In deuterostomy, the developing embryo's first opening (the
blastopore) becomes the anus and
cloaca, while the mouth is formed at a different site later on. This was initially the group's distinguishing characteristic, but deuterostomy has since been discovered among
protostomes as well.[5] This group is also known as enterocoelomates, because their
coelom develops through
enterocoely.
Initially, Deuterostomia included the phyla
Brachiopoda,[6]Bryozoa,[7]Chaetognatha,[8] and
Phoronida[6] based on morphological and embryological characteristics. However, Deuterostomia was redefined in 1995 based on DNA molecular sequence analyses, leading to the removal of the
lophophorates which was later combined with other protostome animals to form the superphylum
Lophotrochozoa.[9] The
arrow worms may also be deuterostomes,[8] but molecular studies have placed them in the protostomes more often.[10][11] Genetic studies have also revealed that deuterostomes have more than 30 genes not found in any other animal groups, but are present in some marine algae and prokaryotes. This could mean they are very ancient genes that were lost in other organisms, or that a common ancestor acquired them through
horizontal gene transfer.[12]
While protostomes as a monophyletic group has strong support, research has shown that deuterostomes may be paraphyletic, and what was once considered traits of deuterostomes could instead be traits of the last common bilaterian ancestor. This suggests the deuterostome branch is very short or non-existent. The Xenambulacraria's sister group could be both the chordates or the protostomes, or be equally distantly related to them both.[13]
In both deuterostomes and protostomes, a
zygote first develops into a hollow ball of cells, called a
blastula. In deuterostomes, the early divisions occur parallel or perpendicular to the polar axis. This is called
radial cleavage, and also occurs in certain protostomes, such as the
lophophorates.
Most deuterostomes display
indeterminate cleavage, in which the developmental fate of the cells in the developing embryo is not determined by the identity of the parent cell. Thus, if the first four cells are separated, each can develop into a complete small larva; and if a cell is removed from the blastula, the other cells will compensate.
Another feature present in both the Hemichordata and Chordata is pharyngotremy; the presence of spiracles or
gill slits into the
pharynx, which is also found in some primitive fossil
echinoderms (
mitrates).[17][18] A hollow nerve cord is found in all chordates, including
tunicates (in the larval stage). Some hemichordates also have a tubular nerve cord. In the early embryonic stage, it looks like the hollow nerve cord of chordates.
Except for the
echinoderms, both the
hemichordates and the
chordates have a thickening of the
aorta, homologous to the chordate
heart, which contracts to pump blood. This suggests a presence in the deuterostome ancestor of the three groups, with the echinoderms having secondarily lost it.[citation needed]
The highly modified nervous system of echinoderms obscures much about their ancestry, but several facts suggest that all present deuterostomes evolved from a common ancestor that had pharyngeal gill slits, a hollow nerve cord, circular and longitudinal muscles and a segmented body.[19]
The defining characteristic of the deuterostome is the fact that the blastopore (the opening at the bottom of the forming gastrula) becomes the anus, whereas in protostomes the blastopore becomes the mouth. The deuterostome mouth develops at the opposite end of the embryo, from the blastopore, and a digestive tract develops in the middle, connecting the two.
In many animals, these early development stages later evolved in ways that no longer reflect these original patterns. For instance, humans have already formed a gut tube at the time of formation of the mouth and anus. Then the mouth forms first[citation needed], during the fourth week of development, and the anus forms four weeks later, temporarily forming a
cloaca.
