The Cambrian explosion is named for the geologically sudden appearance of numerous metazoan body plans (many of living phyla) between about 530 and 520 million years ago, only 1.7% of the duration of the fossil record of animals. Earlier indications of metazoans are found in the Neoproterozic; minute trails suggesting bilaterian activity date from about 600 million years ago. Larger and more elaborate fossil burrows appear near 543 million years ago, the beginning of the Cambrian Period. Evidence of metazoan activity in both trace and body fossils then increased during the 13 million years leading to the explosion. All living phyla may have originated by the end of the explosion. Molecular divergences among lineages leading to phyla record speciation events that have been earlier than the origins of the new body plans, which can arise many tens of millions of years after an initial branching. Various attempts to date those branchings by using molecular clocks have disagreed widely. While the timing of the evolution of the developmental systems of living metazoan body plans is still uncertain, the distribution of Hox and other developmental control genes among metazoans indicates that an extensive patterning system was in place prior to the Cambrian. However, it is likely that much genomic repatterning occurred during the Early Cambrian, involving both key control genes and regulators within their downstream cascades, as novel body plans evolved.

Reference

Abouheif
E.
,
Zaardaya
R.
,
Meyer
A.
(
1998
)
Limitations of metazoan 18S rRNA sequence data: Implications for recnstructing a phylogeny of the Animal Kingdom and inferring the reality of the Cambrian Explosion.
J. Mol. Evol
47
,
394
405
Ayala
F. J.
(
1997
)
Vagaries of the molecular clock.
Proc. Nat. Acad. Sci.USA
94
,
7776
7783
Ayala
F. J.
,
Rzhetsky
A.
,
Ayala
F. J.
(
1998
)
Origin of the metazoan phyla: molecular clocks confirm paleontological estimates.
Proc. Nat.Acad. Sci. USA
95
,
606
611
Balavoine
G.
,
Telford
M. J.
(
1995
)
Identification of planarian homeobox sequences indicates the antiquity of most Hox/homeotic gene subclasses.
Proc. Nat. Acad. Sci. USA
92
,
7227
7231
Bowring
S. A.
,
Grotzinger
J. P.
,
Isachsen
C. E.
,
Knoll
A. H.
,
Pelechaty
S. M.
,
Kolosov
P.
(
1993
)
Calibrating rates of Early Cambrian evolution.
Science
261
,
1293
1298
Bromham
L.
,
Rambaut
A.
,
Fortey
R.
,
Cooper
A.
,
Penny
D.
(
1998
)
Testing the Cambrian explosion hypothesis by using a molecular dating technique.
Proc. Nat. Acad. Sci. USA
95
,
12386
12389
Campos
A.
,
Cummings
M. P.
,
Reyes
J. L.
,
Laclette
J. P.
(
1998
)
Phylogenetic relationships of Platyhelminthes based on 18S ribosomal gene sequences.
Mol. Phylog. Evol
10
,
1
10
Carranza
S.
,
Baguna
J.
,
Riutort
M.
(
1997
)
Are the Platyhelminthes a monophyletic primitive group? An assessment using 18S rDNA sequences.
Mol. Biol. Evol
14
,
485
497
Cavalier-Smith
T.
,
Allsopp
M. T. E. P.
,
Chao
e. E.
,
Boury-Esnault
N.
,
Jacelet
J.
(
1996
)
Sponge phylogeny, animal monophyly, and the origin of the nervous system: 18S rRNA evidence.
Can. J. Zool
74
,
2031
2045
Crimes
T. P.
(
1974
)
Colonisation of the early ocean floor.
Nature
248
,
328
330
Davidson
E. H.
,
Peterson
K. J.
,
Cameron
R. A.
(
1995
)
Origin of adult bilaterian body plans: evolution of developmental regulatory mechanisms.
Science
270
,
1319
1325
Eernisse
D. J.
,
Albert
J. S.
,
Anderson
F. E.
(
1992
)
Annelida and Arthropoda are not sister taxa: A phylogenetic analysis of spiralian metazoan morphology.
