Deuterostomes: The Evolutionary Lineage of Second-Mouth Animals
In the vast tree of animal life, Deuterostomes represent one of the most significant branches of bilaterian animals—organisms characterized by bilateral symmetry and three germ layers. The term originates from the Ancient Greek deúteros (second) and stóma (mouth), a direct reference to the unique way these animals develop in the womb or egg.
While they encompass a diverse array of creatures, from the simplest sea squirts to the complexity of humans, all deuterostomes share a fundamental biological blueprint that separates them from their evolutionary cousins, the protostomes.
Key Facts
- Defining Trait: The first opening of the embryo (the blastopore) becomes the anus; the mouth forms second.
- Major Phyla: Includes Chordata (vertebrates and relatives), Echinodermata (sea stars, urchins), and Hemichordata (acorn worms).
- Development: Typically exhibit indeterminate cleavage, allowing separated early cells to develop into complete larvae.
- Temporal Range: Present from the Cambrian period (approximately 538.8 million years ago) to the present day.
- Coelom Formation: Known as enterocoelomates because their body cavity (coelom) forms via pouching of the gut.
Embryonic Development and Characteristics
The primary distinction of the superphylum Deuterostomia is the fate of the blastopore, the first opening that forms during gastrulation. In deuterostomes, this opening becomes the anus or cloaca, while the mouth develops later at a different site. This contrasts with protostomes, where the mouth typically develops first.
Beyond the blastopore, deuterostomes are characterized by indeterminate cleavage. In this process, the developmental fate of early embryonic cells is not fixed. If the first four cells of a developing embryo are separated, each has the potential to grow into a complete, albeit small, larva. This biological flexibility is the mechanism that allows for the occurrence of identical twins.
Structurally, many deuterostomes share ancestral traits. All chordates possess a hollow nerve cord, a feature also mirrored in the early embryonic stages of some hemichordates. Evidence suggests that the common ancestor of all modern deuterostomes likely possessed a segmented body, circular and longitudinal muscles, a hollow nerve cord, and pharyngeal gill slits.
Taxonomy and Classification
The classification of deuterostomes has evolved significantly with the advent of DNA molecular sequence analysis. In 1995, groups like the lophophorates (Brachiopoda, Bryozoa, Phoronida) were moved to the protostome superphylum Lophotrochozoa after genetic evidence contradicted previous morphological assumptions.
Current consensus divides the superphylum into several key groups:
- Chordata: Includes vertebrates (fish, mammals, birds, reptiles), Cephalochordata (lancelets), and Tunicata (tunicates).
- Ambulacraria: A clade comprising:
- Hemichordata: Such as acorn worms (Enteropneusta) and Pterobranchia.
- Echinodermata: Including sea stars (Asteroidea), sea urchins (Echinoidea), and sea lilies (Crinozoa).
- Cambroernida: An extinct clade of early deuterostomes.
There is ongoing scientific debate regarding the Xenacoelomorpha. While some suggest they are sister to the Ambulacraria, recent transcriptome analyses indicate they may be the basalmost bilaterian clade, meaning they split off before the divergence of protostomes and deuterostomes.
| Phylum | Common Examples | Key Distinguishing Feature |
|---|---|---|
| Chordata | Humans, Fish, Tunicates | Hollow nerve cord, notochord |
| Echinodermata | Sea Stars, Sea Urchins | Pentaradial symmetry (adults), water vascular system |
| Hemichordata | Acorn Worms | Pharyngeal gill slits, worm-like body |
Origins and the Fossil Record
The split between protostomes and deuterostomes likely occurred during the Ediacaran Period, well before the Cambrian explosion. This is supported by the existence of Kimberella (approx. 555 million years ago), which is believed to be a protostome.
The fossil record for deuterostomes is extensive. Early candidates include the Ediacaran Arkarua (possibly an early echinoderm) and tunicate-like organisms such as Burykhia and Ausia, though the latter remain debated. By the Mid-Cambrian (roughly 508 million years ago), all main lineages were established, as evidenced by fossils like Pikaia (a primitive chordate) and Spartobranchus (an acorn worm).
Frequently Asked Questions
What is the main difference between a deuterostome and a protostome?
The primary difference is the development of the embryo: in deuterostomes, the first opening (blastopore) becomes the anus, whereas in protostomes, it typically becomes the mouth.
Why are deuterostomes called enterocoelomates?
They are called enterocoelomates because their coelom, or main body cavity, forms through a process called enterocoely, where the cavity develops from pouches budding off the gut.
How do identical twins occur in deuterostomes?
Identical twins are possible due to indeterminate cleavage. Because the fate of early embryonic cells is not predetermined, a split embryo can result in two separate, complete individuals.
Which animals are considered deuterostomes?
Deuterostomes include all vertebrates (mammals, birds, reptiles, amphibians, fish), as well as invertebrates like sea stars, sea urchins, sea cucumbers, and acorn worms.
When did deuterostomes first appear in the fossil record?
While some disputed fossils exist in the Ediacaran, they are well-established by the start of the Cambrian period, approximately 538.8 million years ago, with diverse forms appearing by 508 million years ago.