Origins and evolution
Bilateria, one of the five major lineages of animals, is split into two groups; the
protostomes and deuterostomes. Deuterostomes consist of chordates (which include the vertebrates) and ambulacrarians.[20] It seems likely that the 555 million year oldKimberella was a member of the protostomes.[21][22] That implies that the protostome and deuterostome lineages split some time before Kimberella appeared — at least 558 million years ago, and hence well before the start of the Cambrian 538.8 million years ago,[20]i.e. during the later part of the
Ediacaran Period (circa 635-539 Mya, around the end of global
Marinoan glaciation in the late
Neoproterozoic). It has been proposed that the ancestral deuterostome, before the chordate/ambulacrarian split, could have been a chordate-like animal with a terminal anus and pharyngeal openings but no gill slits, with active suspension feeding strategy.[23]
The last common ancestor of the deuterostomes had lost all
innexin diversity.[24]
Fossils of one major deuterostome group, the
echinoderms (whose modern members include
sea stars,
sea urchins and
crinoids), are quite common from the start of Series 2 of the Cambrian, 521 million years ago.[25] The Mid
Cambrian fossil Rhabdotubus johanssoni has been interpreted as a
pterobranch hemichordate.[26] Opinions differ about whether the
Chengjiang fauna fossil Yunnanozoon, from the earlier Cambrian, was a hemichordate or chordate.[27][28] Another Chengjiang fossil, Haikouella lanceolata, is interpreted as a chordate and possibly a craniate, as it shows signs of a heart, arteries, gill filaments, a tail, a neural chord with a brain at the front end, and possibly eyes — although it also had short tentacles round its mouth.[28]Haikouichthys and Myllokunmingia, also from the Chengjiang fauna, are regarded as
fish.[29][30]Pikaia, discovered much earlier but from the Mid Cambrian
Burgess Shale, is also regarded as a primitive chordate.[31]
On the other hand, fossils of early chordates are very rare, as non-vertebrate chordates have no
bone tissue or teeth, and fossils of no Post-Cambrian non-vertebrate chordates are known aside from the
Permian-aged Paleobranchiostoma, trace fossils of the
Ordovician colonial tunicate Catellocaula, and various Jurassic-aged and Tertiary-aged spicules tentatively attributed to ascidians.
Phylogeny
Below is a
phylogenetic tree showing consensus relationships among deuterostome taxa. Phylogenomic evidence suggests the enteropneust family,
Torquaratoridae, fall within the
Ptychoderidae. The tree is based on 16S +18S rRNA sequence data and phylogenomic studies from multiple sources.[32][13] The approximate dates for each radiation into a new clade are given in millions of years ago (Mya). Not all dates are consistent, as of date ranges only the center is given.[33]
Support for the clade Deuterostomia is not unequivocal. In particular, the Ambulacraria are sometimes shown to be related to the Xenacoelomorpha. If true, this raises two possibilities: either the Ambulacraria are taken out of the deuterostome-protostome dichotomy (in which case the grouping Deuterostomia dissolves, with Chordata and Protostomia grouped together as
Centroneuralia), or the Xenacoelomorpha are re-positioned next to Ambulacraria within the Deuterostomia as in the above diagram.[13][34][35][36][37][38][39][40]
Fossil record
Deuterostomes have a rich fossil record with thousands of fossil species being found throughout the
Phanerozoic. The earliest undisputed deuterostomes are forms such as the early chordate Pikaia and the early echinoderm Gogia, each from about 515 million years ago. There are also a few earlier fossils that may represent deuterostomes, but these remain debated. The earliest of these disputed fossils are the
tunicate-like organisms Finkoella and Ausia from the
Ediacaran period. While these may in fact be tunicates, others have interpreted them as
cnidarians[41] or
sponges,[42] and as such their true affinity remains uncertain.
References
^Fedonkin, M. A.; Vickers-Rich, P.; Swalla, B. J.; Trusler, P.; Hall, M. (2012). "A new metazoan from the Vendian of the White Sea, Russia, with possible affinities to the ascidians". Paleontological Journal. 46 (1): 1–11.
Bibcode:
2012PalJ...46....1F.
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10.1134/S0031030112010042.
S2CID128415270.
^Fedonkin, M.A.; Simonetta, A; Ivantsov, A.Y. (2007), "New data on Kimberella, the Vendian mollusc-like organism (White sea region, Russia): palaeoecological and evolutionary implications", in Vickers-Rich, Patricia; Komarower, Patricia (eds.), The Rise and Fall of the Ediacaran Biota, Special publications, vol. 286, London: Geological Society, pp. 157–179,
doi:
10.1144/SP286.12,
ISBN978-1-86239-233-5,
OCLC156823511
^Butterfield, N.J. (December 2006). "Hooking some stem-group "worms": fossil lophotrochozoans in the Burgess Shale". BioEssays. 28 (12): 1161–1166.
doi:
10.1002/bies.20507.
PMID17120226.
S2CID29130876.
^M. A. Fedonkin (1996). "Ausia as an ancestor of archeocyathans, and other sponge-like organisms". In: Enigmatic Organisms in Phylogeny and Evolution. Abstracts. Moscow, Paleontological Institute, Russian Academy of Sciences, p. 90-91.