Syst. Biol
41
,
305
330
Erwin
D. H.
,
Valentine
J. W.
,
Jablonski
D.
(
1997
)
The origin of animal body plans.
Amer. Sci
85
,
126
137
Fedonkin
M. A.
,
Waggoner
B. M.
(
1997
)
The late Precambrian fossil Kimberella is a mollusc-like bilaterian organism.
Nature
388
,
868
871
Fortey
R.
,
Briggs
D. E. G.
,
Wills
M. A.
(
1966
)
The Cambrian evolutionary ‘explosion’: decoupling cladogenesis from morphological disparity.
Biol. J. Linn. Soc
57
,
13
33
Friedrich
M.
,
Tautz
D.
(
1997
)
An episode of change of rDNA nucleotide substitution rate has occurred during the emergence of the insect order Diptera.
Mol. Biol. Evol
14
,
644
653
Garcia-Fernandez
J.
,
Holland
P. W. H.
(
1994
)
Archetypal organization of the amphioxus HOX gene cluster.
Nature
370
,
563
566
Gellon
G.
,
McGinnis
W.
(
1998
)
Shaping animal body plans in development and evolution by modulation of Hox expression patterns.
BioEssays
20
,
116
125
Giribet
G.
,
Ribera
C.
(
1998
)
The position of arthropods in the Animal Kingdom: a search for a reliable outgroup for internal arthropod phylogeny.
Mol. Phylogen. Evol
9
,
481
488
Gonzalez-Crespo
S.
,
Levine
M.
(
1994
)
Related target enhancers for dorsal and NF-KB signaling pathways.
Science
264
,
255
258
Grenier
J. K.
,
Garber
T. L.
,
Waren
R.
,
Whitington
P. M.
,
Carroll
S.
(
1997
)
Evolution of the entire arthropod Hox gene set predated the origin and radiation of the onychophoran/arthropod clade.
Curr. Biol
7
,
547
553
Grotzinger
J. P.
,
Bowring
S. A.
,
Saylor
B. Z.
,
Kaufman
A. J.
(
1995
)
Biostratigraphic and geochronologic constraints on early animal evolution.
Science
270
,
508
604
Halanych
K. M.
,
Bacheller
J. D.
,
Aguinaldo
A. M. A.
,
Liva
S. M.
,
Hillis
D. M.
,
Lake
J. A.
(
1955
)
Evidence from 18S ribosomal DNA that the lophophorates are protostome animals.
Science
267
,
1641
1643
Halder
G.
,
Callerts
P.
,
Gehring
W. J.
(
1995
)
New perspectives on eye evolution.
Curr. Op. Gen. Dev
5
,
602
609
Hoffman
P. F.
,
Kaufman
A. J.
,
Halverson
G. P.
,
Schrag
D. P.
(
1998
)
A Neoproterozoic snowball Earth.
Science
281
,
1342
1346
Irvine
S. Q.
,
Warinner
S. A.
,
Hunter
J. D.
,
Martindale
M. Q.
(
1997
)
A survey of homeobox genes in Chaetopterus variopedatus and analysis of polychaete homeodomains.
Mol. Phylogen. Evol
7
,
331
345
Jensen
S.
,
Gehling
J. G.
,
Droser
M. L.
(
1998
)
Ediacara-type fossils in Cambrian sediments.
Nature
393
,
567
569
Knoll
A. H.
(
1996
)
Breathing room for early animals.
Nature
382
,
111
112
Krumlauf
R.
(
1994
)
Hox genes in vertebrate development.
Cell
78
,
191
201
Lafay
B.
,
Smith
A. B.
,
Christen
R.
(
1995
)
A combined morphological and molecular approach to the phylogeny of asteroids (Asteroidea: Echinodermata).
Syst. Biol
44
,
190
208
Lake
J. A.
(
1990
)
Origin of the Metazoa.
