow into a complete organism. This flexibility makes the natural formation of identical twins possible in humans and is a feature not found in organisms with determinate cleavage, where each cell’s fate is sealed early.
The Major Deuterostome Lineages
The deuterostome superphylum is composed of three primary phyla. The most recognizable is Phylum Echinodermata, an exclusively marine group that includes starfish, sea cucumbers, sea urchins, and crinoids. These animals are defined by a five-part (pentaradial) symmetry in their adult form, an endoskeleton of calcium carbonate plates, and a water vascular system used for locomotion, feeding, and respiration.
A smaller, less-known group is Phylum Hemichordata. These marine animals are generally worm-like, with common examples being acorn worms that live on or in the seafloor. A significant feature of hemichordates is the presence of pharyngeal slits, openings in the throat region used for filter-feeding.
The final and most familiar lineage is Phylum Chordata, which includes all vertebrates like fish, amphibians, reptiles, birds, and mammals, as well as invertebrate groups like tunicates and lancelets. All chordates are united by four anatomical features present at some stage of their life cycle:
- A flexible rod called a notochord
- A dorsal hollow nerve cord
- Pharyngeal slits
- A post-anal tail
In many vertebrates, like humans, some of these traits are only apparent during embryonic development.
Reconstructing the Deuterostome Family Tree
The current consensus, heavily informed by genetic data, shows a primary split within the deuterostomes. This branching event separated the ancestors of Phylum Chordata from the ancestors of the other two phyla, which are now grouped together in a distinct clade.
This clade, named Ambulacraria, unites Phylum Echinodermata and Phylum Hemichordata as sister taxa. This means that echinoderms and hemichordates share a more recent common ancestor with each other than either group does with chordates. The establishment of Ambulacraria is a significant revision to our understanding of animal evolution.
This view challenges older models based on comparing adult anatomy. For many years, scientists proposed that hemichordates were more closely related to chordates because both groups possess pharyngeal gill slits. This morphological similarity suggested a direct link, placing echinoderms as a more distant outgroup, but this hypothesis has been overturned by molecular evidence.
The recognition of Ambulacraria clarifies the major evolutionary pathways within the deuterostome lineage. It suggests an ancient deuterostome population split, with one branch leading to chordates and the other to the common ancestor of ambulacrarians. This ancestor then diverged, giving rise to the distinct body plans of echinoderms and hemichordates.
Evidence Supporting the Phylogenetic Model
The construction of the deuterostome family tree relies on integrating evidence from multiple scientific fields. The strongest support for the modern model, particularly the Ambulacraria clade, comes from molecular data. By comparing DNA and protein sequences, scientists can quantify genetic similarity, as organisms with fewer genetic differences are inferred to have a more recent common ancestor.
Analyses of specific genes, such as ribosomal RNA (rRNA) and mitochondrial DNA, consistently show a closer relationship between echinoderms and hemichordates than between either group and chordates. These molecular studies provide statistical support for the sister-group relationship within Ambulacraria. This genetic evidence forced a re-evaluation of how anatomical features were interpreted.
Morphological and developmental evidence also aligns with the molecular findings. For example, the larvae of some echinoderms and hemichordates are strikingly similar, supporting the Ambulacraria hypothesis. The pharyngeal slits shared by hemichordates and chordates are now thought to be an ancient feature of the ancestral deuterostome, which was subsequently lost in the echinoderm lineage.
The fossil record adds a deep-time perspective to these relationships. Fossil discoveries from the Cambrian period, over 500 million years ago, reveal a variety of early deuterostome body plans. While interpreting these ancient fossils can be challenging, they provide physical evidence of early deuterostome evolution and help calibrate timelines of divergence suggested by molecular clocks.