Proc. Nat. Acad. Sci. USA
87
,
763
766
Nikoh
N.
,
Iwabe
N.
,
Kuma
K.-i.
,
Ohno
M.
,
Sugiyama
T.
,
Watanabe
Y.
,
Yasui
K.
,
Zhang
S.-c.
,
Hori
K.
,
Shimura
Y.
,
Miyata
T.
(
1997
)
An estimate of divergence time of Parazoa and Eumetazoa and that ofCephalochordata and Vertebrata by aldolase and triose phosphate isomerase clocks.
J. Mol. Evol
45
,
97
106
Pawlowski
J.
,
Bolivar
I.
,
Farhrni
J. F.
,
de Vargas
C.
,
Gouy
M.
,
Zaninetti
L.
(
1997
)
Extreme differences in rates of molecular evolution of foraminifera revealed by comparison of ribosomal DNA sequences and the fossil reocrd.
Mol. Biol. Evol
14
,
498
505
Peterson
K. J.
,
Cameron
R. A.
,
Davidson
E. H.
(
1997
)
Set-aside cells in maximal indirect development: evolutionary and developmental significance.
BioEssays
19
,
623
631
Quiring
R.
,
Walldorf
U.
,
Kloter
U.
,
Gehring
W. J.
(
1994
)
Homology of the eyeless gene of Drosophila to the Smalleye gene in mice and Aniridia in humans.
Science
265
,
785
789
Schierwater
B.
,
Kuhn
K.
(
1998
)
Homology of Hox genes and the zootype concept in early metazoan evolution.
Mol. Phylogen. Evol
9
,
375
381
Seilacher
A.
,
Bose
P. K.
,
Pfluger
F.
(
1998
)
Triploblastic animals more than 1 billion years ago: trace fossil evidence from India.
Science
282
,
80
83
Shubin
N.
,
Tabin
C.
,
Carroll
S.
(
1997
)
Fossils, genes and the evolution of animal limbs.
Nature
388
,
639
646
Smith
A. B.
,
Lafay
B.
,
Christen
R.
(
1992
)
Comparative variation of morphological and molecular evolution through geologic time: 28S ribosomal RNA versus morphology in echinoids.
Phil. Trans. Roy Soc. London
338
,
365
382
Swedmark
B.
(
1964
)
The interstitial fauna of marine sand.
Biol. Rev
39
,
1
47
Valentine
J. W.
(
1998
)
Cleavage patterns and the topology of the metazoan tree of life.
Proc. Nat. Acad. Sci. USA
94
,
8001
8005
Valentine
J. W.
,
Erwin
D. H.
,
Jablonski
D.
(
1996
)
Developmental evolution of metazoan bodyplans: the fossil evidence.
Dev. Biol
173
,
373
381
Vermeij
G. J.
(
1996
)
Animal origins.
Science
274
,
525
526
Warren
R. W.
,
Nagy
L.
,
Selegue
J.
,
Gates
J.
,
Carroll
S.
(
1994
)
Evolution of homeotic gene regulation and function in flies and butterflies.
Nature
372
,
458
461
Winnepenninckx
B.
,
Backeljau
T.
,
Mackey
L. Y.
,
Brooks
J. M.
,
DeWachter
R.
,
Kumar
S.
,
Garey
J. R.
(
1995
)
18S rRNA data indicate that Aschelminthes are polyphyletic in origin and consist of at least three distinct clades.
Mol. Biol. Evol
12
,
1132
1137
Wray
G. A.
,
Levinton
J. S.
,
Shapiro
L. H.
(
1996
)
Molecular evidence for deep Precambrian divergences among metazoan phyla.
Science
274
,
568
573
Xiao
S.
,
Zhang
Y.
,
Knoll
A. H.
(
1998
)
Three-dimensional preservation of algae and animal embryos in a Neoproterozoic phosphorite.
Nature
391
,
553
558
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